VII. Climate Change

Blackbody modelWindows and saturationFeedbacksAerosols

Blackbody model

Energy In = Energy Out

Energy In = 1368 W/m2 Earth cross-section (1-reflectivity)Energy Out = Earth surface Area SB Tearth

4

SB is Stefan-Boltzmann constant

Tearth = 255 K ignores clouds and greenhouse gases

Energy Balance beyond Blackbody

Earth’s IR Emissions

Energy Balance beyond Blackbody

CO2

Concentration increasing, seasonal variationAbsorptions are nearly saturated

Saturation

Strong CO2 absorptions almost saturated.Window regions between strong absorbances:

Activity: model greenhouse gases X and Ya) Consider [Y] = 2.5 x 1013 molecules cm-3

at 1 in IR, Y = 1 x 10-19 cm2 molecule-1 What is A(1), the absorbance at 1 ?

b) Add [X] = 2.5 x 1011 molecules cm-3

at 1 in IR, X = 4 x 10-18 cm2 molecule-1

at 2 in IR, X = 1 x 10-18 cm2 molecule-1

What is the total A(1) and what is A(2) ?

c) Does the addition of X reduce heat emission more at 1 or 2?

Global mean radiative forcing of climatefor year 2000 relative to 1750 (IPCC)

Human Affects on Radiation Budget

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)

t

t

t

dttIRFAGWP

etIRFtIRF

0

/

)(

)0()(

Greenhouse Gases

Global Warming Potential (w/respect to CO2)

(dimensionless)

t

t

tCO

t

t

ti

t

t

CO

t

t

i

dtetIRF

dtetIRF

dttIRF

dttIRF

iGWPCO

i

0

/2

0

/

0

2

0

2)0(

)0(

)(

)(

)(

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)