9.3 - The Greenhouse Effect

Question 1

Solar Radiation

Answer 1

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

Question 2

The Greenhouse Effect

Answer 2

Gases in earth’s atmosphere trap heat from the sun & radiate it back down to earth
- Without greenhouse effect, earth would be too cold to support life

How it works:
- Solar radiation (light waves like UV & visible light) strike earth’s surface, heating it
- Earth’s surface releases infrared radiation
- Greenhouse gases absorb infrared radiation & radiate it both out into space and back toward earth
- Portion coming back to earth is the “greenhouse effect”

Question 3

Greenhouse Gases and Sources

Answer 3

Most important Greenhouse Gases (GHGs) are:
- CO2 - FF comb, decomposition, 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)

Question 4

Global Warming Potential

Answer 4

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 molecule stays 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 yrs, absorbs more IR than CO2
- N2O remains in atm. around 115 yrs, absorbs much more IR than CO2
- CFCs remain in atm 50-500 yrs, absorb much, much, much more IR than CO2