9. Feedbacks: water vapor, ice and snow, clouds (Climate change)
Description
This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
9. Feedbacks: water vapor, ice and snow, clouds (Climate change)
If nothing except surface and air temperature changed (and if human-
made aerosols vanished), then a CO2 doubling would eventually warm
earth's surface by 1 to 1.2 o C [Hartmann, p 231-2] [IPCC 95, p 30].
However, there are feedbacks, including though not confined to:
water vapor feedback probably positive ice-snow-albedo feedback presumably positive cloud feedback poorly understood biological feedbacks see next section
It is widely assumed that warming, which tends to enhance evaporation,
will increase the water vapor content of the troposphere. This should
amplify the warming, as water vapor is the dominant greenhouse gas
[Hartmann, p 232-4] [IPCC 95, p 197, 200-1, 210]. [Lindzen] proposed
that, in warmer tropics, deep convective clouds might rain out more
thoroughly. This might dry the tropical upper troposphere and curb the
tropical water vapor feedback. The available data on spatial patterns
and short-term changes of upper-tropospheric humidity do not support
Lindzen's notion. However, spatial and short-term variations need not
be reliable surrogates for global climate change. The same data sug-
gest that some part of the feedback formerly ascribed to water vapor
may instead stem from lapse rate changes, the effects of which were
outlined in section 4 [Sinha] [Soden].
Snow and ice reflect much of the incident sunlight back to space,
thus a reduction of snow and ice cover is likely to enhance warming.
Details remain hazy. Feedbacks between cloud cover and changes in
sea ice and snow cover are poorly understood. Another hurdle is
the interplay between atmosphere, surface ocean, and sea ice, in
particular at the always present ice-free patches and near sea ice
margins [IPCC 95, p 156-7, 204, 214, 216, 267, 347].
The cloud feedback may be large, yet not even its sign is known.
Low clouds tend to cool, high clouds tend to warm. High clouds tend
to have lower albedo and reflect less sunlight back to space than
low clouds. Clouds are generally good absorbers of infrared, but
high clouds have colder tops than low clouds, so they emit less
infrared spacewards. To further complicate matters, cloud properties
may change with a changing climate, and human-made aerosols may
confound the effect of greenhouse gas forcing on clouds. With fixed
clouds and sea ice, models would all report climate sensitivities
between 2 and 3 o C for a CO2 doubling. Depending on whether and
how cloud cover changes, the cloud feedback could almost halve or
almost double the warming [Hartmann, p 68, 71-5, 249] [IPCC 94,
p 150-4, 183-5] [IPCC 95, p 34-5, 201-10, 345-6] [Wielicki].
A recent intercomparison of 15 climate models showed mostly small
to modest negative or positive cloud feedbacks. Sadly, the validity
of this result is doubtful [IPCC 95, p 205-6].
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