Radiation’s Role in Anthropogenic Climate Change

AOS 340

Mean Planetary Energy Budget (W/m2)

Trenberth et al. BAMS 2009Lecture 1

Incoming energy must balance outgoing energy

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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Mauna Loa Data

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

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

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

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)

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)

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)

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)

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)

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?