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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 Outgoing > Incoming Positive & Negative Forcing Positive forcing warming Negative forcing cooling Forcing and Feedbacks Radiative forcing (external) Internal response (including feedbacks) Climate system 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 More Anthropogenic increases in greenhouse gases are considered forcings Increases in greenhouse gases that are caused by temperature changes are feedbacks 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 Comparing Causes of Temperature Change Assumption: Larger radiative forcing larger effect on temperature Comparisons follow Source: Intergovernmental Panel on Climate Change (IPCC) Positive Radiative Forcings Largest – by far: increased greenhouse gases Increase is almost entirely anthropogenic Long-Lived Greenhouse Gases Gas Forcing (Wm-2) CO2 1.66 CH4(methane) 0.48 N2O (nitrous oxide) 0.16 Halocarbons 0.34 Total 2.64 More About Greenhouse Gases Radiative transfer model Adding greenhouse gas reduces outgoing longwave radiation (OLR) at top of atmosphere Initial Equilibrium Top of atmosphere Absorbed Shortwave OLR Now, add greenhouse gas Keep temperatures fixed Reduced Upward Flux Top of atmosphere Absorbed Shortwave OLR Net Downward Flux Top of atmosphere Net Flux Result: A positive radiative forcing Negative Radiative Forcings Largest: Increase in sulfate aerosols Mostly anthropogenic 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 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 Evidence for Indirect Effect Bright streaks are areas of enhanced albedo Cause: Emissions from ships Streaks called “ship tracks” Cause of Ship Tracks • Ship exhaust contains aerosols • The aerosols cause more droplets to form • Cloud albedo is increased Total Anthropogenic Effect on Climate Total Anthropogenic Climate Forcing = sum of all anthropogenic forcings Mainly, greenhouse gases (+) + sulfate aerosols (-) Net Anthropogenic Radiative Forcing (1750 – 2005) Best Estimate:1.6 Positive. 2 W/m Solar Irradiance Some evidence suggests solar irradiance may have increased lately Current estimate of forcing: very small Note: Evidence is very weak! Engineering Solution to Global Warming? Sulfate aerosols cool the Earth Why not deliberately increase sulfur emissions? Answer: Sulfur emissions contribute to acid rain Damage to forests Erosion of statues Words on gravestone obliterated Effect on Aquatic Life http://www.elmhurst.edu/~chm/vchembook /195lakeeffects.html Canadian lakes have acidified by U. S. pollution Reducing acid rain Sulfur emissions have been reduced Result: Less acid rain 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/199 9/07/990708075951.htm