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Radiation’s Role in Anthropogenic Climate Change
AOS 340
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Mean Planetary Energy Budget (W/m2)
Trenberth et al. BAMS 2009Lecture 1
Incoming energy must balance outgoing energy
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Carbon Dioxide in the Atmosphere
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Carbon Dioxide
• The current concentration of CO2 is about 385 ppm (parts per million), as compared to the pre-industrial concentration of about 285 ppm.
• This is an increase of about 30%
• This increase is primarily due to the burning of fossil fuels (also some loss of biomass)
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Antarctic Ice Core Data
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1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Year
CO
2 C
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trat
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, pp
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Mauna Loa Data
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6 IPCC Report
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Data analyzed by Hadley Research Centre, United Kingdom
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Ground
Atmosphere Absorbers are: 1) Gases -- CO2, water vapor, and ozone 2) Clouds 3) Aerosols
Thermal radiation from the atmosphere
Thermal energy lost to space; must balance absorbed solar energy
Solar energy in
Greenhouse effect
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Increasing CO2 Decreases Transparency in IR
Increasing CO2 decreases the transparency of the atmosphere,
And lower transparency implies higher optical depth,
Which implies more downwelling infrared radiation,
Which implies more heating at the surface
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Climate Feedback
• Adding CO2 to the atmosphere warms the planet
• That warming alters other parts of the climate system
• Those changes can further enhance the warming or decrease the amount of warming
Feedback
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Water Vapor Feedback
Greenhouse effectMore CO2
in theatmosphere
Warmer surface temperature and atmosphere
Water vapor physics
More H2O in the
atmosphere
Greenhouse effectMuch warmer surface temperature and atmosphere
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Increasing Water Vapor Decreases Atmospheric Transparency…
1 cm PWV
4 cm PWV
And lower transparency implies higher optical depth,
Which implies more downwelling infrared radiation,
Which implies more heating at the surface
This is a “positive feedback” as it amplifies the original perturbations influence; positive feedback increases the warming
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What are some other Feedbacks ?
• Ice – albedo feedback– Warmer surface conditions melt the ice, which
decreases the surface albedo (e.g., in the arctic), which leads to more shortwave absorption by the surface, and hence more warming
– This is a positive feedback (enhances warming)
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What are some other Feedbacks ?
• Cloud feedbacks– More atmospheric water vapor may lead to more
cloud formation. There are multiple different possibilities:
• 1) More low altitude cloud. – These clouds are typically composed of liquid water
droplets with small particle sizes; more clouds like this lead to more scattered solar radiation (back to space) and little impact on the longwave radiation.
– This is a negative feedback (cooling influence)
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What are some other Feedbacks ?
• Cloud feedbacks– More atmospheric water vapor may lead to more
cloud formation. There are multiple different possibilities:
• 2) More high altitude cloud. – These clouds are typically composed of ice crystals
with large particle sizes; more clouds like this lead to more infrared absorption, which results in less IR emission to space and increased IR radiation at the ground. There is little impact on the solar radiation.
– This is a positive feedback (warming influence)
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The wild card: Aerosols
• CO2 emissions are (almost always) accompanied by emissions of aerosols (particulate matter, and ‘precursor gases’ that react in the atmosphere to form these particles)
• Aerosols directly influence radiation– Aerosols are typically small and reflect solar
radiation back to space while having little impact on infrared radiation
– Negative feedback (cooling influence)
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The wild card: Aerosols
• Aerosols have a large “indirect effect” on the radiative balance by affecting the cloud properties. There are at least two different mechanisms for this:
• 1) Cloud albedo affect:– More aerosol particles implies more cloud
condensation nuclei, which leads to many more smaller cloud droplets, which leads to enhanced solar scattering. There is only a small IR impact.
– Negative feedback (cooling influence)
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The wild card: Aerosols
• 2) Cloud lifetime affect:– More aerosol particles implies more cloud
condensation nuclei, which leads to many more smaller cloud droplets, which leads a decrease in precipitation and hence a longer-lived cloud.
– Sign of feedback is unknown, as we don’t know if low-altitude clouds or high-altitude clouds will be affected more
– Certainly has impacts on the precipitation timing, intensity, and location (which we’ve ignored up to now…)
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NOW
Projected CO2 Concentrations
Projecting future CO2 emissions is a projection of human behavior (strong national component).
While this is often discussed, the projections of aerosol emissions is hardly discussed at all!
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Anthropogenic Climate Change• Is REAL -- we will continue to add CO2 to the
atmosphere and the climate will warm.• Is LONG TERM -- a problem for decades, not years =>
OUR CHILDREN’S PROBLEM!• Has IMPLICATIONS FOR SOCIETY -- global warming will
impact water resources, agriculture, energy usage, severe weather damage, sea level, etc., on a regional basis.
• We (the US) are the largest part of the problem but have NO COHERENT STRATEGY to address it
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The Global Greenhouse Problem• PRESENTS DIFFICULT ETHICAL AND MORAL
CHOICES -- in any plausible forecast of the future, there will be losers; there may be some winners.
• The biggest losers will most likely not be those who are most responsible for the change in climate.– Who pays for their losses?– With what currency?– On what time scale?