Radiative Forcing

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Lecture 13

• Radiative Forcing

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Radiative Forcing

• Definition A change in the net radiation at the
  top of the atmosphere due to some external
  factor.

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Net Radiation

• Net radiation Incoming - Outgoing
  
• Positive net radiation
• Incoming gt Outgoing
• Negative net radiation
• Outgoing gt Incoming

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Positive Negative Forcing

• Positive forcing ? warming
• Negative forcing ? cooling

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Forcing and Feedbacks
Radiative forcing (external)
Internal response (including feedbacks)
Climate system
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Forcing and Feedbacks

• Forcing is produced by an external process,
  e.g.
• Changes in solar flux
• Volcanic eruptions
• Human actions
• A feedback is a response to temperature changes
• Example Increased water vapor due to warming

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More

• Anthropogenic increases in greenhouse gases are
  considered forcings
• Increases in greenhouse gases that are caused by
  temperature changes are feedbacks

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• The same gas can be involved in forcings and
  feedbacks, e.g., CO2
• Forcing
• CO2 increase from burning of fossil fuels
• Feedback
• ?temp ? ?decay ? ?CO2

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Comparing Causes of Temperature Change

• Assumption Larger radiative forcing ? larger
  effect on temperature
• Comparisons follow
• Source Intergovernmental Panel on Climate Change
  (IPCC)

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Positive Radiative Forcings

• Largest by far increased greenhouse gases
• Increase is almost entirely anthropogenic

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Long-Lived Greenhouse Gases
1.66
CO2
0.48
CH4(methane)
0.16
N2O (nitrous oxide)
0.34
Halocarbons
2.64
Total
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More About Greenhouse Gases

• Radiative transfer model
• Adding greenhouse gas reduces outgoing longwave
  radiation (OLR) at top of atmosphere

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Initial Equilibrium
Top of atmosphere
OLR
Absorbed Shortwave
Now, add greenhouse gas Keep temperatures fixed
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Reduced Upward Flux
Top of atmosphere
OLR
Absorbed Shortwave
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Net Downward Flux
Top of atmosphere
Net Flux
Result A positive radiative forcing
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Negative Radiative Forcings

• Largest Increase in sulfate aerosols
• Mostly anthropogenic

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Anthropogenic Sulfate Aerosols

• Coal and diesel fuel contain sulfur
• Burning of these fuels produces sulfur dioxide (a
  gas)
• In the atmosphere, this gas is converted into
  particles

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Effect of Anthropogenic Sulfate Aerosols on
Temperature

• Direct effect
• The aerosols themselves reflect sunlight
• This is similar to the effect of volcanic
  aerosols
• Indirect effect
• Sulfate aerosols act as condensation nuclei
• This increases the droplet concentration in
  clouds
• Result Increased cloud albedo
• Both effects tend to increase the Earths albedo

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Evidence for Indirect Effect
Bright streaks are areas of enhanced
albedo Cause Emissions from ships Streaks called
ship tracks
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Cause of Ship Tracks

• Ship exhaust contains aerosols
• The aerosols cause more droplets to form
• Cloud albedo is increased

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Total Anthropogenic Effect on Climate

• Total Anthropogenic Climate Forcing
• sum of all anthropogenic forcings
• Mainly, greenhouse gases ()
• sulfate aerosols (-)

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Net Anthropogenic Radiative Forcing (1750 2005)

• Best Estimate1.6 W/m2
• Positive.

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Solar Irradiance

• Some evidence suggests solar irradiance may have
  increased lately
• Current estimate of forcing very small
• Note Evidence is very weak!

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Engineering Solution to Global Warming?

• Sulfate aerosols cool the Earth
• Why not deliberately increase sulfur emissions?
• Answer Sulfur emissions contribute to acid rain

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Damage to forests
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Erosion of statues
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Words on gravestone obliterated
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Effect on Aquatic Life

• http//www.elmhurst.edu/chm/vchembook/195lakeeffe
  cts.html
• Canadian lakes have acidified by U. S. pollution

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Reducing acid rain

• Sulfur emissions have been reduced
• Result Less acid rain

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Acid Rain vs. Global Warming

• Reducing sulfur emissions reduces acidity
• Reducing sulfur emissions reduces cooling effect
  of sulfate aerosols
• Result Accelerated warming
• http//www.sciencedaily.com/releases/1999/07/99070
  8075951.htm