Transcript EnvSciChapt16

Global Warming
Chapter 16
1
th
20
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
2
3
Global Temperature Changes 1901--1998
4
Global Precipitation Changes 1901--1998
5
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 mm)
– Earth must emit energy in order not to keep heating up
• IR = 4.0-50 mm = 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 Effect6
Spectrum of Sunlight (dotted) and Earth’s Emitted light
7
The Greenhouse Effect
8
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
9
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 mm and above 18 mm
– 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 mm and 15 mm strongly
10
The IR absorption spectrum of CO2
11
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 mm very little IR is absorbed
• Only ozone contributes absorption in this “window”
12
The atmospheric IR absorption spectrum
13
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
14
Trends in CO2 concetration 1955-2005
15
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)
16
World CO2 Emissions from Fossil Fuel 1950-1999
17
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
18
Annual net movement of CO2 in the Atmosphere
19
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
20
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
21
Biological Sources of Nitrous Oxide
22
CFC’s and Water Vapor
• CFC’s = chlorofluorocarbons
–
–
–
–
Absorb in 8-13 mm 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
23
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)
24
– Warmer = Wetter, but where will new clouds form?
Aerosols effects on Global Warming
25
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
26
Estimated Contributions to Global Warming
27
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
28
Two scenarios for future CO2 concentrations
29
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
30
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
31
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
32
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
33
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
34
Top 20 CO2 Emitters from Fossil Fuels, 1995
35
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
36
IPCC Projections of CO2 Emission and Concentrations
37
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
38
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
39