The Greenhouse Effect

A quick review

The Greenhouse Effect

• A natural process that keeps the surface of the earth at a habitable temperature.– Without the greenhouse effect, surface

temperatures would be about 60º F colder. – There would be NO liquid water!

How does it work?

Image source: kids.earth.NASA.gov

How does it work?• Short wavelength (ultraviolet) radiation from

Sun reaches Earth• Earth warms and emits long wavelength

(infrared) radiation back towards space• Greenhouse gases in Earth’s atmosphere “trap”

some of the infrared radiation near Earth– Think of your car on a sunny day: the sunlight enters

through the closed windows and warms the car interior. That heat can’t escape back out through the glass.

What are the main Greenhouse Gases?

Water vapor (H2O)Carbon dioxide (CO2)

Methane (CH4)Nitrous oxide (N2O)

These were all present in atmosphere before humans, but we are adding more

Greenhouse Gases allow ~75% of short wavelength energy from Sun to reach Earth.

Only 15-30% of the energy re-radiated by Earth escapes back through greenhouse gases in the atmosphere.

Image created by Robert A. Rohde / Global Warming Art

http://www.globalwarmingart.com/wiki/File:Atmospheric_Transmission_png

Greenhouse Gas EfficiencyGas Relative

greenhouse efficiency per molecule

Concentration in the atmosphere (ppm)

Relative efficiency x concentration (ppm)

Carbon Dioxide (CO2)

1 350 350

Methane (CH4) 30 1.7 51Nitrous Oxide (N2O)

160 0.31 49.6

Ozone (O3) 2000 0.06 120Chlorofluorocarbons (CFCs)

21,000 0.00026 5.46

Anthropogenic Greenhouse Gas SourcesMethane (CH4)

– Enteric fermentation (cow burps!)

– Landfill emissions– Rice cultivation– Natural gas use– Melting of permafrost in

Arctic

Nitrous Oxide (N2O)– Agricultural processes

(fertilizers)

Buuurp!

Anthropogenic Greenhouse Gas Sources

What about CFCs?Very powerful greenhouse gas, but people have been very successful at reducing emissions (Montreal Protocol)

Effect on Upper Atmosphere

• The Greenhouse Effect causes warming at Earth’s surface, but cooling in upper atmosphere– If more of Earth’s emitted longwave radiation

is held close to the surface, it is not available to cause warming in the upper atmosphere

– More noctilucent cloud formation in the Mesosphere? (see “Noctilucent Clouds.ppt”)