chapter 4-impacts of projected climate changes · this chapter aims to overview the main findings...

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4

Impacts of Projected Climate Changes

This chapter describes potential impacts of the projected changes in climate over

the 21st century for a range of sectors (water, ecosystems, food supply, coasts,

settlements and society, and human health), using examples from diverse

regions to illustrate those impacts. The chapter also discusses the importance of

changes in extreme events for impacts on a range of sectors. This material is

drawn from the Fourth Assessment Report (AR4) of the Intergovernmental Panel

on Climate Change (IPCC), specifically the Working Group II assessment and its

Technical Summary.29 The description of impacts on specific sectors and the

regional examples are generally drawn from the Working Group II assessment of

sectoral impacts (WGII chapters 3–8) and regions (WGII chapters 9–16). The

material contained in the IPCC assessment is very rich and detailed. This chapter

necessarily has to be selective, so focuses on a few issues that I regard as

particularly significant and illustrative of how impacts could occur in certain sectors or regions.

In the closing part of this chapter, we look at the degree to which the

severity or extent of impacts depends on the magnitude and rate of climate

change over the 21st century, and which regions are likely to be especially

affected by the impacts of climate change. This analysis highlights the large

regional and global disparity resulting from climate change impacts. This

information is drawn from the Working Group II Technical Summary and

Summary for Policymakers, and the integration of findings on impacts, adaptation, and mitigation in the AR4 Synthesis Report.

The scientific literature on climate change impacts continues to grow rapidly,

but I have not attempted to provide any updates in this chapter due to the very

large range of subjects that it covers. This chapter, therefore, relies entirely on the assessment of the Working Group II Report and my understanding of it.

Chapter contents

4.1 Introduction ................................................................................................................99

4.1.1 Potential impacts of climate change without major adaptation measures .......99

4.1.2 Capturing the diversity across systems, sectors, and regions ..........................99

4.1.3 Underlying climate and socioeconomic scenarios.........................................100

4.2 Water .........................................................................................................................101

4.2.1 Changes in water supply and demand ...........................................................101

Changes in run-off related to precipitation and temperature .........................101

Role of glaciers and snow fields as seasonal water storage...........................104

Groundwater and salinisation ........................................................................104

4.2.2 Estimates of the number of people affected by increased water stress..........104

4.2.3 Changes in flood risk.....................................................................................105

4.2.4 Changes in water quality ...............................................................................106

29 AR4 comprises four volumes: the Working Group I, II, and III Reports (IPCC, 2007a (WGI), 2007b

(WGII), 2007c (WGIII)) and the Synthesis Report (IPCC, 2007d (SYR)). Each report has a

Summary for Policymakers (SPM) and each Working Group report has a Technical Summary (TS).

Climate Change 101 – An Educational Resource

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4.3 Ecosystems ................................................................................................................106

4.3.1 Selected impacts on key ecosystems .............................................................107

Terrestrial carbon cycling and carbon fertilisation........................................107

Impacts on some large-scale ecosystems.......................................................107

Ecosystem hotspots .......................................................................................108

4.3.2 Risk of species extinction..............................................................................109

4.4 Food supply...............................................................................................................110

4.4.1 Large-scale changes in food production........................................................111

4.4.2 Number of people at risk from hunger and regional differences...................112

4.4.3 Food supply from oceans and freshwater systems ........................................115

4.5 Coastal zone ..............................................................................................................115

4.5.1 Key impacts on different types of coastal systems........................................116

4.5.2 Projected number of people affected by coastal flooding .............................117

4.6 Health ........................................................................................................................118

4.6.1 Main projected impacts and key affected regions .........................................119

Malaria, dengue fever, and other infectious diseases ....................................119

Heat- and cold-related mortality....................................................................120

Flow-on effects of storms, floods, and droughts ...........................................121

Urban air quality............................................................................................121

4.6.2 Global aggregated impacts and costs.............................................................121

4.7 Settlements and society ............................................................................................122

4.7.1 Impacts on industry and services...................................................................123

4.7.2 Impacts on human settlements and vulnerable social groups........................124

4.7.3 Cost estimates................................................................................................125

4.8 Extreme events..........................................................................................................126

4.9 Severity and extent of impacts as a function of temperature ...............................129

4.9.1 Examples of impacts related to global average temperature .........................129

4.9.2 Implications for adaptation and mitigation as options to reduce impacts......131

4.10 Regional distribution of impacts and especially affected regions ........................131

Boxes in chapter

Box 4.1: Importance of seasonal changes in water supply in India......................................102

Box 4.2: Ocean acidification ................................................................................................110

Box 4.3: Carbon dioxide fertilisation – how far can it boost plant growth?.........................114

Box 4.4: Small islands – at the squeeze from coastal and other pressures ...........................118


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4.1 Introduction

Chapter 3 outlined projected future changes in the physical climate system for a range of emissions scenarios that assumed no dedicated climate policies to reduce greenhouse gas emissions. Some of the projected changes in climate are staggering in themselves, but we need to look in more detail at the implications of these changes for specific sectors, systems, and regions to fully understand the potential impact of climate change.

4.1.1 Potential impacts of climate change without major adaptation measures

This chapter assesses the impacts on different sectors and regions if no major steps to adapt to the projected future climate changes are taken. It is important to keep this assumption in mind: the impacts described in this chapter are generally potential impacts, not necessarily the impacts that will occur, because humans and to some extent ecosystems will do their best to adapt to those changes. However, we need a solid understanding of these potential impacts, including their biophysical mechanisms and social consequences, to lay the foundation for understanding the potential for adaptation to those changes, which is the subject of chapter 5.

We also need to remember at the outset that adaptation also has its limits, and some systems and regions will be less able to adapt to climate changes than others. The extent to which the potential impacts described in this chapter will turn out to be a reality in some regions is an open research question and depends on broader societal developments in different parts of the world during the 21st century (chapter 9 discusses this in more detail). In addition, it is important to note that even if we are able to adapt to some of the potential impacts, many adaptation options still represent an opportunity cost to society, because they force us to devote our efforts to dealing with the impacts of a changing climate rather than improving living conditions within a stable climate.

For these reasons, the study of potential impacts in the absence of major adaptation efforts provides an important baseline for understanding the consequences of climate change on different systems, sectors, and regions around the world. We can hope that actual impacts will be somewhat less severe due to adaptation, but it is not currently possible to estimate the effectiveness of adaptation in the real world reliably (see also chapter 5).

It is also worth noting that most studies of the impacts of climate change assume no major efforts to reduce greenhouse gas emissions (generally called ‘mitigation’ efforts). If such efforts are undertaken globally, this could reduce the rate and amount of climate change and thus reduce, avoid, or at least delay many potential impacts. However, mitigation cannot avoid all impacts because some further warming and associated potential impacts are inevitable even under the most stringent mitigation efforts.

4.1.2 Capturing the diversity across systems, sectors, and

regions

This chapter aims to overview the main findings regarding the potential impacts of climate change for a range of systems and sectors, specifically water, ecosystems, food supply, the coastal zone, settlements and society, and human health. We also look at the importance of changing climatic extremes. It is difficult to


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comprehensively describe the impacts of climate change because they vary from one location to the next, depending on local circumstances; what is a negative impact in one place can be beneficial elsewhere. (For example, an increase in rainfall can be beneficial in dry regions but increase flood risk in already wet regions; and the impacts depend on whether you are interested in the consequences for highly adapted ecosystems, which might tolerate only small changes in climate, or modern agriculture, which might be more flexible.) For a more detailed, context-specific discussion of impacts, see WGII chapters 9–16 and the country-specific studies referred to in those chapters.

Based on our survey of key impacts on different sectors and systems, we can then ask the question: who is likely to be especially affected by these potential impacts? Answering this question will help us to understand the large differences in vulnerability to climate change between different regions of the world. This is one reason why it is so difficult to reach a global agreement on what level of climate change and what risks are acceptable – the answer to this question very much depends on who you are and where you live.

4.1.3 Underlying climate and socioeconomic scenarios

Virtually all the assessments discussed here and in the IPCC assessment are based on scenarios of climate change for the 21st century from the Special Report on

Emissions Scenarios (SRES; IPCC, 2000) (see section 3.2). These scenarios and impacts, therefore, form a baseline of the sort of impacts that could occur if no major efforts are made to reduce greenhouse gas emissions. The SRES scenarios do represent a range of alternative socioeconomic developments (eg, different rates of change for population, wealth, and access to technologies). One important conclusion from the literature the IPCC assessed is that future impacts of climate change only partly depend on the amount of climate change itself, they also depend on the socioeconomic changes in those scenarios (eg, the number of people who live in high-risk areas and the amount of access these people have to information, technology, and financial resources to protect themselves against increasing climate-related risks). Most impacts discussed in this chapter are based on a range of SRES scenarios, and results cover not just the range of future climate changes, but also the range of socioeconomic developments that are assumed in these scenarios. Where relevant studies are available, we highlight the extent to which specific impacts depend on the socioeconomic aspects of specific SRES scenarios. Chapters 5 and 9 explore in more detail how vulnerability and the ability to adapt to climate change are influenced by these scenarios. (WGII 2.4)

We already know from chapter 3 that climate change will not stop in 2100, and sea-level rise in particular will continue for many more centuries into the future even if greenhouse gas emissions are reduced significantly, which could lead to several metres of sea-level rise over the next few centuries. However, very few assessments have investigated the impacts of climate change beyond 2100. The main reason for this is that it becomes increasingly problematic to describe impacts that not only depend on changes in the physical climate system but also on the characteristics of the human society that is affected. We simply do not know how agriculture (or food production more generally) may be operating in 2150, so discussing the impacts of climate change on food supply in 2150 would be largely speculative. (WGII 2.4)

Another important limitation of currently available impacts studies is that most studies only investigated a range of climate change of about 1–5°C above 1980–1999


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temperatures. However, particularly in the longer term beyond 2100 (see section 3.5), we know that global average temperatures could increase by more than 5°C. The impacts that a global temperature increase of this magnitude would create are largely unknown, because a change of such a magnitude is so far outside any historical analogues and laboratory studies that again we would end up with speculation rather than scientific study. This in itself may be a good reason for deciding that it is better if the world does not go there. Note that many land regions warm more than the global average, so local temperature increases in excess of 5°C could occur over large continental land masses well before 2100 under mid-range to high emissions scenarios (see Figure 3.3).

Very few studies have quantified the extent to which global efforts to reduce greenhouse gas emissions could reduce the impacts of climate change. Because impacts generally increase with the amount of climate change, we can assume that limiting greenhouse gas emissions, and hence the magnitude and rate of climate change, will also reduce its impacts. However, we need to be clear that this is a simplistic picture because impacts do not necessarily change linearly with the amount of climate change. In addition, a society that makes a concerted effort to reduce greenhouse gas emissions may be quite different in its governance and international collaboration from a society that does not make such efforts, which could in turn influence its vulnerability to any given impact. We look briefly at these issues in section 4.9 (based on WGII 18.4 and 20.6, and SYR 5.7).

Note that the scientific literature exploring potential impacts under mitigation scenarios and their socioeconomic characteristics is very limited and was not assessed and discussed extensively by the IPCC. The next IPCC assessment, due for completion in 2014, is expected to provide much richer information on the degree to which mitigation scenarios could reduce potential and actual impacts.

4.2 Water

One of the most important impacts of climate change will be on freshwater resources. Water in its various forms (as ice, snow, rainfall, lakes, rivers, and groundwater) is vital for all sectors and regions, so understanding how climate change is likely to affect the future availability and demand for water is critical, including the risks that water can present through extreme rainfall and associated flooding, or the acute absence of water during droughts. (SYR 3.3)

4.2.1 Changes in water supply and demand

Changes in precipitation and temperature (see chapter 3) together can lead to changes in run-off; run-off is the part of precipitation that is not absorbed by soils, does not evaporate, and is not transpired by plants but flows off on the surface of the land. The amount of run-off is a critical quantity because it determines the amount of freshwater that is available for human uses (drinking water, irrigation, and industrial and domestic uses). (WGII 3.1)

Changes in run-off related to precipitation and temperature

Rising temperatures increase evaporation. Hence, run-off can decline even in regions where precipitation does not decline or perhaps even increases. At the same time, regionally large increases in water demand for irrigation are likely because of the greater evaporation and transpiration from plants, in particular given that the areas


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affected by drought are likely to increase. Currently, irrigation accounts for about 70% of all global water withdrawals and 90% of consumptive use (ie, the water that cannot be recycled and used again downstream).(WGII 3.2, SYR 3.3)

Figure 4.1 shows projected regional annual average changes in run-off by the end of the 21st century relative to the 1980–1999 baseline for the SRES A1B emissions scenario. The pattern is similar to the pattern of projected changes in annual average precipitation, but with greater changes in some regions because of the compounding effects of reducing precipitation and rising temperatures. In general, large increases in run-off are projected mainly at high latitudes and in some already wet parts of the tropics. Significant decreases in run-off are projected in subtropical and low mid-latitude regions, for example the Mediterranean Basin, western United States, southern Africa, and north-eastern Brazil. Many of those regions are already water stressed and will suffer from an increased risk of drought. Even though the figure shows changes by the end of the 21st century, a large part of those changes is expected to occur by the middle of the 21st century. Note that in some regions, rainfall is highly seasonal, and a small change in annual average run-off could mask a much more significant shift in water availability during the dry season (see Box 4.1 for an example). (WGII 3.4; SYR 3.3)

It is worth noting that confidence in model projections is not uniform across the globe. Confidence is generally highest in high-latitude regions and some mid-latitudes and subtropics, but it is often much lower in the tropics, particularly areas influenced by monsoons, and desert regions. This reflects the difficulties in simulating precipitation in highly dynamic regions of the atmosphere compared with in regions dominated by more stable atmospheric circulation patterns. In addition, changes in run-off can vary significantly from one catchment to the next, and confidence in local-scale projections is often much lower than for broad regional changes.

Box 4.1: Importance of seasonal changes in water supply in India

Changes in annual average run-off are not necessarily the best way to describe

relevant changes in some regions. The Indian subcontinent has a strongly

seasonal water supply: its dry season runs from about October to May when

the only water supply comes from glacier melt and very limited rainfall in

the Himalaya Ranges. Large amounts of rainfall come almost exclusively during

the wet season from June to September when the monsoon brings large

amounts of moist air from the Indian Ocean over the dry land masses.

Most models suggest that the most intense precipitation events during the

wet season would intensify further, so the annual average rainfall would

increase over the region. However, the rainfall during the dry season is

projected to decline. This means the Indian subcontinent may benefit very little

from an increase in rainfall during the wet season, unless it can find ways to

store excess water and use it during the dry season to offset the continuing

drying trend. Without such additional adaptation measures, the projected

increases in annual average precipitation, attractive as they may sound for a

region that is already dry and water stressed, could increase risks of flooding

during the wet season and increase the risk of drought during the dry season,

and thus have mostly negative effects.

Impacts of P

rojected Clim

ate Changes

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Figure 4.1: Changes in run-off in different world regions, 2090–2099 relative to 1980–1999

Note: Projected median changes in run-off by 2090–2099 relative to 1980–1999 for the

A1B emissions scenario from the Special Report on Emissions Scenarios (IPCC, 2000),

from a set of 12 climate models. Areas are stippled where more than 90% of the models

agree on the direction of change, and areas are white where less than 66% of the models

agree on the direction of change. Run-off could vary significantly on very small spatial

scales and changes may also vary between seasons, which is not captured by this global annual average map.

Source: SYR Figure 3.5.


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Role of glaciers and snow fields as seasonal water storage

Apart from the impacts of changes in precipitation patterns and temperature, reductions in glaciers and snow cover are also projected to have significant implications in some regions. More than one-sixth of the world population lives in regions where at least part of the available water supply is drawn from water that is seasonally stored as snow and ice in major mountain ranges, for example, the Hindu-Kush, Himalaya Ranges, and Andes. Also, many cities in the western United States depend on snow and ice melt from the Rocky Mountains. The projected accelerated shrinkage of glaciers and disappearance of many smaller glaciers in these regions is expected to initially lead to greater spring flows and the risk of glacier outburst floods associated with the enhanced melting, followed by reduced summer flows as glaciers recede. (WGII 3.4, 10.4, 13.4, 14.4)

The effects of shrinking glaciers on water supply are expected to be particularly severe for regions whose only water supply is from these sources, and where water demand for agriculture coincides with the current spring peak flow from glacier melt. Some examples for such regions are the dry highlands of the Andes (Colombia, Bolivia, Peru, and Ecuador) and parts of the Himalaya Ranges, where about half a billion of people rely on glacial meltwater and the area of glaciers is projected to reduce to about 20% of its current extent over the next several decades. (WGII 10.4, 13.4)

Groundwater and salinisation

Apart from surface run-off, climate change is also expected to impact on the recharge of groundwater resources, but knowledge about groundwater resources is much more limited than for surface water. The few available studies show a potential for significant decreases in groundwater supply, driven by reduced recharge and increasing demand. (WGII 3.4)

Changes to freshwater supply can occur due to not only changes in precipitation, temperature, or glacier extent, but also sea-level rise. Rising saltwater levels can lead to increasing salinity in freshwater supplies from lakes, estuaries, and aquifers and make water unsuitable for irrigation or human consumption without additional treatment. Such salinisation processes are projected to occur particularly in coastal aquifers and in the freshwater lenses that provide the only source of freshwater in many small islands. We should note that climate change and sea-level rise are not the only reasons for salinisation: excessive withdrawals from groundwater aquifers can also lead to the intrusion from neighbouring saltwater aquifers in coastal regions, which has already been observed in many locations. (WGII 3.4, 16.4)

4.2.2 Estimates of the number of people affected by increased

water stress

The number of people affected by the combined changes in water supply and demand from climate change is significant by any measure. For a temperature increase of up to 1°C and associated precipitation and run-off changes, between 400 million and 1.7 billion additional people are projected to experience increased water stress; for temperature increases of 3°C and associated changes, this number increases to between 1.1 billion and 3.2 billion people. The relatively large range of those estimates is in part a consequence of different assumptions about future socioeconomic developments – how many people are affected by changes in run-off in part depends on how many people live in water resource stressed areas to begin


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with. The highest numbers are projected under the A2 SRES scenario, due to its high population growth and lower economic growth, which leaves many more people in areas susceptible to additional water stress. (WGII 3.4, 20.6)

We should note that these figures describe only the additional people projected to be affected by climate change. Even in the absence of any climate change, the number of people experiencing water stress is expected to grow from currently about 1.5 billion to 2 billion to about 3 billion to 6 billion towards the end of the 21st century, mostly due to increases in population and increases in water demand associated with socioeconomic development. So while climate change is not the only reason for water stress, it is expected to add another billion or two people to the number already under stress. (WGII 3.6, 20.6)

Thus, climate change is expected to significantly exacerbate one of the key challenges of the 21st century, which is to provide access to clean freshwater for human consumption and agricultural production, which is an essential requirement if agricultural production in many developing country regions is to be increased. Improved irrigation techniques could reduce consumptive water usage significantly but require good governance and access to technology and finance for their implementation. Climate-related limits to water supply, therefore, present a major challenge to sustainable development and are likely to particularly affect populations who have limited capacity to deal with current inter-annual variability in rainfall, and whose access to improved technology or whose governance systems for water storage and distribution may be limited. (WGII 3.6, 3.7, 20.6, 20.7)

4.2.3 Changes in flood risk

Over the past decade, floods have been the most reported natural disaster event in Africa, Asia, and Europe and have affected more people (about 140 million people per year on average) than all other natural disasters. The economic impacts of floods have increased by a factor of five during the past half century, but this increase is mostly related to the value of the assets at risk and the increasing concentration of buildings in flood plains. An increase in heavy precipitation events has been observed, but this has not yet translated into clear evidence of a climate-related trend in flooding during the past decades. (WGII 3.2)

As discussed in chapter 3, a continuing rise in temperatures is very likely to lead on average to an increase in extremely heavy precipitation events, which would increase the risk of future flooding. What is currently a 1-in-100 year flood event could occur much more frequently by the end of the century, with reduced return periods of less than 10 years, though quantitative projections of changes in flood risk generally have relatively large uncertainties and vary significantly between different regions and even individual catchments. (WGII 3.4)

Up to 20% of the world’s population is estimated to live in areas that will be affected by such increases in flood risk. The impacts of projected future changes in flood risk are expected to be particularly severe in developing regions that have limited capacity to adapt to and cope with such flooding events by providing physical protection mechanisms and/or early warning systems (see chapter 5). For countries with a flat topography such as Bangladesh, some studies project that a warming of 2°C would increase the area at risk of flooding by about 25%. Extreme rainfall events can also present challenges to large-scale infrastructure, including hydropower stations. (WGII 3.4)


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4.2.4 Changes in water quality

Rising temperatures are also expected to affect water quality, mostly in negative ways. Higher surface temperatures can promote more algal blooms and water borne diseases. Reductions in rainfall and increased risk of drought exacerbate these risks. Regions with an increased incidence of heavy rainfall events could see an increase in erosion and sediment load, which implies a greater load of excess nutrients, heavy metals, sewage, and bacteria in water bodies after storm events. Such events have been linked to increases in cryptosporidium and cholera outbreaks (more on health impacts in section 4.7). (WGII 3.4)

The projected impacts of climate change on water supply will have positive and negative aspects, but overall, the negative aspects are projected to outweigh the positive. This is because areas in which run-off increases (which would be beneficial in principle) are also expected to experience an increased incidence of heavy rainfall and associated increase in flood risk, deterioration of water quality, and seasonal shifts. (WGII 3.4; SYR 3.3)

4.3 Ecosystems

Ecosystems can be defined as a dynamic complex system of interacting plant, animal, and micro-organism communities. Ecosystems provide a range of values and services to human society, including the provision of products (eg, plants, animals, and chemical substances); regulating services (eg, the cleaning and filtration of freshwater, stabilisation of hill slopes from erosion, regulation of diseases and pests by maintaining a balance between predators and food species along the food web, and sequestration of carbon); and cultural services (eg, recreation spaces). Those services are underpinned by biodiversity and basic primary production of organic material from inorganic substances. (WGII 4.1)

In addition to ecosystem services that are directly relevant to human society, we can also value intact ecosystems and individual species per se, even if they do not perform a direct material service to human society. The motivation for doing so could be philosophical or ethical (eg, the belief that animals have the same right to survival and well-being as humans), or simply the precautionary principle that we may not yet have realised the services that some species and ecosystems are performing to human society.

The impacts of climate change can manifest themselves in a change in any of the ecosystems services described above. Some ecosystem services may be possible to be maintained even though some particular species might be lost and their functions replaced by others; but from a cultural perspective, the loss of any particular species may also be a relevant impact.

Impacts on ecosystems can occur through a change in the basic climatic conditions in which ecosystems and individual species function, as well as disturbances to ecosystems that are triggered by climate-related events such as floods, droughts, wildfires, insect outbreaks, and the gradual acidification of the oceans. Apart from direct climate-related impacts, the increasing concentration of carbon dioxide (CO2) in the atmosphere is expected to play a major role in changing ecosystem composition through carbon fertilisation as well as the increasing acidification of the oceans (see Box 4.2). These climate-related stresses add to other human-generated pressures such as logging and deforestation, pollution, the fragmentation of natural systems, and the overexploitation of natural resources, and can influence how resilient ecosystems are against changes in their biophysical environment. (WGII 4.1, 4.2)


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4.3.1 Selected impacts on key ecosystems

The key conclusion from the AR4 Working Group II assessment is that during the 21st century, the resilience of many ecosystems is likely to be exceeded by a combination of climate change, including the direct consequences of rising CO2 concentrations, associated disturbances, and other human-induced pressures.

Terrestrial carbon cycling and carbon fertilisation

Terrestrial ecosystems provide an essential service by absorbing CO2 from the atmosphere and storing it in plant material and soil. If CO2 concentrations were to increase but the climate did not change, we would expect the rate at which the biosphere absorbs CO2 to increase over time because the higher CO2 concentrations act as fertiliser for plant growth. The already observed ‘greening’ of the planet discussed in chapter 1 is consistent with this initial enhancement of primary productivity under moderate warming and rising CO2. The strongest effects of carbon fertilisation are expected where carbon is a limiting factor in plant growth, but a range of field experiments indicate that limitations in other nutrients could lead to a weaker overall response than laboratory studies with young species suggest. (WGII 1.3, 4.4)

However, as discussed in chapter 3, rising temperatures are expected to reduce the effectiveness with which this storage occurs due to increasing nocturnal respiration by plants and soils and the increased risk of disturbances of ecosystems from droughts and consequent increasing risks of insect outbreaks and wildfires. Dynamic vegetation models, based on laboratory data and field experiments, can be used to estimate the combined effect of those future changes. These models indicate that if greenhouse gas emissions and other land-use changes such as deforestation continue at or above current rates, it is likely that the uptake of CO2 by terrestrial ecosystems will peak before mid-century and then begin to decline. While some models show only a gradual decline in the uptake of carbon, other models suggest that the terrestrial biosphere could become a net source of carbon before the end of the 21st century as a result of these combined stresses, so would amplify rather than buffer climate change. (WGII 4.4)

Even though increasing CO2 concentrations generally increase plant growth, their impacts on ecosystems functions are often negative. This is because not all plants respond in equal measure to enhanced atmospheric CO2, which could unravel existing balances between plants and the animals that depend on them as food sources. For example, lianas (a type of vine) in tropical forests have been shown to respond particularly strongly to increased CO2, and a disproportionate increase in lianas could lead to an overall loss of carbon due to the detrimental impacts of these stranglers on trees that store most of the carbon. (WGII 4.4)

Impacts on some large-scale ecosystems

Some large-scale terrestrial and oceanic ecosystems have been identified as particularly sensitive to climate change. These include savannahs, tropical rainforests where precipitation declines, tundra, boreal forests, and the regions of the world dominated by sea ice. These ecosystems are highly sensitive to increasing average temperatures, which are expected to be amplified by disturbance regimes (eg, increasing risk of fire in savannahs or of insect and pest outbreaks in boreal forests) or by biophysical feedbacks (such as the disappearance of sea ice that enhances local warming and leads to changes in ocean temperatures and nutrients). (WGII 4.4, WGII SPM)


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Overall, the warming of the atmosphere is expected to lead to a northward shift in many large-scale ecosystems, but these shifts are unlikely to include all dependent species at the same rate. As a result, many animal and plant communities are expected to be increasingly disrupted because some species move faster than others, and the loss of one species can precipitate changes through a more extended food web to other species. (WGII 4.4)

For increases in global average temperature above 1.5–2.5°C relative to 1980–1999 and associated increases in CO2 concentrations, the IPCC assessment projects major changes in ecosystem structure and function, the ecological interactions between species, and shifts of species ranges. These changes are expected to have predominantly negative implications for biodiversity and the goods and services provided by these ecosystems, such as water and food supply. As one example, several studies have highlighted the risk of collapse of the Amazon rainforest, because drying trends projected by a group of climate models would lead to a transition from evergreen rainforest cover to rain-green forests or even grasslands; greater occurrence of droughts and forest fires would further accelerate such shifts in this important ecosystem and could be associated with a large loss of carbon. (WGII 4.4, SYR 3.3)

The consequences of projected changes in these ecosystems are also generally negative for large fauna in the relevant regions. For example, large reductions in the order of tens of percent in mammal species richness are projected for many of Africa’s savannah regions due to the combined effects of reduced rainfall and increasing drought risk, fire disturbances, and the need to change migration routes. In the north polar region, reductions in sea ice are expected to present a significant threat to the survival of polar bears and migratory birds that rely on tundra for their summer breeding grounds. (WGII 4.4, Box 4.3, Box 4.5)

Ecosystem hotspots

Some ecosystems are expected to be particularly affected because they occupy particular climatic and/or ecological niches and cover only limited regions, so are highly vulnerable to changes in the climatic conditions to which they are adapted. Examples of such highly specialised ecosystems are mountain regions, coastal mangroves and salt marshes, coral reefs, and Mediterranean-type ecosystems. Effects of climate change on these highly sensitive systems are expected even at relatively low levels of warming of less than 1°C above current temperatures. (WGII 4.4, SYR.3.3)

Mountain ecosystems are particularly affected because of their sensitivity to warming, slow growing rates, and thus limited ability to migrate and keep up with changes, and reliance on snow and/or cloud cover and associated moisture regimes. Their ability to adjust to changes by migrating is also limited by simple physical constraints – they eventually run out of suitable habitat at the top of a mountain if they migrate upwards in response to climate change. (WGII 4.4)

Coastal mangroves and salt marshes are affected by sea-level changes, but their particular sensitivity comes from the combination of this with other stresses, such as changes in sediment loads and flood risk from rivers and coastal storms, and human modifications to the coastal zone that reduce habitat and prevent inland migration with rising sea levels (which otherwise would be a natural and effective adaptation response by mangroves). For a sea-level rise of 36–72 cm, a loss of 33–44% of all coastal wetlands is estimated. Mangrove habitats act as natural buffers against high


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tides and coastal erosion, so the loss of the mangrove habitat can have significant consequences for human society. (WGII 4.4, 6.4, 16.4)

Coral reefs have long been identified as sensitive to climate change through increasing sea surface temperatures, combined with ocean acidification. Human pressures in the form of nutrient loads, sedimentation from agriculture and erosion in nearby land areas, over-fishing, and pollution add to these stresses (see also Box 1.5). The most recent IPCC assessment found that temperature increases of 1°C would increase coral bleaching, while for temperature increases of about 2°C, most corals would be bleached regularly. If temperatures exceed 3°C above 1980–1999 levels, widespread coral mortality is expected, with a shift towards algae-dominated communities and substantial reductions in reef biodiversity and food productivity. Some studies suggest a potential for corals to adapt to increasing temperatures, but the evidence for this is still limited, and the more frequent occurrence of extreme temperatures could overwhelm any capacity for corals to adjust to gradual changes. The flow-on consequences of such impacts for human societies, mostly in developing small island states, that depend on coral reefs as food sources and as a core attraction for tourism are significant. (WGII 4.4, 6.4, 16.4)

Mediterranean-type ecosystems are found in many mid-latitudes around the world and are characterised by high biodiversity, wet winters, and dry summers, often on nutrient-poor soils. Projected changes in precipitation in many of the relevant regions are expected to lead to significant increases in fire risk, shifts in the areas that particular species can occupy, and an increasing risk of extinctions due to the combined effect of rising temperatures and drying. (WGII 4.4)

4.3.2 Risk of species extinction

The structural changes and shifts in a large range of ecosystems have major implications for biodiversity and the risk of species extinction. Over the past few years, the first comprehensive studies have emerged that estimate the number of species at risk of extinction as a function of temperature. (WGII 4.4)

The overall conclusion from the IPCC assessment based on a large number of studies is that about 20–30% of all species assessed are at an increased risk of extinction if increases in global temperature exceed 1.5–2.5°C above 1980–1999 levels. The confidence in these numbers is still limited since they often rely on observed correlations between species distribution and climate parameters and may underestimate the importance of species interactions and take limited account of autonomous adaptation; changes in particular ecosystems could range from 1% to 80% based on the particular systems studied. For warming above about 3.5°C, models suggest significant extinctions of 40–70% of species assessed around the world. (WGII 4.4)

This assessment makes it clear that even if those numbers are only roughly correct, climate change poses an extinction risk of geological proportions to the world’s ecosystems over the coming century. These risks are compounded by the fragmentation of landscapes due to human activities, which makes the migration of species between ecosystems increasingly difficult and suggests that past observed changes may not equally hold in future or that significant human intervention may be required to facilitate such migration (see also Box 5.3). We need to be clear though that those extinctions will not happen instantaneously as soon as temperatures reach a certain limit. In many cases, rising temperatures or changing precipitation patterns could simply limit the reproductive success or suitable range of some species, or the


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successful interaction of a range of species. This could mean that their populations are set on a gradual but inevitable decline, possibly over many species’ generations, so that the actual extinction process could take many years to decades. (WGII 4.4)

The irreversibility of global species extinctions is a key factor in risk assessments related to climate change. They imply that even if some aspects of climate change, such as melting of the Greenland ice sheet or changes in rainfall patterns, might be limited if temperatures temporarily overshoot a certain threshold but then decline again, a large number of species and their ecosystems could be irreversibly lost in the process.

Box 4.2: Ocean acidification

One of the emerging risks from climate change for ecosystems and their goods

and services is the gradual acidification of the world’s oceans. This acidification

occurs because rising atmospheric carbon dioxide (CO2) concentrations lead to

an increasing absorption of carbonate ions in surface sea water, where it forms

a weak acid and reduces the availability of carbonate ions in sea water.

Calcium carbonate is the key building block for many shell-forming organisms

(see section 1.5.2; WGI Box 7.3; WGII 4.4).

The pH of the world’s ocean surface water is expected to decline by another

0.14–0.35 units over the course of the 21st century, in addition to the drop by

0.1 unit already observed since the pre-industrial period. If CO2 concentrations

remain elevated, the process of acidification is expected to continue over

several more centuries and the pH could fall to levels not reached for several

hundreds of millions of years. (WGI Box 7.3, 10.4)

As yet no effects have been documented of the observed ocean acidification

on marine organisms, and identifying vulnerable species and the extent to

which they would respond is an area of active research. The most affected

species in the longer term are expected to be corals and other shell-forming

organisms such as phytoplankton and zooplankton; many of these species play

vital roles in the overall food chain and support the sedimentation of carbon to

the ocean floor. (WGII 1.3, 4.4)

Aragonite is a meta-stable form of calcium carbonate that is produced by

corals and planktonic snails, and whose formation is particularly sensitive to

changes in pH. A range of experiments indicates that doubling atmospheric CO2

concentrations would reduce calcification rates in corals that use aragonite by

20–60%. By 2070, many reefs could have reached critical aragonite saturation

states, which implies reduced coral formation and weakening of reef

frameworks. The negative effect of rising temperatures could further enhance

the detrimental impact of ocean acidification on corals. (WGII 4.4, Box 4.4)

4.4 Food supply

Food production is a core need for human society, and the rapid rise in food prices in 2007 and 2008 has shown how sensitive many groups, including in developed countries, are to food prices. Given that agricultural systems are subject to human management decisions, it is important to be clear in any impacts assessment whether statements about impacts are made on the assumption of no adaptation (ie, farmers continue to act exactly as they do now – which is not a realistic assumption but an important baseline), autonomous adaptation (ie, farmers change, for example, planting


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dates or cultivars as the climate warms, or switch from rain-fed agriculture to irrigation), or proactive adaptations (ie, farmers and governments invest in breeding drought resistant plants in expectation of a greater need for such plants in future).

This chapter discusses impacts in the absence of adaptation, and the potential for autonomous adaptations as defined above to offset some of those impacts. Proactive adaptation options and their limits are discussed in chapter 5.

The key factors influencing food production in the context of climate change are the sensitivities of individual crops to changes in temperature (including hot or cold extremes), precipitation, and CO2 fertilisation. Given the large differences between climatic zones and differences in projected changes in precipitation, the impacts of climate change on food production can be expected to vary significantly between geographical regions, and even on much smaller scales. Water supply for agricultural crops can come from irrigation or direct rain, which determines the extent to which local shifts in precipitation patters would have a direct impact on food production.

Secondary impacts from climate change can occur through changes in pasture species composition, which influences the nutritional value of pastures and their resilience to drought events, changing susceptibility to pests and diseases, and changes to nutrient requirements to sustain any autonomous or planned change in crops and pastures.

4.4.1 Large-scale changes in food production

The recent IPCC assessment confirmed the expected strong regional differences in the impacts of climate change on food production. The assessment showed that for increases in local temperature of 1–3°C, average crop yields in mid- to high-latitude regions can be expected to increase due to the combined influence of a longer growing season, tendency for increased precipitation, and fertilisation from higher CO2 concentrations. For warming above 3°C, crop yields are expected to decrease in some of those regions. In contrast, the yields of major cereals in seasonally dry and low-latitude regions tend to decrease even for warming of 1–2°C, with greater losses for greater temperature increases. Globally, food production has the potential to increase for local temperature increases of 1–3°C, but to decline for greater rises. (WGII 5.4)

These latitude-dependent differences in food production are also expected to be noticeable within continental regions. For example, crop yields could increase up to 20% in east and south-east Asia, but could decrease up to 30% in central and south Asia by the middle of the 21st century. Similarly, wheat crop yields in northern Europe are expected to increase at least for moderate warming while in southern Europe, rainfall reductions and increased drought are expected to reduce crop productivity. (WGII 11.4, 12.4)

Note that the temperature increases used here are local, not global, average temperatures. Local temperature increases over land are expected to be larger than the global average temperature increase for virtually all land regions, with the difference depending on the region. For example, for the A1B scenario, an increase in global average temperature of 2.8°C results in a local temperature increase of about 4°C in large parts of continental North and South America, Africa, and Asia, and even greater local warming of 5°C or more in high northern latitudes (see Figure 3.3). This land enhancement comes from the simple fact that land surfaces heat up more quickly from solar radiation and thus radiate more heat back into the atmosphere (see also chapter 2).


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Figure 4.2 shows the modelled average changes in production for the key crops maize, wheat, and rice in mid- to high-latitude and low-latitude regions for different levels of warming. The figure also includes estimates for changes when autonomous adaptation through changes in planting dates, cultivars, and shifts from rain-fed to irrigated cropping has been taken into account. These latter adaptation options are only hypothetical and have not been assessed for their feasibility in each case (ie, whether farmers have access to capital and governance structures to establish irrigation systems, and whether water abstraction for irrigation is ecologically sustainable). The potential barriers and limits to adaptation are further explored in chapter 6. The main conclusion from these studies is that autonomous adaptation could offset the negative impacts of climate change in low-latitude regions up to a local warming of about 3°C, but above this level yields would decline even with adaptation. In mid- to high-latitude regions, autonomous adaptation could achieve crop production above current levels for local temperature increases up to about 5°C. (WGII 5.4)

4.4.2 Number of people at risk from hunger and regional

differences

What do these effects on food production mean for global food supply? The increasingly negative impacts with rising temperatures imply that, towards the end of the 21st century, global food supply could be reduced by climate change. However, such a conclusion must be seen in the context of the overall number of people at risk from hunger, and how this number is expected to change due to socioeconomic developments that are unrelated to climate change. At present, about 820 million people globally are regarded as undernourished. In the absence of climate change, but assuming other socioeconomic developments are to proceed, this number is expected to drop to between 100 million and 230 million for the A1, B1, and B2 SRES scenarios, and to 770 million under the A2 scenario by the 2080s. Taking climate change into account, these numbers would become 100 million to 380 million under the A1, B1, and B2 scenarios, and between 740 million and 1,300 million under the A2 scenario. In other words, in most scenarios (except A2), we expect absolute numbers of undernourished people to drop significantly compared with today as global development progresses, and the impacts of climate change would only slightly reduce those overall gains. (WGII 5.4)

Global food production is only part of the story. The regional disparity in food production between mid- to high latitudes and low latitudes could put severe pressure on food supply and exacerbate poverty and malnutrition in some low-latitude regions and challenge global food distribution systems. Much of the hunger experienced in the world today is not due to a global lack of food, but a lack of adequate food distribution systems that cater to those at the bottom of the socioeconomic ladder. Regions that are most negatively affected by climate change impacts tend to be those regions that are already most at risk of food shortages and malnutrition. For example, in sub-Saharan Africa, yields from rain-fed agriculture could drop by up to 50% in some countries even by 2020, which would severely compromise access to food. In addition, climatic extremes such as droughts and pest outbreaks could increase food insecurity in many regions. As a result, the global trade in food is expected to increase throughout the 21st century, with most developing countries (particularly in low latitudes) becoming increasingly dependent on food imports even if global food production continues to increase. (WGII 5.4, 5.6, 9.4, 13.4)


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Figure 4.2: Projected changes in maize, wheat, and rice yield at different

latitudes

Note: Changes are shown for a range of temperatures and associated carbon dioxide

(CO2) increases. The studies span a range of projected changes in rainfall. Orange dots

and lines represent impacts in the absence of adaptation, green dots and lines represent

impacts with adaptation. Studies vary to the extent that they have considered CO2-

fertilisation (see also Box 4.3). Adaptation options considered in these studies include

changes in planting dates and cultivars, and switching from rain-fed to irrigated

cultivation. The solid lines are polynomials fitted to the individual data points; they are

illustrative of broad trends with temperature, not specific predictions or interpolations for all measurements.

Source: WGII Figure 5.2.


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Box 4.3: Carbon dioxide fertilisation – how far can it boost plant growth?

A key question often asked is whether increases in carbon dioxide (CO2) and

resulting carbon fertilisation will offset the potential negative impacts of climate

change on crops in low latitudes. A large number of studies have attempted to

answer this question. In so-called free air carbon enrichment (FACE)

experiments, CO2 is artificially blown over open plots to measure the response

by plants without introducing artefacts from greenhouses or other controlled

environments. These studies have shown that some crops respond much more

effectively than others, and that the availability of nutrients (particularly water

and nitrogen) and other air pollutants (such as ground-level ozone) play an

important role in determining the overall outcome. (WGII 5.4)

C3-plants, which are characterised by the particular type of carbohydrates

they produce during photosynthesis and include wheat and rice, tend to

respond with increased growth of 10–25% for doubled CO2 concentrations (but

constant climate conditions). In contrast, C4-crops, which include many

forage grasses and the agriculturally important crops maize, sugar cane, millet,

and sorghum, respond much less by 0–10%. Their responses also depend on

the level of water and nitrogen available to sustain any increased growth rate.

(WGII 5.4)

Carbon fertilisation also interacts with and is often limited by climate change.

Higher temperatures during flowering can reduce grain number, size, and quality

and increase water demand, which may more than offset the beneficial effect of

improved water use efficiency from CO2-fertilisation. In pastures, C4 grasses tend

to respond more to higher temperatures, while C3 grasses are more stimulated

by increases in CO2. The combined effect of rising CO2 concentrations and

temperatures is, therefore, likely a shift in pasture compositions, but the net

effect on productivity is uncertain and very likely depends on location-specific

nutrient levels and changes in precipitation. (WGII 5.4)

FACE experiments with trees have shown that young trees, and trees at the

beginning of an experiment, respond with significant increases in growth rates

(about 10–25% depending on tree species and availability of nutrients and

water). However, this initial growth spurt seems to level off after a few years,

and mature trees in long-term experiments tend to show much weaker growth

enhancement under elevated CO2. An extension of such experiments to a

broader range of species and over longer time horizons would be critical to

deliver more robust and comprehensive assessments of the long-term effect of

higher CO2 concentrations on the net primary productivity in forests. (WGII 5.4)

Some groups of people are considered to be particularly vulnerable to shifts in food production and climate variability even for low levels of warming. These are mostly small-holder and subsistence farmers and fishers, who have little capacity to adapt to changes in weather and climate patterns or cope with disruptions to their harvests from extreme events. These groups of people are also vulnerable to longer-term consequences from climate change such as reductions in water supply in areas dependent on glacial snowmelt, and changes in disease risks associated with climate change. Some regions will also be exposed in the nearer future to rapid changes imposed by globalisation. The net results of these various trends will be complex and location-specific, so it is not currently possible to make quantitative projections about the net impacts of climate change on small-holder farmers other than for very specific case studies. (WGII 5.4, 17.3)


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4.4.3 Food supply from oceans and freshwater systems

Most of this section has focused on food supply from agriculture. For many regions though, an important source of protein comes from ocean and freshwater fisheries. Unfortunately, experimental data and models for the impacts of climate change on fisheries and aquaculture are still lacking, so no reliable quantitative projections can currently be made. (WGII 5.4)

Qualitatively, climate change is expected to lead to changes in marine fish distributions, with regional extinctions of some species and the appearance of others as species groups move poleward. Such projected patterns are consistent with those already observed for many species (see chapter 1). The impact of such changes on fisheries, including economic implications, will largely depend on the capacity of fisheries to monitor and adapt to such changes and ensure sustainable catch rates within changing environmental conditions. (WGII 5.4)

Freshwater fisheries could be more directly affected by climate change due to their geographical constraints. Warming water temperatures generally increase the risk of diseases, though data are still limited to quantify the systematic spread of pathogens. One study has shown a positive impact of higher water temperatures on rainbow trout during winter, but negative impacts of high summer temperatures. The overall consequences of such seasonally varying impacts are still regarded as speculative. (WGII 5.4)

4.5 Coastal zone

Coasts are vulnerable to the impacts of climate change in the form of erosion and inundation through sea-level rise, more frequent and intense storms, and the combination of these impacts with river flooding and changing sediment supply from rivers. In addition, rising air and sea surface temperatures have the potential to alter water quality, the salinity of freshwater, and natural habitats and reduce sea ice, which can accelerate erosion processes. Human modifications of coastlines have also played a major role in the observed loss and erosion of coastal zones and will continue to do so by directly altering coastal habitats and reducing sediment supply from rivers with dams and other infrastructure (WGII 6.1, 6.2, 6.3).

In many cases, the impacts of climate change are generated not only by the change in climate itself but also by the inability of fixed societal structures (buildings and other infrastructure) to respond to these changes. This issue is particularly prevalent in coastal development areas, where it is known as ‘coastal squeeze’: if housing developments along the coast are easily movable inland, the sea-level rise and coastal erosion is a much smaller problem than if a dynamic shoreline is clashing with settlement structures that are fixed in place.30 In such cases, sand dunes, wetlands, or tidal areas that once acted as natural buffers against high tides and storm surges are gradually being squeezed out of existence until the waves roll up directly against the foundations of buildings. This situation is illustrated in Figure 4.3. (WGII 6.6)

30 Incidentally, the state of Texas in the United States has implemented just such a policy, where

public ownership of coastal land is agreed to move inland as the shoreline changes in response to

sea-level rise. Such a policy is of course feasible only where private ownership and residential

development of coastal land does not create a rigid barrier to coastline changes. (WGII 17.3)


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Figure 4.3: Natural and human influences on the coastal system

Note: Major influences of climate change on the coastal system, including its natural and human components.

Source: Based on WGII Figure 6.1.

4.5.1 Key impacts on different types of coastal systems

Even though sea-level rise is a common factor in the vulnerability of all coastal systems to climate change, it is generally only one part of the climate-related changes that determine the overall impacts of climate change on those systems. (WGII 6.4)

For beaches, rocky shorelines, and cliffed coasts, the amount of sediment that is transported along the coastline is a critical factor, particularly for sandy beaches. Upstream modifications of rivers through dams, or specific coastal protection measures that reduce the availability of sediment, can greatly accelerate the amount of erosion that occurs for a given increase in sea level. In addition, changes in wave height and storm intensities can alter the degree to which beaches and cliffs erode with rising sea level. Most of the world’s sandy coastlines retreated during the past century, and this trend is expected to continue as sea-level rise accelerates unless the compounding human pressures are significantly reduced. (WGII 6.4)

Large river deltas with large human populations (so-called mega-deltas) are highly sensitive to sea-level rise because rising sea levels are often combined with land subsidence. This occurs for two reasons: one is the compaction of the underlying sediment by buildings, and the other is the abstraction of ground water. As an example, more than 1,500 km2 were converted in the Mississippi River delta from intertidal coastal marshes to open water between 1978 and 2000, mostly due to land subsidence. Changes in sediment supply from the rivers themselves can further exacerbate land loss, while exposure to episodic flooding and coastal storms makes them vulnerable to extreme events. The most vulnerable mega-deltas are those in Asia and Africa due to their very high population densities in mega-cities, but limited capacity to upgrade coastal protections and manage settlement patterns because of the large proportion of poor populations. Based on digital terrain modelling and population projections, it is estimated that more than 1 million people will be directly


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affected by increased coastal flooding in 2050 in just three mega-deltas: the Ganges-Brahmaputra in Bangladesh, Mekong in Vietnam, and Nile in Egypt. (WGII 6.4)

Estuaries, lagoons, mangroves, and saltmarshes are susceptible not just to changes in sea-level rise but also to changes in the balance between fresh- and saltwater, water temperatures, storm events, and nutrient and sediment supply, all of which can alter the ecological balance and sustainability of these systems. Increasing temperatures and CO2 concentration can have positive effects in some systems. As natural ecosystems, they could be able to persist through a range of sea-level variations as long as sediment supply is not limited and migration inland is possible. However, human modifications to the coastal zone and inland often limit such responses. As a result, most of these systems are expected to decline with sea-level rise, but local responses depend on the particular combination of the above factors. The severe threat to coral reefs from rising temperatures and ocean acidification was discussed in section 4.3. (WGII 6.4)

4.5.2 Projected number of people affected by coastal flooding

The range of impacts on different coastlines will have wide-ranging implications for human society. Global estimates suggest that, by the 2080s, millions of additional people will be subjected to flooding every year. The specific number of people depends greatly on the assumed amount of sea-level rise over the 21st century and socioeconomic developments (both in terms of the total number of people living in the coastal zone, and the amount of money available and invested to enhance coastal protection measures). Depending on these assumptions, the additional number of people affected by coastal flooding by the 2080s (from sea-level rise alone; disregarding river flooding in deltas and estuaries) ranges from a few million to almost 100 million every year. Chapter 9 provides more information on the importance of socioeconomic developments in this range of numbers. (WGII 6.4, 6.6, 20.6)

Apart from direct flooding impacts, sea-level rise is also expected to directly affect humans by compromising freshwater supplies from shallow aquifers due to the gradual intrusion of saltwater. Further impacts include those on human health as a direct consequence from flood and storm events, and flow-on effects on food supplies from coastal fisheries. The evidence for other health-related impacts is still speculative. Even though the absolute numbers of people affected is comparatively low, small islands are expected to be particularly affected by impacts from sea-level rise as they combine with other climate change impacts and socioeconomic development pressures within very confined spaces (see Box 4.4). (WGII 6.4)


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Box 4.4: Small islands – at the squeeze from coastal and other pressures

Impacts related to the coastal zone are expected to be most severe for small

islands. This is because small islands have characteristics that make them

especially vulnerable, including small land areas to sustain agriculture and

other economic activities independent of the coast, limited capacity to adapt to

changes, and high costs of coastal protection relative to average national

income levels. In the Caribbean and Pacific islands, more than 50% of the

population live within 1.5 km from the shore, and most major roads and other

infrastructure are situated along the coast. (WGII 16.2)

Impacts on the coastal zone are expected to interact with many other

pressures and thus make small islands highly vulnerable to climate change.

The strong dependence of small islands on fisheries means that changes in the

coastal environment can have large impacts on their economies and

nutrition status. Reliance on rainfall for freshwater supply means that many

islands are highly sensitive to possible changes in rainfall patterns and

increases in demand with rising temperatures, with negative flow-on effects for

agriculture. By mid-century, water resources are projected to become

insufficient to meet demand during low-rainfall periods in the Caribbean and

Pacific. (WGII 3.4, 16.4)

Projected increases in the intensity of tropical cyclones combine with sea-

level rise to pose major risks to infrastructure, agriculture, and ecosystems in

tropical small islands. While tropical islands could suffer large biodiversity

losses resulting from coral bleaching, mid-latitude and high-latitude islands will

become increasingly affected by invasion from non-native species that are

better adapted to higher temperatures than species belonging to local native

ecosystems. These pressures are expected to combine to mostly negative

direct and indirect impacts on tourism, which acts as a main source of foreign

investment and income for many small islands. (WGII 16.3, 16.4)

4.6 Health

Human health is influenced by climate in many ways. Apart from the direct effects of changing weather patterns (average seasonal and extreme temperatures, moisture and heavy precipitation, and direct impacts of extreme events such as storms and floods), climate also affects human health indirectly through its effects on agriculture (particularly combined with droughts and consequent malnutrition), ecosystems, industry, and settlements, and economic consequences. There is also a growing literature on psychological impacts related to climatic extremes, such as post-traumatic stress disorders and the stress associated with extended drought periods. Figure 4.4 shows the various ways in which climate change can affect human health. (WGII 8.1, 8.2)

Evidence of impacts related to the observed changes in climate is still very limited, although the consequences of individual extreme events including droughts are well known. The main areas where changes have already been observed are the distribution of infectious disease vectors (such as malaria-transmitting mosquitoes), the seasonal distribution of allergenic pollen, and heat-wave–related deaths. (WGII 1.3, 8.1, 8.2)


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Figure 4.4: Pathways of climate change impacts on human health

Note: Schematic diagram of the pathways by which climate change affects health, including direct and modifying influences.

Source: Based on WGII Figure 8.1.

4.6.1 Main projected impacts and key affected regions

Malaria, dengue fever, and other infectious diseases

Malaria is spread by mosquitoes, which in turn are controlled by climatic conditions that provide suitable breeding grounds. The overall effects of climate change on the potential spread of malaria are projected to be mixed: regions that become warmer and wetter are generally at increased risk, while regions that become warmer and drier could see a reduction in the suitable range. The largest changes in risk are expected to occur at the edge of currently suitable regions for mosquitoes, for example, an upward spread in the mountain ranges of Africa. The actual spread of malaria itself is determined not only by the suitability of habitat for mosquitoes but also by prevention and control measures. These depend to a large extent on surveillance, monitoring, and eradication programmes as well as preventative measures such as mosquito nets. As a result, the net changes in the actual incidence of malaria are highly dependent on socioeconomic conditions and the future state of health services in the most at-risk regions, which include much of tropical Africa and several countries in south and south-east Asia and central and south America. (WGII 8.4)

Several other infectious diseases are also controlled by the suitability of habitat for the vectors (mainly insects) that transmit the diseases. These include dengue fever, tick-borne encephalitis, and Lyme disease. Most available studies show a potential poleward spread of these diseases with rising temperatures, particularly for dengue fever in urban centres. Several studies have estimated the number of people at risk from dengue fever in the 2080s to be about 5 billion to 6 billion people,


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compared with about 3.5 billion in the absence of climate change. However, changes in the actual occurrence of these diseases are thought to be more strongly affected by other human-induced changes to the habitats for the main vectors (ticks and mosquitoes). Perhaps more importantly, the state of local health systems and prevention measures has a major influence on whether a risk factor translates into an actual occurrence of infectious diseases. Because the evolution of health systems in different parts of the world is very difficult to project, projections of health impacts tend to focus on change in exposure risk rather than projections of actual changes in health impacts. (WGII 8.4)

Heat- and cold-related mortality

Rising temperatures generally will reduce cold-related mortality but increase heat-related mortality, but the balance between these effects will vary strongly with regions. In high-latitude countries (such as the United Kingdom), reduced cold-related mortality is expected to outweigh increases in heat mortality. In mid-latitudes, particularly Europe and the United States, the effects of increased heat waves could be significant. The 2003 European heat wave led to about 35,000 deaths (see Figure 4.5), with a disproportionate toll on elderly people. Further increases in extreme summer temperatures are estimated to lead to a 25–100% increase in heat-wave days in the United States, with similar changes in Europe. The resulting health toll of such increases could be moderated significantly by adaptation and acclimatisation of populations, but the overall impact is still expected to be negative. Developing countries with lesser capacity to adapt to increasing heat stress, and those in lower latitudes that are already at high risk from heat extremes (eg, India), are likely to be most vulnerable and affected by increasing extreme temperatures, including through reduced productivity of outdoor workers. (WGII 8.4, 10.4, 12.4, 14.4)

Figure 4.5: Impacts of 2003 European heat wave on mortality

Note: Mean daily summer temperatures and mortality in France during the 2003 heat

wave, compared to average temperatures and mortality during 1999–2002.



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Flow-on effects of storms, floods, and droughts

Increases in the frequency and/or severity of climatic extremes such as storms, floods, and droughts (see chapter 3) would also directly impact on human health through injuries and loss of life, as well as through the spread of diarrhoeal diseases due to contamination of drinking water and food supplies, post-traumatic stress disorders, and social tensions. Droughts and other climate-related impacts on food production in low-latitude regions, particularly Africa, are expected to reduce food security and increase malnutrition, which can have significant long-term effects on population health. (WGII 8.2, 8.4)

As one example, the population at risk of hunger in Mali is projected to grow from 34% in the early 2000s to 64–72% by the 2050s in the absence of effective adaptation strategies. Very few studies have quantified the potential overall changes, and most studies are restricted to small-scale case studies or to qualitative estimates of increased health burdens. Actual outcomes would depend on both the exposure of populations to specific risks as well as their ability to adapt to and cope with increased extremes. (WGII 9.4, 8.2, 8.4)

Urban air quality

Climate change can affect urban air quality directly, insofar as the emission of greenhouse gases is often locally coupled with the emission of harmful pollutants such as fine particulates and toxic gases, and indirectly by influencing weather and climate patterns that lead to high-pollution episodes particularly in urban regions. The main impact in this area is expected to come from increased ground-level ozone concentrations in urban regions, which would increase cardiorespiratory diseases. (WGI 7.4, Box 7.4; WGII 8.2, 8.4)

The link between the emission of greenhouse gases and other pollutants that have direct impacts on human health is one of the main co-benefits of reducing greenhouse gas emissions, because this will generally also reduce the emission of those other pollutants, especially fine particulate matter. A range of studies shows that the benefits of improved health outcomes can offset a significant fraction of the overall costs of reducing greenhouse gas emissions in many regions (more on this in chapter 8). (WGII 8.3, 8.4; WGIII 11.8)

4.6.2 Global aggregated impacts and costs

A global study by the World Health Organization estimated that, in 2000, climate change had already caused the loss of over 150,000 lives. The same report estimated that most of the projected future health impacts from climate change by 2030 would be due to increases in diarrhoeal diseases and malnutrition, primarily in regions with low-income populations and populations already suffering from health problems. The study also suggests that, globally, the negative impacts of climate change would outweigh the benefits associated with a warmer climate, especially due to the disproportionately negative effects on developing countries. (WGII 8.4)

Diarrhoeal diseases were estimated to increase by about 2–5% in very low-income countries (with per capita incomes below US$6,000). Countries with average incomes above this level were assumed to suffer no additional risk from diarrhoea. These average figures hide significant diversity in health risks within populations, since many case studies have shown that older people, already ill people, and socially marginalised groups (eg, children and poor people) are at higher risk than the average person even in developed societies (as was clearly demonstrated, for


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example, by the disproportionate impact of the 2003 European heat wave on elderly people). (WGII 7.2, 7.4, 8.4)

A few studies have attempted to place costs on the global health impacts on climate change. While they show figures in the order of tens of billions of dollars, their evidence base is generally weak as cost estimates depend very heavily on assumptions. In particular, one problem is how to measure the value of a human life in economic terms. Valuation methods that are based on ‘willingness to pay’ (ie, empirical studies of the amount of money people are prepared to pay for health services and insurance) systematically find lower values for people in developing countries. This reflects not a lower value of human life in these regions but primarily a lower ability to pay for services in developing countries, which can lead to significant distortions of estimates and raises ethical concerns about the value and use of such studies. Studies that assume a globally uniform value of human life tend to increase the global cost estimates by as much as a factor of five. Cost estimates also tend to ignore or underestimate the health impacts on members of society who are not in the labour force, such as children and older people, so that at present there are no widely accepted global cost estimates for the projected health impacts. However, some countries have estimated costs within their national boundaries. (WGII 8.2, 8.4)

It is important to note that, for health impacts in particular, adaptation and the state of national health services can make a major difference on the impacts that actually occur. Diarrhoeal diseases and malnutrition could be reduced significantly by targeted measures in the areas of sanitation, water supplies, health services, and food distribution. This demonstrates that while reducing the rate and magnitude of climate change is important to keep climate-related pressures at a manageable level, improving basic human services in the areas of health, sanitation, and nutrition offers important and effective near-term ways of increasing resilience to future climate change and dealing with existing problems in these areas. Apart from public health initiatives that could directly address the potential health impacts of climate change, underlying factors such as education, infrastructure, governance, and economic development, as well as the state of the health care system overall and related monitoring and intervention strategies, will be critical in determining which of the potential impacts will become a reality. More details on adaptation to health impacts, and their interaction with other pressures, are discussed in chapter 5. (WGII 8.3, 8.6; SYR 3.3)

4.7 Settlements and society

The preceding sections looked at climate impacts mainly from a sectoral perspective, that is, in terms of impacts on water, ecosystems, food, and human health. We can also ask what the impacts are likely to be on different aspects of human society, which will depend on the types of settlements in which humans live and overall societal structures.

Studies that focus on human settlements indicate that the most noticeable impacts, at least in the next few decades, are likely to come from changes in extreme events such as droughts, floods, and storms. This is not surprising, since human settlements by definition are fixed in space, with ongoing resource requirements (eg, food, water, and protection from rivers and coastal floods) and discharge of waste. Increasing variations in the climatic conditions within which these fixed settlements exist are, therefore, most likely to exert pressure on those systems and lead to damages. (WGII 7.2, 7.4)


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At the same time, climate is hardly ever the only source of stress on settlements. Changing demographics and consequent resource use, along with depletion of natural resources and pollution, are often the dominant source of stresses. Therefore, climate change must be seen in the context of a large array of changes where, rather than introducing a new problem, climate change often exacerbates existing and already growing problems in the areas of flood risk from rivers and coasts, storms, health, drainage, and water quality and supply. (WGII 7.2)

Perhaps the more relevant question arising from studies about climate change impacts on settlements and societies is: who is likely to be most affected by climate change, and why? The following sections briefly discuss this question.

4.7.1 Impacts on industry and services

The IPCC assessment concluded that industrial sectors are generally thought to be less vulnerable to climate change impacts than sectors such as water supply and agriculture, because they are less sensitive to changes in climate and tend to have larger resources to adapt to changes. However, it also suggested that there would be major exceptions to these general assumptions, particularly for industries located in climate-sensitive areas (such as major flood plains), industries that rely on climate-sensitive inputs (such as food processing plants), and industries with long-lived infrastructure. Flow-on effects of climate change impacts on essential services (such as energy and water supply and transport) can be significant for industry. Climate change is likely to affect both energy demand and supply, but the balance of impacts will be highly region specific, depending on the main sources of energy supply and the changing regional demand for energy for cooling and heating. (WGII 7.4)

For service and import/export industries, one of the biggest impacts could lie in the changing terms of trade. This might be particularly so for industries that will be affected worldwide not only by climate change itself but also differences in climate change mitigation policies (ie, efforts to reduce greenhouse gas emissions associated with those industries) and adaptation measures that may change the demand for certain products, services, and locations. However, quantification of such impacts still tends to be speculative due to the large number of variables and assumptions that influence future outcomes. A prime example of this is agriculture, where regional production patterns, and hence changing supply and demand, will be affected by long-term climate trends, climatic and weather extremes such as droughts, policies to reduce direct greenhouse gas emissions from agriculture, changing domestic and international consumer demand, rising energy prices, competition for water, policies that govern land and water use, and interactions between policies to increase the use of biofuels, which can create competition with using land for food production.

A similarly complex industry is tourism, which is likely to be affected both by climate change itself (eg, much of the Mediterranean is expected to become too hot for comfort during the summer months over the next few decades) and by transport fuel prices and consumer attitudes to short- and long-distance travel. In both cases, changes in overall socioeconomic status of main and emerging markets will also affect future trade and travel patterns. More information on recent experiences regarding the interaction between biofuels and food production is in chapter 6. (WGII 7.4)


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4.7.2 Impacts on human settlements and vulnerable social

groups

Human settlements and utilities will be affected by the range of impacts already discussed in the preceding sections, particularly changes in water supply and demand and changing natural hazards related to flood risk, erosion, and landslides following heavy rainfall events. In rapidly growing cities, particularly those of south-east and east Asia, changes in climate will compound pressures on natural resources and exposure to hazards that result from rapidly growing populations and industrialisation, for example, water supply, sanitation and drainage, transport systems, and housing development in zones of high climate-related risks of flooding and erosion. The long-term nature of sea-level rise in particular suggests the importance of avoiding development in areas of increasing future risks, but population pressures directly counteract such sustainability measures. (WGII 7.4, 7.6, 10.4)

While it is, therefore, difficult to quantify future impacts of climate change within a highly dynamic environment, recent research offers some important conclusions about which groups of people are likely to be most at risk in such rapidly changing environments. The key conclusion from recent studies is that the poor and marginalised sections of any society (including wealthy and highly developed societies) are most at risk. For example, poor people are less likely to have access to or be able to afford air conditioning during heat waves, in part also due to their typical work environments. Urban neighbourhoods with green spaces and that are well served by health facilities and public utilities are more resilient to climatic extremes than areas with a very high population density without these services. In addition, access to insurance to protect individuals from the consequences of individual extreme events can support recovery after disasters, although a steady increase in weather and climate extremes is widely seen as a key challenge to the insurance industry. (WGII 7.4, 7.6)

As a result, poor people tend to have less access to mechanisms that can protect them from natural disasters and help them recover. In addition, particularly in rapidly growing urban areas in developing countries, poor people tend to live in areas at greater risk from flooding, erosion, landslides, and in areas that are more prone to negative health impacts such as spread of dengue fever due to pools of stagnant water near urban drainage channels. Similar risks also apply to other marginalised groups in society with limited access to resources; these include older people, children, indigenous people in urban areas, and recent immigrants, particularly where they lack necessary language skills and are not part of community decision-making processes that determine the distribution of resources. Such groups have a greater reliance on public interventions to protect them from climate-related impacts and to assist their recovery after disasters. They also rely more on public transport and subsidised access to services such as health and water supplies. (WGII 7.4, Table 7.3, Box 7.4)

Experiences with recent climate-related disasters, such as the European heat wave in 2003, hurricane Katrina in the United States in 2005, and the impact and recovery from more recent hurricanes in developing countries, strongly support these conclusions for both developing and developed countries.

The essentially static nature of human settlements also makes them highly vulnerable to potential abrupt changes in climate. As discussed in chapter 3, we know that regionally abrupt climate changes have occurred, but currently it is not possible to reliably project or quantify the probability of possible future abrupt changes. However, if sea levels were to rise by more than 1 m within the 21st century, or precipitation reduced by tens of percent for several decades in some


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regions, the impacts on large settlements and societies that cannot easily retreat from shorelines or reduce their water demand would be significant and affect large numbers of people. At present though, the likelihood of such events occurring is highly speculative, although there is increasing evidence that sea-level rise over the 21st century is likely to be higher than the figures provided in the AR4 due to the accelerated loss of ice through glaciers that drain the polar ice sheets (CCSP, 2008).

4.7.3 Cost estimates

Estimating costs of climate change for specific settlements and sub-national regions or groups of people is difficult, because most macroeconomic variables apply to total national economies. They also suffer from the same problems already discussed in the context of human health, namely that many important impacts cannot easily be quantified in monetary terms and such quantifications rely heavily on assumptions. Some studies have attempted to quantify the costs to specific sectors of the economy, for example, a study in the United Kingdom estimated that the annual average damage to land and properties could increase by a factor of 3–9 by the 2080s. (WGII 7.5)

Most cost estimates have focused on the impacts of specific extreme events on regional economies, because in these cases a relevant baseline in the absence of such an event can be more robustly defined. As an example, Figure 4.6 shows the actual change in the gross domestic product (GDP) growth rate for Honduras following the impact of hurricane Mitch in October 1998, and compares this with the expected GDP growth in the absence of this impact. Other studies of the impact of extremes on national economies show similar impacts, with reductions in GDP between 3% and 6% for flooding in Mozambique and Central America. (WGII 7.5)

GDP figures capture only a small part of the total humanitarian impact, and they also obscure increases in government borrowing or the reliance on external aid sources for recovery. The monetary and humanitarian impact of extreme events also tends to differ between developed and developing countries, so may change over time as countries develop. Interesting cases in point are hurricane Andrew, which hit south Florida in 1992, and hurricane Mitch; both hurricanes were of similar magnitude. Hurricane Andrew is remembered in the United States as the first hurricane to cause tens of billions of United States dollars in damage, but led to only 37 deaths, while hurricane Mitch caused over 11,000 deaths in Honduras, but much less absolute monetary damage. Note though that the impact of Mitch on the GDP of Honduras was much higher than that of Andrew on the GDP of the United States.31

Extrapolating from current events to future changes is, therefore, generally difficult; while we know that the frequency and/or severity of damaging extremes such as hurricanes and floods is likely to increase, it is not yet possible to quantify those changes at small scales, and increasing adaptation and preparedness for such events could reduce their future impacts. At the same time, as we discuss in chapter 5, the ability of societies to adapt to such changes depends on several factors that make it by no means a certainty that effective adaptation to changing extremes will occur in all places and for all parts of society. The recovery from disasters also depends on the amount of aid and disaster relief provided to countries from the outside, which may change over time. Hence, the economic impact of past events rarely offers a reliable basis for long-term extrapolations into the future, but they can reveal important insights about the most vulnerable and most affected parts of society, and the most effective prevention and recovery mechanisms. (WGII 7.5, 7.6)

31 See the National Weather Service website (www.srh.noaa.gov/jetstream//tropics/tc_notable.htm).


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Figure 4.6: Economic impact of hurricane Mitch in 1998 and drought in 1998/99

on the Honduran economy

Note: The actual economic growth is compared with the economic growth that would have

been expected in the absence of the hurricane and drought.


4.8 Extreme events

The preceding section in particular showed that significant impacts of climate change could occur through the changing frequency and intensity of extreme events, in addition to long-term gradual changes in average climate conditions. All the systems and sectors discussed so far are susceptible to projected changes in weather and climatic extremes: water supply is negatively affected by droughts, and the risk of flooding is increased with heavier rainfalls. Agricultural production is also affected by extreme temperatures, droughts, and flooding; incorporating such changes in extremes into projected impacts generally shows significantly more negative impacts on agriculture than considering changes in only climatic averages. Similarly, human health is affected by climatic extremes such as heat waves, floods, and storms and their consequences on food and water supplies. While sea-level rise itself is a gradual long-term phenomenon, its impacts are often manifested through high tides and resulting inundation and erosion during particular events, often when combined with storm surges and high tides. (WGII 3.4, 4.4, 5.4, 6.4, 7.4, 8.4, 17.3)

A common metric to express changes in extreme events is the change in the event’s return period. A return period of 20 years for a flood or drought means that, on average (for a stable climate), a flood or drought of a given magnitude would occur only about once in 20 years. Such a measure is useful because it can be related to practical experiences; for example, a 1-in-20 year drought is a drought that a farmer might expect to have to suffer through perhaps twice over his or her working life. A 1-in-100 year flood is the sort of flood that you may never experience yourself but there are records of it in recent history.

Climate model simulations can be used to estimate how the return periods of heat waves, floods, droughts, and storms would change with a changing climate. As outlined in chapter 3 (section 3.3) and shown in Figure 4.7, changes in the frequency of extremes can be much larger than changes in climate means, so could have significant impacts even where the change in average climate does not appear all that large. For example, climate projections for Europe suggest that the return period for


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what is currently a 1-in-100 year drought (ie, the sort of drought that people still talk about a generation or two later), could occur once every 10 years or even more often in much of the Mediterranean region by the 2070s. A similar study for New Zealand has shown that the return period of what is currently a 1-in-20 year drought could reduce to once every two to five years in some already dry eastern regions. Likewise, the return periods for extreme floods are expected to reduce significantly in many regions, especially (but not only) where average rainfall is also projected to increase. (WGII 11.4, 12.4, TS.4.4)

It is a common misunderstanding to say that an increase of, say, 1°C would lie within the natural climate variability that a country normally experiences, and, therefore, an increase in the mean climate of 1°C over the next 30 years or so would not really matter. It is true that, for many regions, a currently abnormally hot year is about 1°C warmer than the long-term average, so 1°C roughly represents the range of natural climate variability. However, if mean temperatures increase by 1°C, this would mean that every average year in future would be regarded as abnormally hot. More importantly, an abnormally hot year in the future would lie entirely outside the range of temperatures that are currently experienced. Changes in the frequency of extreme events are also significant because it usually takes some time for systems to recover from extreme events (eg, an abnormally hot year). If extremes happen more and more frequently, damages increase but in addition the period available for recovery reduces, which could result in less and less capacity to absorb such shocks.

Figure 4.7: Schematic illustration of changes in extremes as a result of changes

in mean climate (reproduced Figure 3.5)

Note: For any given mean climate, there is a small percentage of extremely hot days as

well as extremely cold days. If the mean climate becomes warmer, the temperature of

the most extreme hot days will also increase, but, more importantly, the number of days that exceed a given threshold may increase significantly.

Source: Based on WGI Box TS.5 Figure 1.


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Projected changes in extreme events have very few positive and mostly negative impacts. The main positive impacts, which involve a reduction in extreme events, include a reduced frequency of late spring frosts in temperate regions and cold-air outbreaks in high-latitudes (which can damage crops and pose threats to human life and animals). Most other projected changes in extremes lead to negative impacts in a variety of sectors. Table 4.1, reproduced from the Working Group II assessment and Synthesis Report, shows examples of impacts from extreme events on agriculture, forestry, ecosystems, water resources, human health, industry, settlements, and society. (WGII TS.4.4; SYR 3.3)

Table 4.1: Possible impacts due to changes in extreme weather and climate events

Examples of major projected impacts by sector

Phenomenona

and direction

of trend

Likelihood of

future trends

based on

projections for

21st century using SRES

scenarios

Agriculture,

forestry, and

ecosystems Water resources Human heath

Industry,

settlement and

society

Over most land

areas, warmer

and fewer cold

days and

nights, warmer

and more

frequent hot

days and

nights

Virtually

certainb

Increased yields

in colder

environments;

decreased yields

in warmer

environments;

increased insect

outbreaks

Effects on water

resources

relying on

snowmelt;

effects on some

water supplies

Reduced human

mortality from

decreased cold

exposure

Reduced energy

demand for

heating;

increased demand

for cooling;

declining air

quality in cities;

reduced

disruption to transport due to

snow, ice; effects

on winter tourism

Warm spells/

heat waves. Frequency

increases over

most land

areas

Very likely Reduced yields

in warmer regions due to

heat stress;

increased

danger of

wildfire

Increased water

demand; water quality

problems, eg,

algal blooms

Increased risk of

heat-related mortality,

especially for the

elderly,

chronically sick,

very young, and

socially isolated

Reduction in

quality of life for people in warm

areas without

appropriate

housing; impacts

on the elderly,

very young, and

poor

Heavy precipitation

events.

Frequency

increases over

most areas

Very likely Damage to crops; soil

erosion; inability

to cultivate land

due to

waterlogging of

soil

Adverse effects on quality of

surface and

ground water;

contamination of

water supply;

water scarcity may be relieved

Increased risk of deaths, injuries

and infections,

respiratory and

skin diseases

Disruption of settlements,

commerce,

transport and

societies due to

flooding;

pressures on urban and rural

infrastructures;

loss of property

Area affected

by drought increases

Likely Land

degradation; lower yields/

crop damage

and failure;

increased

livestock

deaths;

increased risk of

wildfire

More widespread

water stress

Increased risk of

food and water shortage;

increased risk of

malnutrition;

increased risk of

water- and food-

borne dieses

Water shortages

for settlements, industry, and

societies; reduced

hydropower

generation

potentials;

potential for

population

migration

Intense

tropical

cyclone activity

increases

Likely Damage to

crops;

windthrow

(uprooting) of

trees; damage

to coral reefs

Power outages

causing

disruption of

public water

supply

Increased risk of

deaths, injuries

and water- and

food-borne

diseases, post-

traumatic stress disorders

Disruption by

flood and high

winds; withdrawal

of risk coverage

in vulnerable

areas by private insurers;

potential for

population

migrations; loss

of property


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Examples of major projected impacts by sector

Phenomenona

and direction

of trend

Likelihood of

future trends

based on

projections for 21st century

using SRES

scenarios

Agriculture,

forestry, and

ecosystems Water resources Human heath

Industry,

settlement and

society

Increased

incidence of

extreme high

sea level

(excludes

tsunamis)c

Likelyd Salinisation of

irrigation water,

estuaries and

freshwater

systems

Decreased

freshwater

availability due

to salt water

intrusion

Increased risk of

deaths and

injuries by

drowning in

floods;

migration-related

health effects

Costs of coastal

protection versus

costs of land-use

relocation;

potential for

movement of

populations and

infrastructure; also see tropical

cyclones above

Notes: SRES = Special Report on Emissions Scenarios (IPCC, 2000).

a See WGI Table 3.7 for further details regarding definitions.

b Warming of the most extreme days and nights each year.

c Extreme high sea level depends on average sea level and on regional weather systems. It

is defined as the highest 1% of hourly values of observed sea level at a station for a given reference period.

d In all scenarios, the projected global average sea level at 2100 is higher than in the

reference period. The effect of changes in regional weather systems on sea-level extremes has not been assessed.

Entries are based on projections for the mid- to late 21st century, in the absence of additional adaptation measures. Statements about likelihood in the second column refer to the probability that the change in extremes listed in first column will occur, and are based on the formal IPCC system of quantifying probabilities: ‘virtually certain’ means a probability of greater than 99%; ‘very likely’ means a probability greater than 90%; ‘likely’ means a probability greater than 66%.

Source: SYR Table SPM.3.

4.9 Severity and extent of impacts as a function of temperature

4.9.1 Examples of impacts related to global average temperature

One of the most significant advances of climate impacts research over recent years is the increasing ability to relate the severity and extent of impacts to the amount of temperature change over the 21st century. Most impacts increase in severity and become increasingly negative with rising temperatures. Some impacts are initially positive for low levels of warming (such as increased water availability and cereal productivity in mid- to high latitudes), whereas others are negative for almost any amount of warming (such as increased coral bleaching and increased mortality from heat waves).

Examples of impacts for different systems and sectors are shown in Figure 4.8 compared with the amount of global average temperature increase by the end of the 21st century under different emissions scenarios. (WGII Figure SPM.2; SYR Figure SPM.7)


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Figure 4.8: Impacts associated with global average temperature change

Note: Impacts will vary by extent of adaptation, rate of temperature change, and

socioeconomic pathway. Upper panel: Illustrative examples of global impacts projected

for climate changes (and sea level and atmospheric carbon dioxide where relevant)

associated with different amounts of increase in global average surface temperature in

the 21st century. The solid lines link impacts; broken-line arrows indicate impacts

continuing with increasing temperature. Entries are placed so that the left-hand side of

the text indicates the approximate level of warming that is associated with the onset of a

given impact. Quantitative entries for water scarcity and flooding represent the additional

impacts of climate change relative to the conditions projected across the range of

scenarios A1FI, A2, B1 and B2 from the Special Report on Emissions Scenarios (SRES;

IPCC, 2000). Adaptation to climate change is not included in these estimations.

Confidence levels for all statements are high. Lower panel: Dots and bars indicate the

best estimate and likely ranges of warming assessed for the six SRES marker scenarios

for 2090–2099 relative to 1980–1999.

Source: SYR Figure SPM.7.


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4.9.2 Implications for adaptation and mitigation as options to reduce impacts

Relating the timing and magnitude of impacts to the timing and magnitude of temperature rise allows us to consider how effective limiting or reducing greenhouse gas emissions and hence limiting further temperature increases would be as a measure to reduce future impacts of climate change. For example, reducing the amount of temperature increase during the 21st century to the lower end of the ranges shown in Figure 4.8 would clearly reduce the risk of climate change affecting a large number of ecosystems and causing significant extinction of species, and would reduce the number of people affected by water stress and the likelihood that cereal productivity would fall. (SYR 5.7)

However, no simple one-to-one relationship exists between temperature and impacts, because impacts also depend on the underlying socioeconomic structure of the society that is affected by climate change and the degree to which society will adapt to these changes. Note that the impacts in Figure 4.8 do not include adaptation. For some impacts and regions, adaptation is unlikely to be effective and cannot avoid all damages, but in other regions and sectors, adaptation and sustainable development initiatives could reduce impacts significantly and thus complement any measures to reduce greenhouse gas emissions.

The question of what amount of global temperature change and which severity of impacts is manageable by adaptation, and which impacts are unmanageable and hence must be avoided by mitigation (ie, by reducing global greenhouse gas emissions), therefore, does not allow a simple answer. Nonetheless, this question is critical and lies at the heart of any climate change debate. We, therefore, revisit this question in chapter 8, after discussing detailed options, effectiveness, and costs for adaptation and mitigation in chapters 5 and 6.

We also need to remember that, as discussed in chapter 3, some further warming during the 21st century is already unavoidable even if we were to reduce greenhouse gas emissions sufficiently to keep their concentrations constant at current levels. Unfortunately, even the most ambitious scenarios of global greenhouse gas emission reductions imply some further growth in greenhouse gas concentrations, and hence even more warming of at least about 1.5°C above 1980–1999 levels. As a result, the IPCC concluded that there are some impacts for which adaptation is the only available and appropriate response, because even the most ambitious efforts to reduce emissions can no longer avoid them. Examples of such potential impacts are shown on the left-hand side of Figure 4.8 (ie, those below about 1.5°C). (WGII SPM)

4.10 Regional distribution of impacts and especially affected regions

Most of the discussion of impacts in this section has focused on different systems and sectors. We could have equally structured this discussion around impacts on different regions of the world. I have chosen the former approach because impacts even within regions are highly dependent on local circumstances. It appears more important to me to understand the way in which different systems and sectors are affected, because this allows us to assess the potential impacts in any particular location that we wish to focus on.

A condensed overview of some key impacts on different regions of the world is in Table 4.2, which is copied directly from the IPCC Synthesis Report. The listing of impacts in this table is necessarily selective, and readers are encouraged to consult


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the underlying chapters of the AR4 (WGII chapters 9–15) and national impacts reports for further detail.

Table 4.2: Examples of projected impacts on different world regions

Region Projected impacts

Africa By 2020, between 75 and 250 million people are projected to be exposed to increased water stress due to climate change.

By 2020, in some countries, yields from rain-fed agriculture could be reduced by up to 50%. Agricultural production, including access to food, in many African countries is projected to be severely compromised. This would further adversely affect food security and exacerbate malnutrition.

Towards the end of the 21st century, projected sea level rise will affect low-lying coastal areas with large populations. The cost of adaptation could amount to at least 5–10% of gross domestic product (GDP).

By 2080, an increase of 5–8% of arid and semi-arid land in Africa is projected under a range of climate scenarios.

Asia By the 2050s, freshwater availability in central, south, east and south-east

Asia, particularly in large river basins, is projected to decrease.

Coastal areas, especially heavily populated mega-delta regions in south, east and south-east Asia, will be at greatest risk due to increased flooding from the sea and, in some mega-deltas, flooding from the rivers.

Climate change is projected to compound the pressures on natural resources and the environment associated with rapid urbanisation, industrialisation and economic development.

Endemic morbidity and mortality due to diarrhoeal disease primarily associated with floods and droughts are expected to rise in east, south and south-east Asia due to projected changes in the hydrological cycle.

Australia and New Zealand

By 2020, significant loss of biodiversity is projected to occur in some ecologically rich sites, including the Great Barrier Reef and Queensland Wet Tropics.

By 2030, water security problems are projected to intensify in southern and eastern Australia and, in New Zealand, in Northland and some eastern regions.

By 2030, production from agriculture and forestry is projected to decline over much of southern and eastern Australia, and over parts of eastern New Zealand, due to increased drought and fire. However, in New Zealand, initial benefits are projected in some other regions.

By 2050, ongoing coastal development and population growth in some areas

of Australia and New Zealand are projected to exacerbate risks from sea level rise and increases in the severity and frequency of storms and coastal flooding.

Europe Climate change is expected to magnify regional differences in Europe’s natural resources and assets. Negative impacts will include increased risk of inland flash floods, and more frequent coastal flooding and increased erosion (due to storminess and sea level rise).

Mountainous areas will face glacier retreat, reduced snow cover and winter tourism, and extensive species losses (in some areas up to 60% under high emissions scenarios by 2080).

In southern Europe, climate change is projected to worsen conditions (high temperatures and drought) in a region already vulnerable to climate variability, and to reduce water availability, hydropower potential, summer tourism and, in general, crop productivity.

Climate change is also projected to increase the health risks due to heat waves and the frequency of wildfires.


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Region Projected impacts

Latin America

By mid-century, increases in temperature and associated decreases in soil water are projected to lead to gradual replacement of tropical forest by savannah in eastern Amazonia. Semi-arid vegetation will tend to be replaced by arid-land vegetation.

There is a risk of significant biodiversity loss through species extinction in

many areas of tropical Latin America.

Productivity of some important crops is projected to decrease and livestock productivity to decline, with adverse consequences for food security. In temperate zones, soybean yields are projected to increase. Overall, the number of people at risk of hunger is projected to increase. (TS; medium confidence)

Changes in precipitation patterns and the disappearance of glaciers are projected to significantly affect water availability for human consumption, agriculture, and energy generation.

North

America

Warming in western mountains is projected to cause decreased snowpack,

more winter flooding and reduced summer flows, exacerbating competition for over-allocated water resources.

In the early decades of the century, moderate climate change is projected to increase aggregate yields of rain-fed agriculture by 5–20%, but with important variability among regions. Major challenges are projected for crops that are near the warm end of their suitable range or that depend on highly utilised water resources.

Cities that currently experience heat waves are expected to be further challenged by an increased number, intensity and duration of heat waves during the course of the century, with potential for adverse health impacts.

Coastal communities and habitats will be increasingly stressed by climate change impacts interacting with development and pollution.

Polar regions

The main projected biophysical effects are reductions in thickness and extent of glaciers, ice sheets and sea ice, and changes in natural ecosystems with detrimental effects on many organisms, including migratory birds, mammals, and higher predators.

For human communities in the Arctic, impacts, particularly those resulting from changing snow and ice conditions, are projected to be mixed.

Detrimental impacts would include those on infrastructure and traditional indigenous ways of life.

In both polar regions, specific ecosystems and habitats are projected to be

vulnerable, as climatic barriers to species invasions are lowered.

Small islands

Sea level rise is expected to exacerbate inundation, storm surge, erosion, and other coastal hazards, thus threatening vital infrastructure, settlements, and facilities that support the livelihood of island communities.

Deterioration in coastal conditions, for example, through erosion of beaches and coral bleaching is expected to affect local resources.

By mid-century, climate change is expected to reduce water resources in many small islands, eg, in the Caribbean and Pacific, to the point where they become insufficient to meet demand during low-rainfall periods.

With higher temperatures, increased invasion by non-native species is

expected to occur, particularly on mid- and high-latitude islands.

Note: Unless stated explicitly, all entries are from Working Group II SPM text, and are

either very high confidence or high confidence statements, reflecting different sectors

(agriculture, ecosystems, water, coasts, health, industry, and settlements). The Working

Group II SPM refers to the source of the statements, timelines, and temperatures. The

magnitude and timing of impacts that will ultimately be realised will vary with the amount and rate of climate change, emissions scenarios, development pathways, and adaptation.

Source: SYR Table SPM.2.


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A key message from this assessment is the extremely uneven distribution of impacts across different regions. Some regions are clearly going to be more negatively affected by climate change than others, either because the changes in climate and potential impacts themselves are more negative and significant, or because the affected societies and ecosystems have a very low ability to cope with additional pressures and to adapt to the climatic changes. It is an unfortunate fact that, for some regions, both those aspects combine, mostly in the least developed regions of the world.

There is no purely scientific answer to the question which regions of the world are going to be most affected by climate change, but we can use expert judgement to consider which regions combine high magnitudes and rates of climate change, are particularly sensitivity to such changes and resulting impacts, and have limited capacity to adapt. The IPCC undertook such an expert assessment and concluded that the following regions are likely to be especially affected by climate change. (SYR 3.3)

• The Arctic: The Arctic is likely to warm more and faster than any other region of the planet, and this warming will have significant impacts on natural and human systems, particularly the tundra, sea-ice–related ecosystems and species, and indigenous communities whose way of life depends on snow and ice.

• Africa: Africa is likely to be affected by a range of adverse climate changes and impacts (eg, on ecosystems, agriculture, and human health) that will often combine to create multiple stresses. In addition, poverty, under-development, conflict, and a lack of resources mean that most African countries have very limited ability to adapt to the projected changes and cope with the additional pressures from climate change.

• Small islands: Small island developing states have a particularly high exposure of their populations and infrastructure to a range of climate change impacts, where increasing storms combine with sea-level rise and limited resources and natural capital to adapt to the changes.

• Asian and African mega-deltas: The Asian and African mega-deltas have large populations that are exposed simultaneously to the effects of sea-level rise, storm surges, and river flooding. At the same time, high population densities and continued population growth, especially in poor suburbs, limit the ability of those regions to adapt to those changes and to implement protection and prevention measures.

Singling out these four key regions does not imply that other regions are unaffected by climate change, or that they will not suffer significant impacts in some respects. The preceding sections already emphasised particularly affected systems and sectors that may be of particular importance in some regions and locations. In fact, in all regions of the world, some people, areas, and activities can be particularly at risk from climate change. For example, mountain communities that rely on glacier meltwater for agriculture will be hit extremely hard by the gradual disappearance of many smaller glaciers, but those impacts are often more specific and localised than for the larger regions listed above.

It is also worth noting again that even in the most developed countries, some regions and parts of society are at particularly high risk from climate change, as was demonstrated by hurricane Katrina in 2005 and the European heat wave in 2003. Of course, we cannot claim that these two individual events were caused by human-induced climate change (see Box 2.3), but they clearly demonstrated the relatively high vulnerability of particular locations and population groups to extreme events that are projected to become more frequent and/or severe with climate change. (WGII TS.3, TS.4; SYR 3.3, 4.2, 5.3)

chapter 4-impacts of projected climate changes · this chapter aims to overview the main findings...

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