Climate Change Processes - Climate Dynamics - page 168

But if surface evaporation is allowed to respond to the surface warming, and
more water vapor is loaded into the atmosphere as a result (see Eq. 5.21), the
warming is doubled or tripled.
Why doesn't this feedback continue indefinitely? Could this feedback run
away, loading more and more water into the atmosphere and causing more
warming of the planet? No. All other things being equal, the feedback must
end when the air reaches saturation (see Fig. 2.32 ) and water condenses out
as precipitation. But there are also competing processes that can contain water
vapor levels below saturation, for example, the land surface may dry out or
plants may resist evapotranspiration through stomatal resistance .
ICE ALBEDO-TEMPERATURE FEEDBACK
The ice albedo-temperature feedback is sketched in Figure 11.1b. Surface
warming due to increases in atmospheric CO 2 melts permanent snow and ice,
or delays the onset of the snow season, or hastens the end of winter. The result-
ing darkening of the surface (decrease in surface albedo) permits the absorp-
tion of more solar radiation, which further warms the surface. Conversely,
decreases in T * lead to more snow and ice on the surface, a higher albedo, and
less solar absorption. This is another positive feedback, since it amplifies the
initial temperature change.
Note the minus signs in the ice albedo-temperature feedback schematic.
The first leg of the loop, marked with a minus sign, indicates that T * and sur-
face snow and ice change in opposite directions. An increase in T * leads to a
decrease in surface snow and ice, and a decrease in T * leads to an increase in
surface snow and ice. There is a second minus sign in the relationship between
the surface albedo and the solar radiation absorbed, so the net effect is a posi-
tive feedback.
The ice albedo-temperature feedback introduces latitudinal dependence
into the climate change signal, since this feedback does not operate at low lati-
tudes, except near tropical mountain glaciers. This is one of the two processes
responsible for the polar amplification of the global warming signal. The other
is the vertical stability of the polar atmosphere, especially in winter. In Figure
2.9 we noted the presence of temperature inversions at high latitudes. When
increased greenhouse gas concentrations increase the longwave back radiation
to the surface, the associated warming is concentrated near the surface when
vertical mixing is inhibited by these temperature inversions. The amplification
of the global warming signal due to the ice albedo-temperature feedback tends
to be strongest at lower latitudes, for example, near ice and snow margins, and
larger than the amplification due to the polar inversion.
The ice albedo-temperature feedback also introduces seasonality into the
climate change dynamics. For example, in middle latitudes this feedback am-
plifies the temperature response in the late fall and early spring, shortening the
length of time that snow is on the land.
The ice albedo-temperature feedback changes the amount of solar radiation
absorbed by the earth system by changing the planetary albedo. This modifies
the radiative equilibrium temperature of the earth system.