Geoscience Reference
In-Depth Information
forcing. But this kind of analysis can be applied to consider other climate vari-
ables and other forcing factors as well.
In the schematic drawings of
feedback loops
, climate factors are connected
by arrows to indicate the order of the cause and effect. Each arrow is labeled
with a + or a sign, which indicates how the two factors are correlated. A +
sign indicates that the factors are positively correlated, so that an increase in
one leads to an increase in the other, or a decrease in one leads to a decrease
in the other. A negative sign means that the factors are negatively correlated,
so that an increase in the first factor leads to a decrease in the second factor,
or vice versa.
WATER VAPOR-TEMPERATURE FEEDBACK
Figure 11.1a
is a schematic representation of the chain of events in the
water
vapor-temperature feedback
. Start at the top, and read this feedback loop as
follows:
An increase (decrease) in atmospheric CO
2
concentration leads to an increase
(decrease) in
T
*, which leads to an increase (decrease) in evaporation and at-
mospheric water vapor, which leads to an increase (decrease) in longwave back
radiation at the surface, which leads to an increase (decrease) in
T
*.
Recall that the positive and negative signs are not the sign of the change but,
rather, the sign of the correlation.
The water vapor-temperature feedback is a
positive feedback
—the original
T
*
increase (or decrease) is amplified. This is a very powerful feedback process
in the climate system. Estimates using climate models indicate that in the ab-
sence of this feedback, doubling atmospheric CO
2
concentrations would lead
to an increase in
T
* of approximately 1.2 K (similar to our simple calculation
using the surface heat balance equation in the absence of climate feedbacks).
(a) Water vapor/temperature feedback
(b) Ice albedo/temperature feedback
CO
2
+
CO
2
+
T
*
T
*
+
-
+
+
longwave back
radiation
evaporation from
the surface
solar radiation
absorbed
snow/ice on
the surface
+
+
-
+
atmospheric
water vapor
surface
albedo
Figure 11.1 Schematic representations of the (a) water vapor/temperature
and (b) ice albedo/temperature feedbacks.
