GHG Water vapor

converted to heat

the
Atmosphere emit
Infrared radiation
Back towards earth

gases:
Ne, He, Kr, H, O3
Methane, Nitrous Oxide

atmosphere
WAVELENGTH (micrometers)
Infrared
Visible
UV

to space.

Carbon dioxide, methane, and nitrous oxide are very good

at capturing energy at wavelengths that other compounds miss

a molecule must:
- absorb light in the infrared region (must

have dipole moment for vibration mode)
- 3 modes of vibration for CO2 shown

O=C=O

O=C=O

O=C=O

Symmetric vibration not allowed

roughly 7 to 25 μm region
- however, in some regions

(5 to 7 and 13 to 17 μm), essential no light from surface makes it to space due to current gases present
- for this reason, CO2 is less effective as a greenhouse gas (at least for additional CO2)

regions are more effective
Additional CO2 is not as effective as

additional N2O (absorbs at 7.5 to 9 μm) on a forcing per ppm basis

From Girard (old text)

Anthropogenic increase: 30%
Average atmospheric residence time: 200 years
Methane (CH4)
Source:

Rice cultivation, cattle & sheep ranching, decay from landfills, mining
Anthropogenic increase: 145%
Average atmospheric residence time: 7-10 years
Nitrous oxide (N2O)
Source: Industry and agriculture (fertilizers)
Anthropogenic increase: 15%
Average atmospheric residence time: 140-190 years

are not the same thing.
Global warming refers to a

rise in the temperature of the surface of the earth

An increase in the concentration of greenhouse gases leads to an increase in the the magnitude of the greenhouse effect. (Called enhanced greenhouse effect)
This results in global warming

The northern latitudes have an energy deficit and the low

latitude/ equator has an excess. But the low latitudes don’t indefinitely get hotter and the northern latitudes don’t get colder.

Why?

The atmosphere and ocean transfer energy from low
latitudes to high

not follow longitude lines (Coriolis effect)
Speed of rotation highest at

equator
Winds travelling polewards get a bigger and bigger westerly speed (jet streams)
Air becomes unstable
Waves develop in the westerly flow (low pressure systems over Northern Europe)
Mixes warm tropical air with cold polar air
Net transport of heat polewards

close to the surface That energy drives climate and weather
50 percent

of mass of the atmosphere is within 6 km of the surface

trap heat (warming)
High clouds
more clouds trap heat (warming)

high: 5-14 km;

low < 2km

responsible for the most greenhouse effect heating
- the third most

prevalent molecule in the atmosphere (on average, but composition is variable)
- direct anthropogenic sources are insignificant (at least outside of deserts and the stratosphere)
- also responsible for cooling through increasing albedo (in clouds) so normally kept separate from other greenhouse gases
- water vapor is important indirectly as planet heating increases water vapor (this is covered under feedbacks)

think that atmospheric warming is not due to an increase

in energy output from the sun
Since 1975
Troposphere has warmed
Stratosphere has cooled

Warmer temperatures create more clouds
Thick, low altitude cumulus clouds – decrease surface temperature
Thin, cirrus clouds at high altitudes – increase surface temperature

the climate system. A greenhouse gas, like carbon dioxide, it

represents around 80 percent of total greenhouse gas mass in the atmosphere and 90 percent of greenhouse gas volume.
Water vapor and clouds account for 66 to 85 percent of the greenhouse effect, compared to a range of 9 to 26 percent for CO2. So why all the attention on carbon dioxide and its ilk? Is water vapor the real culprit causing global warming?
The answer is that water vapor is indeed responsible for a major portion of Earth’s warming over the past century and for projected future warming. However, water vapor is not the cause of this warming. This is a critical, if subtle, distinction between the role of greenhouse gases as either forcings or feedbacks. In this case, anthropogenic emissions of CO2, methane, and other gases are warming the Earth. This rising average temperature increases evaporation rates and atmospheric water vapor concentrations. Those, in turn, result in additional warming.