un cc:learn · this section provides an introduction to basic concepts of climate change science...

This section provides an introduction to basic concepts of climate change science such as weather, climate, the greenhouse effect, climate forcings and natural climatefluctuations. It then discusses main causes and elements of anthropogenic (human caused) climate change, including observed and projected changes in the surface temperature. The section concludes with a short discussion of climate change science, its importance and historical development.

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It is important to understand the difference between “weather” and “climate”. What is happening in the atmosphere at any given time is considered “weather” (including e.g. wind speed and direction, precipitation, barometric pressure, temperature, and relative humidity). Weather changes in the short term (e.g. daily, weekly, monthly). Climate is average weather and occurs over long time frames (e.g. 30 years). A common confusion between weather and climate arises when scientists are asked how they can predict climate 50 years from now when they cannot predict the weather a few weeks from now. The chaotic nature of weather makes it unpredictable beyond a few days. Projecting changes in climate (i.e., long-term average weather) due to changes in atmospheric composition or other factors is a very different and much more manageable issue. As an analogy, while it is impossible to predict the age at which any particular man will die, we can say with high confidence that the average age of death for men in industrialised countries is about 75.

IPCC (2007): Frequently Asked Questions - What is the Relationship between Climate Changeand Weather?

Further information:

The IPCC defines climate as follows: “Climate in a narrow sense is usually defined as the average weather, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization. The relevant quantities are most often surfacevariables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.”

IPCC (2013). Climate Change 2013: The Physical Science Basis, Working Group I Contribution to the IPCC Fifth Assessment Report, Glossary

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In a broader sense, climate is the status of the climate system which comprises the atmosphere, the hydrosphere, the cryosphere, the surface lithosphere and the biosphere. These elements all determine the state and dynamics of the Earth’s climate. The graphic illustrates a number of both natural and human factors that have an influence on the climate. An important mechanism within the climate system is the greenhouse effect which is explained in the following slide.

IPCC (2007). Climate Change 2007: The Physical Science Basis


The atmosphere is the envelope of gas surrounding the Earth. The hydrosphere is the part of the climate system containing liquid water at the Earth’s surface and underground (e.g. oceans, rivers, lakes…). The cryosphere contains water in its frozen state (e.g. glaciers, snow, ice…). The surface lithosphere is the upper layer of solid Earth on land and oceans supporting volcanic activity which influence climate. The biosphere contains all living organisms and ecosystems over the land and in the oceans.

WMO website

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The temperature of the Earth results from a balance between energy coming into the Earth from the Sun (solar radiation) and the energy leaving the Earth into outer space. About half the solar radiation striking the Earth and its atmosphere is absorbed at the surface. The other half is absorbed by the atmosphere or reflected back into space by clouds, small particles in the atmosphere, snow, ice and deserts at the Earth’s surface. Part of the energy absorbed at the Earth’s surface is radiated back (or re-admitted) to the atmosphere and space in the form of heat (or thermal) energy. The temperature we feel is a measure of this heat energy. In the atmosphere, not all thermal radiation emitted by the Earth reaches outer space. Part of it is absorbed and reflected back to the Earth’s surface by greenhouse gas(GHG) molecules and clouds (the greenhouse effect) leading to a global average of around 14°C, well above the -19°C which would be felt without the natural greenhouse effect. The concentrations of some GHGs, such as carbon dioxide (CO2), are significantly influenced by humans, others, such as water vapor, are not.

WMO website.


The two most abundant gases in the atmosphere, nitrogen (comprising 78% of the dry atmosphere) and oxygen (comprising 21%), exert almost no greenhouse effect. Instead, the greenhouse effect comes from molecules that are more complex and much less common. Water vapour is the most important greenhouse gas, and carbon dioxide (CO2) is the second-most important one. Methane, nitrous oxide, ozone and several other gases present in the atmosphere in small amounts also contribute to the greenhouse effect.

IPCC (2007): Frequently Asked Questions - What is the Greenhouse Effect?

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Do you know how the greenhouse effect maintains the surface temperature of the Earth? This Nasa’s Earth Observatory video explains it in a graphical way.

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The natural greenhouse effect is part of a balanced system of energy transfer and transformation within the atmosphere, at the Earth’s surface and in the oceans. The Earth’s climate remains largely stable because the energy received is equal to that lost (the energy budget is balanced). However, there are a factors that have caused major changes in the climate system. Since these factors drive or “force” the system to change they are called "forcings”. The change in energy fluxes caused by these drivers are quantified by radiative forcing (RF). Positive RF leads to surface warming, negative RF leads to surface cooling.

During the last millennium, changes in the output of energy from the sun, volcanic eruptions and increased concentration of greenhouse gases in the atmosphere have been the most important forcings. Total radiative forcing has been positive and has led to an up-take of energy by the climate system. The graphic shows that theincrease in the atmospheric concentration of CO2 since 1750 has become the largest contributor to total radiative forcing.

IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers , p11

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Changes in the climate system due to climate forcings are not to be confused withnatural climate fluctuations. Indeed, even within a relatively stable period, the systems that make up and influence the global climate still naturally fluctuate. These fluctuations or “oscillations” as they are often called (because they oscillate between two main states) can have a large affect on the climate, both locally and on a global scale. One example is El Niño, La Niña and the El Niño Southern Oscillation (ENSO).ENSO is a climate pattern that occurs, roughly every 5 years, accross the tropical Pacific Ocean. El Niño (Spanish for little boy) describes extensive warming of the ocean surface across the eastern and central equatorial Pacific lasting three or more seasons (see red area near the equator on the left globe). When this oceanic region switches to below normal temperatures, it is called La Niña (Spanish for little girl , see blue area near the equator on the right globe).

WMO website

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Since the beginning of the 20th century, scientists have been observing a change in the climate that can not be attributed to any of the “natural” influences of the past only. This change in the climate, also known as global warming, has occurred faster than any other climate change recorded by humans.

The main cause of global warming is the increased concentration of greenhouse gases in the atmosphere since the industrial revolution in the late 18th century. The increased amount of gases which absorb and re-emit thermal radiation, has directly led to more heat being retained in the atmosphere and thus an increase in global average surface temperatures. The increase in temperature is also leading to other effects on the climate system. Together these affects are known as anthropogenic (human caused) climate change.

WMO website


Adding more of a greenhouse gas, such as CO2, to the atmosphere intensifies the greenhouse effect, thus warming Earth’s climate. The amount of warming depends on various feedback mechanisms. For example, as the atmosphere warms due to rising levels of greenhouse gases, its concentration of water vapour increases, further intensifying the greenhouse effect. This in turn causes more warming, which causes an additional increase in water vapour, in a self-reinforcing cycle. This water vapour feedback may be strong enough to approximately double the increase in the greenhouse effect due to the added CO2 alone.

IPCC (2007): Frequently Asked Questions - What is the Greenhouse Effect?

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This diagram gives an indication of the observed change in average surface temperature between 1901 and 2012. It shows that almost the entire globe has experienced surface warming.

According to the IPCC, the average global surface temperature has increased by 0.85°C over the period 1880 to 2012.

IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers, p3

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Scientists have not only observed past changes in the climate but also try to analyzepossible future changes. For this purpose they have developed a number of tools. Just as an architect might build a scale model of a building to understand and predict its behaviour, so too climate scientists can build a computer-based model of the climate system to understand and predict its behaviour. One of the inputs to a climate model are emission scenarios, which estimate future releases of greenhouse gases and aerosols to the atmosphere based on assumptions concerning, for example, future socioeconomic and technological developments. The outputs of a climate model feed into a climate projection, i.e. a simulated response of the climate system to a certain emission scenario. This dependence on emission scenarios differentiates climate projections from climate predictions which are based on conditions that are known at present and assumptions about the physical processes that will determine future changes.

WMO website

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This figure illustrates the projected (i.e. future) change in average surface temperature for two different scenarios. The projections are for the end of the 21st

century (2081-2100) and are given relative to 1986-2005. The projection on the left is based on a scenario with relatively limited greenhouse gas emissions (RCP 2.6), while the projection on the right is based on a scenario with very high greenhouse gas emissions (RCP 8.5). RCP 2.6 projects an increase of 0.3 to 1.7°C in mean surface temperature compared to preindustrial times, while RCP 8.5 projects an increase of 2.6 to 4.8°C by 2081-2100.

IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers


Projections of changes in the climate system are based on a hierarchy of climate models that range from simple climate to intermediate complex models, to comprehensive climate models as well as Earth System Models. Based on a set of scenarios of anthropogenic forcings, the modules simulate changes. The Representative Concentration Pathways (RCPs) is a new set of scenarios that are used for the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).

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Climate change has an impact on almost every aspect of our lives. Our ecosystems suffer biodiversity and habitat loss and human systems like health will be negatively impacted, for example by the spread of disease vectors like mosquitos. Climate change also challenges us to rethink our urban systems (including transport and buildings) and the way we do business (including green business opportunities). The impacts of climate change might also result in conflict or force people to migrate (for example from low-lying coastal areas).

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Scientists worldwide are trying to better understand how the climate is changing, what changes we can expect in the future, and what role human activities play. While there is a debate about the likelihood of certain changes and their causes, broad scientific consensus exists that (1) warming of the climate system is unequivocal, and (2) that human influence on the climate system is clear.


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Climate change science provides important information for decision-making at various levels. For example, sound weather data and forecasts can help us determine when the best time would be to harvest our lands. It can also help us plan appropriately for emergency responses, in case of climate-related hazards, such as cyclones. Climate models help to forecast long term climate scenarios and are important for proactive planning.

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As far back as 1824, the French physicist Joseph Fourier is the first to describe the Earth’s natural “greenhouse effect”. In 1861, the Irish physicist John Tyndall shows that CO2 and H2O can cause changes in the climate. In 1895, Swedish chemist SvanteArrhenius concludes that industrial-age coal burning will enhance the natural greenhouse effect. In 1938 the British engineer Guy Callendar shows that temperatures had risen over the previous century due to increased CO2

concentrations. The "Callendar effect" is widely dismissed. In 1958 the geochemist Charles David Keeling was employed to continuously monitor CO2 levels in the atmosphere, with an increase in Antarctica visible after only two years. During the 1970’s other greenhouse gases, CH4, N2O and CFCs, were widely recognized as important anthropogenic greenhouse gases and in 1979 the First World Climate Conference was held in Geneva, leading to the establishment of the World Climate Programme. In 1988 the Intergovernmental Panel on Climate Change (IPCC) was set up by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP). In 1990 the IPCC delivered its First Assessment Report on the state of climate change, predicting an increase of 0.3 °C each decade in the 21st century.

IPCC (2007). Fourth Assessment Report, Chapter One - Historical Overview of Climate Change Science

Zillman, J. (2009). A History of Climate Activities

Knight, M. for CNN (2008). A Timeline of Climate Change Science

BBC Website

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This section discusses the main driver of anthropogenic climate change, i.e. the increased concentration of greenhouse gases in the atmosphere. The section startswith an overview of the most important greenhouse gases emitted by humans. It then discusses each gas in more detail, looking at (1) how significant the gas is in terms of global warming, and (2) how its concentration in the atmosphere has developed over time. The section concludes with a graph depicting the extent of the human influence on the climate system.

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The Intergovernmental Panel on Climate Change (IPCC) has produced a video on the physical science basis of climate change. It looks at evidence of how the climate has changed in the past and present, what the causes of these changes are, and whatpossible future scenarios exist.

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Greenhouse gases (GHGs) are trace gases in the atmosphere that absorb and emit long wave radiation. They naturally blanket the earth and keep it at about 33° C warmer than it would be without these gases in the atmosphere. The table features the seven most important greenhouse gases as regulated under the Kyoto Protocol. The seven gases each have a different capacity to trap heat in the atmosphere, or a so-called “global warming potential” (GWP). They all belong to the group of long-lived greenhouse gases (LLGHGs), because they are chemically stable and persist in the atmosphere over time scales of a decade to centuries or longer, so that their emission has a long-term influence on climate. Some of the GHGs occur naturally (e.g. CO2, CH4 and N2O) but increases in their atmospheric concentrations over the last 250 years are due largely to human activities. Other greenhouse gases are entirely the result of human activities (e.g. HFCs, PFCs, SF6 and NF3).

IPCC (2007). Fourth Assessment Report, Technical Summary – Changes in Human and Natural Drivers of Climate

UNEP (2012). Emissions Gap Report


Kyoto Protocol: The Kyoto Protocol sets out legally binding targets for developed countries to limit or reduce their GHG emissions. It was adopted in 1997 and entered into force in 2005. Water vapor is the most important greenhouse gas but since it is not produced by humans in any significant quantity, we have no control over its concentration in the atmosphere. It is therefore not regulated under the Kyoto Protocol.

Global Warming Potential: Carbon dioxide is the baseline unit to which all other greenhouse gases are compared and therefore has a GWP of exactly 1. A GWP is calculated over a specific time interval (commonly 20, 100 or 500 years), because some gases remain longer in the atmosphere than others. For example, the 100 year GWP of methane is 25, which means that if the same mass of methane and carbon dioxide were introduced into the atmosphere, that methane will trap 25 times more heat than the carbon dioxide over the next 100 years.

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The most important anthropogenic GHG is carbon dioxide (CO2). It accounts for around 64% of total radiative forcing due to LLGHGs. Carbon dioxide does not have a specific lifetime because it is continuously cycled between the atmosphere, oceans and land biosphere and its net removal from the atmosphere involves a range of processes with different time scales. CO2 is primarily emitted as a result of burning of fossil fuels, deforestation and forest degradation and iron and steel production. Oceans and forests are the main sequesters of carbon i.e sinks that can absorb CO2

from the atmosphere. Carbon dioxide is the gas to which all other gases are compared when speaking of Global Warming Potential. Emissions of other greenhouse gases can be converted into CO2 equivalent emissions.

IPCC (2007). Fourth Assessment Report, Technical Summary – Changes in Human and Natural Drivers of Climate

WMO (2013). Greenhouse Gas Bulletin


The contribution of each greenhouse gas to radiative forcing is determined by itsglobal warming potential and the change in its concentration in the atmosphere over time.

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CO2 levels in the atmosphere have been increasing steadily over the past 200 years. This is due to the increased use of fossil fuels as well as an increase in deforestation, both of which release vast amounts of CO2 into the atmosphere. Today’s concentration of CO2 is believed to be the highest it has been in the past 800,000 years. CO2 concentrations increased by about 100 ppm (parts per million) since the industrial revolution and exceeded the symbolic 400 ppm threshhold at several atmosphere watch stations during 2012. Figure a) shows the increase in CO2

concentrations between 1984 and 2012. Figure b) shows the annual growth rate of CO2 concentrations during the same time period.

WMO (2013). Greenhouse Gas Bulletin, p3 WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record

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The second most significant anthropogenic GHG is methane (CH4) which contributes to approximately 18% of total radiative forcing due to LLGHGs. Approximately 40% of methane is emitted into the atmosphere by natural sources (e.g. wetlands and termites). About 60% comes from human activities (e.g. cattle breeding, rice agriculture, fossil fuel exploitation, landfills and biomass burning). Methane is mostly removed from the atmosphere by chemical reactions, persisting for about 12 years. Thus although methane is an important greenhouse gas, its effect is relatively short-lived.

IPCC (2007). Fourth Assessment Report, Working Group I - The Physical Science Basis

WMO (2013). Greenhouse Gas Bulletin, p3

WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record

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This slide shows the atmospheric concentration of methane from 1984 to 2012 in figure (a), while figure (b) shows the annually averaged growth rate. The concentration of CH4 has more than doubled since pre-industrial times (from approximately 700 parts per billion in 1750 to 1819 ppb in 2012 ). Since 2007, atmospheric methane has been increasing again after a temporary period of levelling-off.

WMO (2013). Greenhouse Gas Bulletin, p3 WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record

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Carbon in its various forms (such as CO2 and CH4) is continually recycled on Earth and is never destroyed. The diagram illustrates the many ways in which carbon is released and stored in the environment. Carbon can be stored over relatively short periods in living organisms (i.e. plants and animals) or over thousands of years in the oceans. It can also be stored over millions of years in rocks or fossils. The diagram also shows how humans are affecting the carbon cycle. Before humans used fossil fuels for energy, the carbon cycle was relatively balanced (i.e. the total amount carbon in the atmosphere stayed constant). By removing carbon from the long term storage underground (oil, gas, etc.) and putting it into the atmosphere, humans have tipped the balance of the carbon cycle, which in turn affects the global climate.Removing stored carbon through deforestation also further exacerbates this process.

UNEP (2009). Climate in Peril, p14

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Nitrous oxide is the third most significant GHG, contributing to about 6% of radiativeforcing due to LLGHGs. The primary human sources of N20 are fertilizer production and use in agriculture and various industrial processes. It is estimated that N20 stays in the atmosphere for an estimated 114 years. Its impact on climate, over a 100-year period, is 298 times greater than equal emissions of carbon dioxide. It also plays an important role in the destruction of the stratospheric ozone layer which protects us from the harmful ultraviolet rays of the sun.


WMO (2013). Greenhouse Gas Bulletin, p3

WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record

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The concentration of N2O has been steadily increasing over the past 30 years. Its atmospheric concentration in 2012 was about 325.1 parts per billion, which is 20% more than the pre-industrial level (270 ppb).

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Fluorinated gases are a family of man-made gases used in a range of industrial applications.Sources include refrigerants, air-conditioning, solvents, aluminium and magnesium production, etc. Many fluorinated gases have very high global warming potentials (GWPs) relative to other greenhouse gases. That means small atmospheric concentrations can have large effects on global temperatures. They can also have long atmospheric lifetimes, in some cases, lasting thousands of years. Fluorinated gases are removed from the atmosphere only when they are destroyed by sunlight in the far upper atmosphere. In general, fluorinated gases are the most potent and longest lasting type of greenhouse gases emitted by human activities. There are three main categories of fluorinated gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

EPA website


Hydrofluorocarbons (HFCs) are the most common group of F-gases. They are used in various sectors and applications, such as refrigerants in refrigeration, air-conditioning and heat pump equipment; as blowing agents for foams; as solvents; and in fire extinguishers and aerosol sprays.

Perfluorocarbons (PFCs) are typically used in the electronics sector (for example for plasma cleaning of silicon wafers) as well as in the cosmetic and pharmaceutical industry. In the past PFCs were also used in fire extinguishers and can still be found in older fire protection systems.

Sulphur hexafluoride (SF6) is used mainly as an insulating gas, in high voltage switchgear and in the production of magnesium and aluminium.

EC website

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Some greenhouse gases are not included in the Kyoto Protocol because they are already regulated under the Montreal Protocol on Substances that Deplete the Ozone Layer which entered into force in 1989. The Montreal Protocol includes, for example, chlorofluorocarbons (CFCs) which contribute about 12% to total radiativeforcing by LLGHGs. CFCs can stay in the atmosphere for more than 1,000 years. CFCs have a global warming potential (GWP) that ranges between 4,750 and 14,400 (over 100 years time span). CFCs are used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants.

NOAA website



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This slide shows the evolution of atmospheric concentrations of sulfur hexafluoride(SF6) (graph on the left) and numerous CFC and F-gases (graph on the right). Concentrations of CFCs, which are controlled by the Montreal Protocol, are in decline. However, hydro-chlorofluorocarbones (HCFCs) and hydro-fluorocarbones(HFCs), which are also potent greenhouse gases, are increasing at relatively rapid rates.


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This figure illustrates the important impact of human activities on the climate. It shows the contribution of different anthropogenic and natural factors to the observed temperature increase of about 0.6°C since 1951 (black bar). The graph shows that GHGs such as carbon dioxide, methane and nitrous oxide (green bar) are the main causes of the observed temperature change. The yellow bar depicts the influence of aerosols (tiny particles found in the atmosphere) which have a negative forcing effect (cooling) on the climate. In fact, aerosols and their interaction with clouds have offset a substantial portion of positive radiative forcing by GHGs. Atmospheric aerosols are not to be confounded with aerosol sprays which often contain GHGs and hence have a positive radiative forcing effect.

Overall, human activity has led to positive radiative forcing (global warming) as indicated by the orange bar. Radiative forcing due to changes in energy output from the sun and volcanic eruptions only played a minor role in the reference period.

IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers , p11-12


Atmospheric aerosols are able to alter climate in two important ways:

• They scatter and absorb solar and infrared radiation;

• They may change the microphysical and chemical properties of clouds and possibly their lifetime and extent.

The scattering of solar radiation acts to cool the planet, while absorption of solar radiation by aerosols warms the air directly instead of allowing sunlight to be absorbed by the surface of the Earth. The direct radiative forcing summed over all aerosol types is negative. Aerosols also cause a negative radiative forcing indirectly through the changes they cause in cloud properties.

WMO website

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This section describes some of the main observed changes in the climate since the industrial revolution. It looks at changes in surface temperature, precipitation levels, ocean warming and acidification, sea-level rise, Arctic sea ice extent, as well as observed changes in physical and biological systems. The section concludes with a discussion of whether the recent increase in extreme weather events (such as cyclones and floods) can be attributed to anthropogenic climate change.

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This figure illustrates how the averaged land and ocean surface temperature has changed between 1850 and 2012. The annual temperature average has always varied, with cold and warm periods alternating. However, it is clear that each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.

The temperature increase is widespread across the world, but there are important regional variations. Warming has been most marked in the northern Polar Regions.

IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers , pp 3-4

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Observations show that changes are occurring in the amount, intensity, frequency and type of precipitation. These aspects of precipitation generally exhibit large natural variability, and El Niño and other natural climate fluctuations have a substantial influence. Over the past century, however, pronounced long-term trends in precipitation amounts have been observed: significantly wetter in eastern North and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. Moreover, widespread increases in heavy precipitation events have been observed, even in places where total amounts have decreased. The two maps show the observed precipitation change from 1901 to 2010 and 1951 to 2010.


IPCC (2007): Frequently Asked Questions – How Is Precipitation Changing?

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Ocean warming dominates the increase in energy stored in the climate system. Oceans account for more than 90% of the energy accumulated between 1971 and 2010. 60% of the net energy increase is stored in the upper ocean (0-700 m) and about 30% is stored in the ocean below 700 m. The ocean warming is largest near the surface, am the upper 75 m warmed by 0.11°C per decade over the period 1971 to 2010.


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About 30% of anthropogenic CO2 emissions has been absorbed by the oceans. This leads to ocean acidification. The green curve in the figure shows the decreasing pH of ocean surface water since the late 1980s. According to the IPCC, the pH of ocean surface water has decreased by 0.1 since the beginning of the industrial era.


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The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. Over the past century, global mean sea level rose by 0.19m. Glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea-level rise since the early 1970s.


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Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice has continued to decrease in extent. The graph illustrates the decrease in summer sea ice extent in the Artic between 1900 and 2010. The spatial extent has decreased in every season since 1979.


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This slide shows a number of changes that have been observed in physical (blue) and biological (green) systems between 1970 and 2004. Biological changes include, forexample, species loss and ecosystem alterations. Physical changes include, for example, changes in snow cover, changes in glacier density/coverage and run-off. The percentages associated with the observations indicate how many of the reported changes are due to a warming environment.

UNEP (2009). Climate in Peril

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Whenever an episode of extreme weather – heatwave, flood, drought, etc. – hits the headlines, many people tend to blame human-induced climate change. But what scientific evidence exist to explain individual extreme events, like a cyclone, with global warming? First of all, determining whether a specific, single extreme event is due to a specific cause, such as increasing greenhouse gases, is difficult, if not impossible, for two reasons: 1) extreme weather events are caused by a combination of factors, and 2) a wide range of extreme events is a normal occurrence even in an unchanging climate.

At the same time, observations have shown a large increase in the number of strong hurricanes globally since 1970. Specifically, the number of strong hurricanesincreased by about 75% since 1970. The IPCC points out that a trend towards longer storm duration and greater storm intensity is closely correlated with tropical sea surface temperature. This might be an indication of a causal link between global warming and hurricane destructiveness. However, the high variability in tropical storms and hurricanes over several decades and a lack of systematic high quality observation before satellite observations make it difficult to detect long-term trends.

IPCC (2007): Frequently Asked Questions – Has There Been a Change in Extreme Events?

UNEP (2009). Climate in Peril

WMO website

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This section presents projected future trends and impacts of climate change on surface temperature, precipitation, ocean pH, sea-level and Arctic sea-ice extent. The extent of these impacts depends on how anthropogenic emission levels willdevelop over the next decades. Therefore, the section presents impacts under a low-emission scenario, as well as under a high-emission scenario. The section ends witha discussion of cumulative CO2 emissions, and what different amounts of CO2

emissions would mean for the future climate.

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For the Fifth Assessment Report of the IPCC, the scientific community has defined a set of four new scenarios, denoted Representative Concentration Pathways (RCPs).These four RCPs include one mitigation scenario leading to a very low forcing level (RCP2.6), two stabilization scenarios (RCP4.5 and RCP6), and one scenario with very high greenhouse gas emissions (RCP8.5). The RCPs can thus represent a range of 21st century climate policies, as compared with the no-climate-policy of the Special Report on Emissions Scenarios (SRES) used in the Third and the Fourth Assessment Reports.


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Global surface temperature change for the end of the 21st century is likely to exceed 1.5°C relative to pre-industrial levels for all RCP scenarios except RCP 2.6. It is likely to exceed 2°C for RCP 6.0 and RCP 8.5. Since the Third IPCC Assessment Report confidence has increased that a 1.5-2.5°C increase in global mean temperature above pre-industrial levels poses significant risks to many unique and threatened systems, including many biodiversity hotspots. Approximately 20-30% of species are at increased risk of extinction if global average warming exceeds 1.5-2.5°C. In terms of food security and human health, crop productivity for cereals in low latitudes would decrease and the distribution of some disease vectors (like mosquitostransmitting malaria) might change.

UNEP (2009). Climate in Peril, pp 27-29


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The IPCC projects that over the 21st century the contrast in precipitation between wet and dry regions and between wet and dry seasons will increase. That means global warming increases risks of both drought and floods. Extreme precipitation events are particularly likely to become more intense and more frequent over most of the mid-latitude land masses and over wet tropical regions.

The two maps illustrate projected changes in annual mean precipitation for the end of the 21st century under different scenarios. Under a low-emission scenario (RCP 2.6 – left map) changes in annual mean precipitation will not exceed 20% compared to 1986-2005 levels. However, under scenario RCP 8.5 (right map) significant changes in annual precipitation are to be expected. The high latitudes and the equatorial Pacific Ocean will experience an increase in annual mean precipitation, while in many mid-latitude and subtropical dry regions, mean precipitation will decrease.


IPCC (2007): Frequently Asked Questions – How is Precipitation Changing?


As climate changes, several direct influences alter precipitation amount, intensity, frequency and type. Warming accelerates land surface drying and increases the potential incidence and severity of droughts, which has been observed in many places worldwide. However, a well-established physical law (the Clausius-Clapeyron relation) determines that the water-holding capacity of the atmosphere increases by about 7% for every 1°C rise in temperature. Because precipitation comes mainly from weather systems that feed on the water vapour stored in the atmosphere, this has generally increased precipitation intensity and the risk of heavy rain and snow events.

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By combining data from various climate models, used by the IPCC, this video shows the expected changes in temperature and precipitation for the 21st century.

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Today the average ocean surface pH is about 8.1. Projections suggest a further acidification of the oceans over this century due to increasing atmospheric carbondioxide concentrations. Under a low-emission scenario (RCP 2.6 – left map) ocean acidification will be relatively limited. However, under scenario RCP 8.5 (see right map) a reduction in average global surface ocean pH of between 0.30 and 0.32 unitsis to be expected. This progressive acidification will harm marine creatures which form shells, for instance corals, and the species which depend upon them.


UNEP (2009). Climate in Peril, p30

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Global mean sea level will continue to rise during the 21st century (see graph). The projected rise ranges between 0.26 m to 0.98 m depending on the scenario. Under all RCP scenarios, the rate of sea level rise will exceed that observed during 1971 to 2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets.


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During the 21st century the Arctic sea ice cover will continue to shrink and thin as global mean surface temperature rises. Under scenario RCP 8.5 it is likely that the Arctic Ocean will be nearly ice-free in the month of September before mid-century.


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Africa is one of the most vulnerable continents to climate change. Most of Africa will see less precipitation, with only the east central region seeing an increase. By 2080, an increase of 5 to 8% of arid and semi-arid land is projected under a range of scenarios. Already by 2020, between 75 million and 250 million people will be exposed to increased water stress due to climate change. Agricultural production, including access to food is projected to be severely comprised. In some countries, yields from rain-fed agriculture could be reduced by up to 50%. Sea level rise will affect major cities in low-lying coastal areas, such as Alexandria, Cairo, Lomé, Cotonou, Lagos and Massawa.

UNEP (2009). A Climate in Peril

IPCC (2007). Climate Change 2007: Synthesis Report, p11

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This slide details some of the impacts of climate change projected for the Asian region. The light blue colour indicates permafrost regions that are at risk of thawing.Glacier melt will increase flooding and rock avalanches, and affect water resources in Tibet, India and Bangladesh, this will then cause decreased river flows and fresh water availability as the glaciers recede. More than a billion people could have water shortages by the 2050s. South East Asia, especially the heavily-populated mega delta regions will be at risk from flooding. Around 30% of Asia’s coral reefs are likely to be lost in the next 30 years due to multiple stresses and climate change. Changes in rainfall will increase diarrheal diseases mainly associated with floods and droughts. The green colour indicates the possible increase in malaria distribution.



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This slide presents climate change impacts projected for the Latin American region. As a consequence of reduced precipitation and retreating glaciers, Latin America may have less water, affecting human consumption, agriculture and energy generation. Yields of food crops may diminish, with adverse consequences for food security. Latin America also faces the risk of significant biodiversity loss through species extinction in many tropical areas. Decreases in soil water are projected to lead to gradual replacement of tropical forest by savanna in eastern Amazonia.Another ecosystem situated in the Caribbean at threat are coral reefs, home to many marine living resources. Sea-level rise will increase the risk of flooding in low-lying areas, particularly the Caribbean.



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Many small islands, e.g. in the Caribbean and the Pacific, will experience reduced water resources to the point where they become insufficient to meet demand during low-rainfall periods. Sea-level rise will lead to the infiltration of freshwater resources by saltwater, making it undrinkable. Sea-level rise is also 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. The deterioration in coastal conditions and coral bleaching will reduce the value of these destinations for tourism.


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This diagram depicts four potential climate futures depending on what policies governments adopt to cut emissions. It is based on the four scenarios used in the 2013 IPCC report on the physical science basis of climate change.

Cambridge University (2013). Climate Change: Action, Trends and Implications For Business

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The graph illustrates the linear relationship that exists between cumulative human-induced CO2 emissions (horizontal axis) and increases in global mean surface temperature (vertical axis). In other words, more CO2 emissions lead to higher mean surface temperatures. Because we cannot tell exactly how much carbon dioxide we will emit in the future, different scenarios exist for what global temperatures will be at the end of this century. These scenarios range from 2°C (blue line – RCP 2.6) to nearly 5°C (red line – RCP 8.5). In order to have a greater than two in three chance of keeping global temperatures below 2°C (thereby avoiding ‘dangerous climate change’), cumulative emissions of CO2 must not exceed 1,000 Gigatonnes of carbon (GtC). However, as of 2011 more than half this amount, over 500 GtC, has already been emitted (black line).

IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers, pp 25-26


The horizontal axis on the bottom of the graph indicates cumulative emissions in gigatonnes of carbon (GtC) while the top axis indicates emission levels in gigatonnesof carbon dioxide (GtCO2). 1 GtC corresponds to 3.667 GtCO2.

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This section provides an overview of main source of scientific climate information, relevant programmes and institutions.

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The IPCC is the leading international body which synthesizes and assesses climate change knowledge. Established in 1988 by UNEP and the World MeteorologicalOrganization (WMO) the IPCC assesses all peer reviewed and published climate change information. Awarded with the Nobel Peace prize in 2007 it calls on a network of climate scientists, biologists, economists, sociologists etc. from every continent on Earth to produce reports on the state of knowledge of climate change science, analyze the social and economic impacts of climate change, and identify possible adaptation and mitigation options.

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The IPCC publishes various reports that are relevant to the climate change regime. Widely cited, the comprehensive assessment reports review the latest science on climate change, its impacts, vulnerability and adaptation, as well as mitigation options. In addition to these reports the IPCC also publishes special reports on selected topics, for example: renewable energy sources, extreme events and disasters, emission scenarios etc. It also produces methodological guidance documents and technical papers.

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The WMO runs a number of global climate programmes that provide policy makers and technical staff with information, necessary to successfully respond to climate change. The World Climate Programme (WCP) aims to produce skillful climate predictions and projections, develop operational structures to provide climate services, and to develop an essential global observing system to meet climate information needs. The WCP underpins the Global Framework for Climate Services (GFCS) which aims to incorporate science-based climate information into planning, policy and practice on the global, regional and national scales. Another major programme is the Atmospheric Research and Environment Programme (ARE) which co-ordinates and stimulates research on the composition of the atmosphere and weather forecasting, focusing on extreme weather events and socio-economic impacts. The Commission for Climatology (CCI) advises and guides the activities of the WCP, while playing a key role in implementation of the GFCS.

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The GFCS is a global partnership of governments and organizations that produce and use climate information and services. It seeks to enable researchers and the producers and users of information to join forces to improve the quality and quantity of climate services worldwide, particularly in developing countries. The presentation provides an introduction to the main elements of the GFCS.

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The Global Atmosphere Watch (GAW) programme of WMO provides reliable scientific data and information on the chemical composition of the atmosphere, its natural and anthropogenic change, and helps to improve the understanding of interactions between the atmosphere, the oceans and the biosphere. The backbone of the GAW programme is a global network of more than 80 measurement stationsas seen on this map.

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WMO Regional Climate Centres (RCCs) are centres of excellence that create regional products including long-range forecasts that support regional and national climate activities. RCCs use data and products from Global Producing Centres for Long-range Forecasts (GPCs), incorporating regional-scale information. An additional main source of information for RCCs are data, products, know-how and feedback they receive from National Meteorological and Hydrological Services (NMHSs).

Regional Climate Outlook Forums (RCOFs) bring together national, regional and international climate experts, on an operational basis, to produce regional climate outlooks based on input from national, regional, and global institutions. By bringing together countries having common climatological characteristics, the forums ensure consistency in the access to and interpretation of climate information. The information provided by RCOFs is being applied to reducing climate-related risks and supporting sustainable development.

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National Meteorological and Hydrological Services (NMHSs) provide weather, climate, and water information to emergency managers, national and local administrations, the public, and critical economic sectors. The NMHSs make a significant contribution to safety, security, and economic well-being by observing, forecasting, and warning of pending weather, climate, and water threats.

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un cc:learn · this section provides an introduction to basic concepts of climate change science...

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