Solar Radiation

• Not all incoming solar radiation reaches earth’s surface

  • 26% reflected back into space by clouds & atm. 

  • 19% absorbed by atm. & clouds & radiated out into space & down to earth

  • The rest reaches earth’s surface where it can be absorbed or reflected (depending on the albedo of the surface it strikes)

    • Darker, lower albedo surfaces absorb sunlight & release infrared radiation (which we feel as warmth)

    • Lighter, higher albedo surfaces reflect sunlight, directly back out into space, or into clouds/GHGs that absorb it

Greenhouse effect

• Gases in earth’s atmosphere trap heat from the sun & radiate it back down to earth

  • Without it, earth would be too cold

• How it works:

  • Solar radiation (light waves like UV & visible light) strike earth’s surface, heating it

  • Earth’s surface releases infrared radiation

  • GHGs absorb infrared radiation & radidate it both out into space and back towards earth

  • Portion coming back to earth is the “greenhouse effect”

Greenhouse Gases & Sources

• Most important Greenhouse Gases (GHGs) are:

  • CO₂ - FF comb, decompostion, deforestation

  • Methane (CH₄) - natural gas extraction & combustion, animal agriculture, anaerobic decomp. (especially permafrost thaw)

  • Nitrous Oxide (N₂O) - agricultural soils (denitrification of nitrate, especially in overwatered, over fertilized soils)

  • CFCs/HCFCs/HFCs - refrigerants, blowing agents in aerosol products

• *Water vapor (H₂O) - evaporation & transpiration from plants

  • *Technically a GHG by definition, but doesn’t drive atm. Temp change (other way around - temp. Controls atm. H₂O vapor level)

Global Warming Potential (GWP)

• Measure of how much a given molecule of gas can contribute to the warming of the atmosphere over a 100 year period, relative to CO₂

  • Based on 2 factors:

    • 1) Residence time: how long molecules stay in the atmosphere

    • 2) Infrared absorption: how well the gas absorbs & radiates infrared radiation (IR)

  • CO₂ has a GWP of 1 (all other gases are measured in relation to CO₂)

  • Methane (CH₄) remains in atm. Around 12 years, absorbs more IR than CO₂

  • N₂O remains in atm. Around 115 years, absorbs much more IR than CO₂

CFCs remain in atm 50-500 years, absorb much, much, much more IR than CO₂