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Lecture 13

Lecture 13. Radiative Forcing. Radiative Forcing. Definition: A change in the net radiation at the top of the atmosphere due to some external factor. Net Radiation. Net radiation = Incoming - Outgoing Positive net radiation Incoming > Outgoing Negative net radiation

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Lecture 13

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  1. Lecture 13 Radiative Forcing

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

  3. Net Radiation • Net radiation = Incoming - Outgoing • Positive net radiation • Incoming > Outgoing • Negative net radiation • Outgoing > Incoming

  4. Positive & Negative Forcing • Positive forcing  warming • Negative forcing  cooling

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

  6. 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

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

  8. 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

  9. Comparing Causes of Temperature Change • Assumption: Larger radiative forcing  larger effect on temperature • Comparisons follow Source: Intergovernmental Panel on Climate Change (IPCC)

  10. Positive Radiative Forcings • Largest – by far: increased greenhouse gases • Increase is almost entirely anthropogenic

  11. Gas Forcing (Wm-2) Long-Lived Greenhouse Gases CO2 1.66 0.48 CH4(methane) N2O (nitrous oxide) 0.16 Halocarbons 0.34 Total 2.64

  12. More About Greenhouse Gases • Radiative transfer model • Adding greenhouse gas reduces outgoing longwave radiation (OLR) at top of atmosphere

  13. Initial Equilibrium Top of atmosphere OLR Absorbed Shortwave Now, add greenhouse gas Keep temperatures fixed

  14. Reduced Upward Flux Top of atmosphere OLR Absorbed Shortwave

  15. Net Downward Flux Top of atmosphere Net Flux Result: A positive radiative forcing

  16. Negative Radiative Forcings • Largest: Increase in sulfate aerosols • Mostly anthropogenic

  17. 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

  18. 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 Earth’s albedo

  19. Evidence for Indirect Effect Bright streaks are areas of enhanced albedo Cause: Emissions from ships Streaks called “ship tracks”

  20. Cause of Ship Tracks • Ship exhaust contains aerosols • The aerosols cause more droplets to form • Cloud albedo is increased

  21. Total Anthropogenic Effect on Climate • Total Anthropogenic Climate Forcing = sum of all anthropogenic forcings • Mainly, greenhouse gases (+) + sulfate aerosols (-)

  22. Net Anthropogenic Radiative Forcing (1750 – 2005) Best Estimate:1.6 W/m2 Positive.

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

  24. Engineering Solution to Global Warming? • Sulfate aerosols cool the Earth • Why not deliberately increase sulfur emissions? • Answer: Sulfur emissions contribute to acid rain

  25. Damage to forests

  26. Erosion of statues

  27. Words on gravestone obliterated

  28. Effect on Aquatic Life • http://www.elmhurst.edu/~chm/vchembook/195lakeeffects.html • Canadian lakes have acidified by U. S. pollution

  29. Reducing acid rain • Sulfur emissions have been reduced • Result: Less acid rain

  30. 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/1999/07/990708075951.htm

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