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Feedback Processes
Understanding climate change is complicated by the many
interactions that occur within the climate system. The next image gives
examples of what are known as "feedback" processes in climate. Feedback
effects occur when a change in one climate system parameter changes another,
which, in turn, causes changes in the initial variable. The following are
examples of feedback processes:
The
Stefan-Boltzmann equation links temperature and radiated energy
and shows that if the temperature of an object goes up, the energy
radiated
also goes up. But this results in a loss of thermal energy from the
object, so its temperature decreases. When an initial increase leads to an
eventual decrease, we call the process a negative feedback.
Relative humidity is reasonably constant despite variations in
absolute humidity. That is, an increase in temperature leads to more
evaporation and increase in the absolute amount of water vapor in the air (increase absolute
humidity).
But since the warmer air has a higher saturation vapor pressure (can hold
more water vapor), the relative humidity stays approximately constant. The
increased absolute humidity, however, increases absorption of infrared
radiation by the atmosphere and hence increases the greenhouse effect.
Note that this increased greenhouse effect raises the surface temperature,
which further increases evaporation. This feedback mechanism is not
self-regulating, so we call it a positive feedback.
An increase in temperature decreases surface area of snow and ice,
which decreases the albedo or reflectivity, which increases the amount of
radiation absorbed and further raises the temperature. This also is a
positive feedback mechanism.
As temperature increases, evaporation and absolute humidity both
increase leading to more cloudiness. But the increase in clouds leads to
increased reflection of solar energy and also leads to more trapping of infrared energy from the surface of the
earth. The net effect (which depends on the altitude of the clouds) is
thought to lead to a cooling, which makes this a negative feedback process.
Increased radiation absorbed at high latitudes due to reduced area
of polar ice will raise temperatures more than at low latitudes, which will
tend to weaken frontal zones and the intensity of frontal storms. This
would lead to complicated changes that might individually be positive or
negative feedback mechanisms.
