Radiation & Climate
Oct. 11
• Greenhouse Gases
• Aerosols
• Clouds
• Earth Radiation Budgets
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Greenhouse Gases: CO2, and…… CH4, N2O, CFCs
Currently,
1/2 of greenhouse
effect from CO2,
other 1/2 from
everything else
(CFCs are #2 after
CO2)
While CO2 conc. has increased, so have the concentrations
of other greenhouse gases
Methane
N2O
How can that be ?
Radiatively-active Gases, by volume
On a per-molecule basis, CFCs have the strongest greenhouse effect
And in addition destroy O3, changing stratosphere temperature profile
Biologically active ultraviolet radiation reaching
The earth’s surface increasing (as strat.ozone decreases)
sunburn
Plant
damage
DNA damage
UV at TOA
TOA
UV at Surface
with O3 150DU
Surface, 150 DU
Surface, 300 DU
UV at Surface
with O3 300DU
Greenhouse Gases: can ignore solar
Aerosols and Clouds: interact strongly w/ solar
Aerosols: range in size from 10-4 to 10 micron
Aerosols both scatter and absorb solar and infrared radiation
The broad categories:
Sulfate (from ocean). Primarily scatters solar, cooling planet
Both at surface and at top of atmosphere
Black carbon (smoke, soot). Scatters and absorbs solar. Cools
Surface but warms at top of atmosphere.
Organic carbon
Dust
Sea salt (large size, thought to encourage precipitation)
Aerosols come in 3
Distinct size groups:
Nuclei mode
Accumulation mode
Coarse mode
Only the last 2
Become cloud
Nuclei; nuclei
Mode must first
Undergo more
chemistry
Sulfates arguably the most straightforward
SPM 3
Aerosol distribution variable
Greenhouse
forcing
Sulfate
forcing
Clouds.
As a first approximation, infrared emissivity and
Cloud albedo can be parameterized as a function of
Liquid water path.
A further improvement is drop size
Note dependence on LWP (and optical depth) becomes
unimportant for thick clouds
Impact of clouds on climate:
Clouds cover ~ 65% of globe.
Surface temperature, vertical temperature profile sensitive
To cloud amount and cloud height
Low clouds cool, high clouds warm planet. This conclusions
Rests on optically-thick low clouds, optically-thin high cloud
Current interest in high and low cloud trends
Global shortwave cloud forcing ~ -45 W m-2
10X that from CO2 doubling !
The ISCCP Project
ISCCP trends not necessarily consistent w/ other data
Uses of Earth Radiation Budget Data
• measure and understand radiation balance driving climate
Such as, what do clouds and aerosols do, land use changes
• infer energy transports in atmosphere/ocean
• radiative parameterizations for energy balance models,
such as, regress LW flux, albedo against surface temperature, cloudiness
• validate global circulation models
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CERES
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Net measured radiation used
To estimate energy transport
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Next Wedn., Oct. 19, Brian Mapes
Will discuss more cloud influences
On climate
In simple models,
Aerosol,cloud scattering
Often parameterized
By the
Henyey-Greenstein
Phase function