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Global Warming

Chapter 16

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20th Century Climate Changes• Global Warming

– Average Global Temp. = 24 h, all seasons, all locations• Increased 0.7 oC (1.25 oF) from 1900—2000

• Not uniform: some places cooler (S. E. United States)

• Largest increase is in the daily low temperature

• Not a continuous change over time (largest change last 25 yr)

• Other Global Weather Changes– Increased Average Precipitation (not uniform)– Shorter winters in N. Hemisphere (blooming/leaf loss)– Shrinking glaciers and polar ice caps– Dieing coral, rising sea level, more extreme weather

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Global Temperature Changes 1901--1998

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Global Precipitation Changes 1901--1998

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Causes of Global Warming• Keeping Earth Warm

– The source of Earth’s energy is sunlight• Contains UV (50-400 nm), Visible (400-750 nm)

• Contains IR = Infrared = 750-4000 nm (0.75-4.0 m)

– Earth must emit energy in order not to keep heating up• IR = 4.0-50 m = thermal infrared = emitted energy

• Some from the surface, some from the atmosphere

– The Greenhouse Effect helps keep Earth warm• Not all of Earth’s emitted IR escapes

• Atmospheric gas molecules can reabsorb some emitted IR– Absorbed IR is transformed into heat, warming the atmosphere

– Temperature of Earth’s atmosphere is warmer due to Greenhouse Effect

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Spectrum of Sunlight (dotted) and Earth’s Emitted light

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The Greenhouse Effect

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The Greenhouse Effect• Why doesn’t Earth’s temperature keep rising?

– Equilibrium = increases and decreases balance out– Air emits more energy as light (not heat) at temp. rises– Some can escape to space, decreasing the warming effect– A temperature is reached where Heat in/out is balanced

• Presence of gases raise the equilibrium temp. of the atmosphere

• Avg. temp. = +15 oC instead of –15 oC without gases

• Gases contribute as much warming as direct sunlight itself

• Enhanced Greenhouse Effect = pollution gases increases the amount of heat absorbed and released, raising the temp. of the Earth

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Greenhouse Gases (G.G.)• Definition = atmospheric gas that efficiently absorbs

IR light emitted by Earth– N2, O2, Ar = main atmospheric gases = not G. G.

– Water Vapor (H2O) is the most important G.G.• Naturally present from evaporation• Concentration is variable depending on season, geography• Absorbs around 7 m and above 18 m

– Carbon Dioxide (CO2) next most important G.G.• Most from natural sources: decay of plant matter• About one-third comes from man’s combustion of fossil fuels• Absorbs at 4 m and 15 m strongly

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The IR absorption spectrum of CO2

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Other Greenhouse Gases• Methane (CH4): naturally occurring, increasing due

to man

• Nitrous Oxide (N2O): natural biological sources

• Ozone (O3): naturally occurring

• The Atmospheric IR Absorption Spectrum– Water and CO2 are the strongest absorbers

– The other G.G.’s contribute at specific wavelengths– Atmospheric Window

• From 8-13 m very little IR is absorbed

• Only ozone contributes absorption in this “window”

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The atmospheric IR absorption spectrum

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Carbon Dioxide Concentrations• Analysis: how do we know what the CO2 level is?

– now = clean air sample– past = bubbles trapped in glaciers

• Results– Pre-Industrial Revolution (before 1750) = 280 ppm CO2

– 2001: 371 ppm CO2

– 1.6 ppm per year increase– Seasonal Concentrations

• Low in summer as plants grow and use up CO2

• High in winter as plants die and decay releasing CO2

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Trends in CO2 concetration 1955-2005

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Anthropomorphic CO2 Sources• Fossil Fuel Combustion—major source

– Coal, oil, natural gas, gasoline produce CO2 when burned

– Propane: C3H8 + 5 O2 ----> 3 CO2 + 4 H2O

– Has slowed down recently due to collapse of Soviet Block economies

• Quicklime Production—minor source (few %)– Used in cement production

– CaCO3 + Heat ----> CaO (quicklime) + CO2

• Burning Vegetation: forests, farmland (25% of CO2)

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World CO2 Emissions from Fossil Fuel 1950-1999

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Carbon Dioxide Sinks• CO2 emission from fossil fuels + Cement = 23 Gt/yr

• Forests– Deforestation (burning) adds to CO2 emissions

– New forest growth removes CO2 from air

– Net effect is removal of about 5 Gt/yr

• Oceans– Shallow oceans are temporary sink

• Plants, dissolving remove CO2, but will be returned to air

• Net effect is zero (equilibrium)

– Deep Oceans and Sediments = permanent sink 6 Gt/yr• Metal carbonates (CaCO3) are formed and deposited

• Very slow removal means CO2 builds up at about 12 Gt/yr

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Annual net movement of CO2 in the Atmosphere

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Methane and Global Warming• Concentration doubled since 1750 (most since 1900)• CH4 21 times worse than CO2 for IR ----> Heat

– CO2 concentration is 80 times that of CH4 in air– CH4 not as important overall as CO2

• Sink = oxidation to CO2 – CH4 + 2 O2 ----> CO2 + 2 H2O – Takes about 10 years for this to happen on average

• Methane sources– Anaerobic Respiration: 2 CH2O ----> CH4 + CO2

• Swamps, marshes (swamp gas, marsh gas)

– Ruminant animal digestion (sheep, cows)– Landfill decomposition, pipelines, coal mines, oil wells

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Nitrous Oxide and Global Warming• N2O is 206 times worse than CO2 for IR ----> Heat

• Increased 13% since 1750 (Now about 0.25%/yr)

• Sources– Tropical oceans and soils (60% natural sources)

• Denitrification in an oxygen-rich environment

• Nitrification in an oxygen-poor environment

– Nitrogen based fertilizers (majority of man-made 40%)– Nylon synthesis (now cleaned up)– Automobile catalytic converters

N2 + O2 ----> N2O

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Biological Sources of Nitrous Oxide

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CFC’s and Water Vapor• CFC’s = chlorofluorocarbons

– Absorb in 8-13 m IR window range– Each CFC > 10,000 worse than a CO2 molecule– Destroying ozone actually cools (O3 is a G.G.)– CFC replacements are much less problematic

• Water Vapor– Produced by combustion of fossil fuels, but small amount– Evaporation of oceans, snowpack, clouds major source– Concentration in air increases as air warms– Leads to more global warming = Positive Feedback– Doubles the effects of the other Greenhouse Gases

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Aerosols and Clouds• Aerosol = tiny suspended particles in the air

– Most reflect light before reaches surface = cooling effect– Dark colored particles change light to heat (ash, soot)– Overall effect of Global Aerosols is cooling

• Helps reduce global warming

• Mt. Pinatubo volcano 1991, H2SO4 aerosol, 0.2 oC globally

– Do not accumulate in air, like CO2 does (rain washes out)

– Not enough to completely counter Greenhouse Gases

• Cloud Cover– Water droplets reflect sunlight (cooling = low clouds) – Reflect emitted IR back to Earth (warming =high clouds)– Warmer = Wetter, but where will new clouds form?

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Aerosols effects on Global Warming

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Estimating Contributions• Man’s effect not proven to cause Global Warming

– Some scientists are not convinced (references)– Vast majority of scientists agree we are likely cause

• IPCC = International Panel on Climate Change of the U.N.

– Estimates of the contributions to Global Warming• Water vapor not listed, but accounts for half of each contributor

• Land Use = tilled farmland reflects more light, rather than turning it into heat

• Individual components > 100%, due to negative contributors

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Estimated Contributions to Global Warming

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The Future of Global Warming• Predicted CO2 Concentrations

– Long lifetime without permanent sinks = decades in air– Mainly influenced by emissions

• If CO2 emissions remain constant, 500 ppm by 2100

• If CO2 emissions continue to increase, 700 ppm by 2100

• Predicted Air Temperatures– Computer models predict 1.9-2.9 oC increase by 2100– Current avg. temp. is about 6.0 oC warmer than ice age– Modeling is difficult because the problem is complex– Results: more hot days, fewer frost days, melting ice caps

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Two scenarios for future CO2 concentrations

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The Runaway Greenhouse Effect• Several positive feedback loops may work together

– No matter what we do, we couldn’t stop warming– Natural processes would enhance man-made effect– This is possible, but too complex to say for sure

• Methane Positive Feedback– As temperature rises, natural CH4 emission will rise

– Plant matter decay is speeded up

– Thawing permafrost releases frozen CH4

– Methane hydrate released from sea floor• CH4 • 6 H2O = clathrate of CH4 trapped like in solid ice

• Warm ocean floor: massive amounts of CH4 will be released

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Predicted Effects of Global Warming• Predictions are difficult due to complexity

• Rising Sea Levels– Increase by 50 cm globally by 2100– Melting + Thermal Expansion (warmer water expands)– This effect will be delayed

• Takes time for warmth to reach cold ocean depths

• Will continue even if we fix global warming today

– ½ of the global population is in low-lying coastal areas• Outright flooding of the land

• Tropical storm damage increased

• Salt water contamination of fresh groundwater

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Changes in Precipitation Patterns• Global Precipitation will increase

– Warmer air can hold (then release) more water– 2% increase in water vapor per 1 oC temperature rise– Storms will be more violent: warmer = more energy– Precipitation changes will be inconsistent

• More water-scarce areas

• More flooding

• Agriculture and Ecosystems Effected– Soil will hold less water as it becomes warmer

– More efficient photosynthesis as CO2 increases

– Longer growing seasons; Fewer pest-killing freezes– Collapse of Ecosystems: coral reefs, hardwood forests

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Effects on Human Health• Heat Kills

– Young and old effected most– Heart disease, respiratory disease, high blood pressure

• Photochemical Smog will increase

• Fewer cold-related illnesses– May cancel out heat related illnesses

• Insect-born diseases will increase– Malaria born by mosquitoes will spread north

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Controlling CO2 Emissions• Energy Use and CO2 Emissions

– GNP = Gross National Product = economic strength • Large GNP = more energy used = more CO2 emitted• Developed countries (US, Europe, Japan) have large GNP’s• Developing countries (S. Am., Africa, Asia) have small GNP’s

– U.S.A. largest emitter in total(#) and per capita (bar size)– 4 tonnes CO2 emitted per person per year on Earth

• 11 tonnes/person/year for Developed countries• 2 tonnes/person/year for Developing countries

– China/India: low per capita, but huge populations– Predicted emissions:

• Developed countries: continue slow growth about 1%/year• Developing countries: 4%/yr

– 30% of total now; 50% by 2008; 70% by 2030

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Top 20 CO2 Emitters from Fossil Fuels, 1995

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Setting Targets• IPCC recommends stable target of 550 ppm CO2

– Various ways to reach this target• Learn how to reduce CO2 emission by 2030

• Atmospheric CO2 concentrations continue to rise until 2100

– National CO2 Allocations?• Rio, 1992: reduce to 1990 emissions by 2000 (goal not met)

• Kyoto, 1997: reduce by 5% below 1990 level by 2010– 39 Developed countries signed, US did not join

– Remains to be seen if successful (not likely without US)

• Buying CO2 rights: trading in an international market?– Incentive to do better = $

– Money available to invent new technologies

– Countries over 4 tonnes/person/year must pay fines

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IPCC Projections of CO2 Emission and Concentrations

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Reducing CO2 Emissions• Fuel Switching

– Provide reason for industry to use less emitting fuels

– Carbon Tax (C--->CO2)

– Natural gas better than oil better than coal

– Solar Energy produces H2: no CO2 emitted (only H2O)

• Burying CO2 (will cost energy and money)

– Develop methods to trap CO2 instead of emitting it

– Sequester CO2 (prevent its release to atmosphere)

• CO2 under pressure is more dense than water: under ocean

• 3500 m of water over the CO2 keeps it in a dense “lake”

• CO2 + H2O + CaCO3 ----> Ca(HCO3)2 calcium bicarbonate

• Stays dissolved in water = permanent sink

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Removing CO2 from the Atmosphere• Iron Fertilization

– Plankton need only iron to grow rapidly in oceans

– Plant growth removes CO2, turns it into plant matter

– Dead plankton sink to ocean floor, sequestering CO2

• Plant Forests: as they die, CO2 eventually returned

• Increase Energy Efficiency– Better light bulbs, more efficient cars = less CO2

– Will we just drive farther? No help if we do.

• Doing something about Global Warming– No definitive proof we really need or can do anything– Trying now is like buying insurance for the future