Global Mean Temperature Responses - Climate Stabilization Targets - page 83

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Global Mean Temperature Responses
3.1 OVERVIEW OF TIME SCALES AND CLIMATE SENSITIVITY
The rapid addition of carbon dioxide and other greenhouse gases to the
atmosphere by human activities throws Earth's energy budget substantially
out of balance. For a time, Earth receives more energy from the Sun than it
loses by emission of infrared radiation to space. The climate system rectifies
this imbalance through processes acting over a range of different time scales.
Restoring balance invariably results in a warmer climate, and the amount of
warming associated with a prescribed addition of carbon dioxide is called
the climate sensitivity . One obtains different climate sensitivities over differ-
ent time scales, because additional processes come into play at long time
scales, which are not important over shorter time scales.
In this report, we will deal with three kinds of climate sensitivity. The
first kind characterizes the equilibrium response to changes in CO 2 , includ-
ing relatively fast feedbacks, primarily water vapor, clouds, sea ice, and
snow cover. This equilibrium response assumes that the oceans have had
time to equilibrate with the new value of carbon dioxide and with these fast
feedbacks, an equilibration that is estimated to require multiple centuries
to a millennium. When the term climate sensitivity is used without further
qualifiers, it should be understood as meaning this form of relatively fast-
feedback equilibrium climate sensitivity; it will be discussed in Section 3.2.
The term “equilibrium” in this chapter will always refer to the equilibrium
response incorporating only these feedbacks.
The second form of sensitivity we shall deal with— Transient Climate
Response —characterizes the early stages of warming, when the deep ocean
is still far out of equilibrium with the warming surface waters. Transient
climate response is of great importance because it is appropriate to under-
standing climate variations and impacts of the 20th and 21st centuries. This
form of sensitivity will be discussed in Section 3.3.
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