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 Efficiency
Gas
Relative
greenhouse
efficiency per
molecule
Concentration in Relative
the atmosphere efficiency x
(ppm)
concentration
(ppm)
Carbon Dioxide
(CO2)
1
350
350
Methane (CH4)
30
1.7
51
Nitrous Oxide
(N2O)
160
0.31
49.6
Ozone (O3)
2000
0.06
120
Chlorofluoro
carbons (CFCs)
21,000
0.00026
5.46
Anthropogenic Greenhouse Gas
Sources
Methane (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”)