VII. Climate Change Blackbody model Windows and saturation Feedbacks Aerosols

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VII. Climate Change Blackbody modelWindows and
saturationFeedbacksAerosols
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Blackbody model
Energy In Energy Out Energy In 1368 W/m2 ?
Earth cross-section ? (1-reflectivity) Energy Out
Earth surface Area ? sSB? Tearth4 sSB is
Stefan-Boltzmann constant Tearth 255 K
ignores clouds and greenhouse gases
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Energy Balance beyond Blackbody
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Earths IR Emissions
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Energy Balance beyond Blackbody
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CO2
Concentration increasing, seasonal
variation Absorptions are nearly saturated
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Saturation
Strong CO2 absorptions almost saturated. Window
regions between strong absorbances Activity
model greenhouse gases X and Y a) Consider Y
2.5 x 1013 molecules cm-3 at l1 in IR, sY 1 x
10-19 cm2 molecule-1 What is A(l1), the
absorbance at l1 ? b) Add X 2.5 x 1011
molecules cm-3 at l1 in IR, sX 4 x 10-18 cm2
molecule-1 at l2 in IR, sX 1 x 10-18 cm2
molecule-1 What is the total A(l1) and what
is A(l2) ? c) Does the addition of X reduce heat
emission more at l1 or l2?
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Human Affects on Radiation Budget
Global mean radiative forcing of climatefor year
2000 relative to 1750 (IPCC)

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Greenhouse Gases
See Coursepack Section E Table 3 Seinfeld and
Pandis Figures 21.17-19 Instantaneouse
Radiative Forcing (IRF) of a compound (Watts
m-2 kg-1) Absolute Global Warming Potential (W
m-2 kg-1 yr)
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Greenhouse Gases
Global Warming Potential (w/respect to CO2)
(dimensionless)
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Key Points

• Radiative balance is complicated
• Greenhouse Gas effect real, global
• Greenhouse Gas effects not isolated
• - feedbacks with biosphere
• - feedback with geosphere
• Aerosol effects messy, local (temporary)
• Climatic effects hard to see (weather)