Geoscience Reference
In-Depth Information
While climate feedbacks can be either positive
or negative, they can also be broadly differentiated
regarding how quickly they operate. In the frame-
work of global radiative forcing appropriate to
understanding human-induced global climate
change, it is the fast feedbacks which are relevant.
The most important are changes in water vapor
and albedo (mentioned above). Both can operate
over timescales of days and even less. Cloud cover
can also change very quickly (hours). Examples
of slow feedbacks are changes in the extent of
continental ice sheets (influencing planetary
albedo) and greenhouse gases during the Pleisto-
cene in response to Milankovich periodicities.
Records from ice cores show that these glacial-
interglacial cycles were nearly coincident with
fluctuations in both atmospheric carbon dioxide
(±50ppm) and methane (±150ppb). The nature of
these trace gas feedbacks remains incompletely
resolved. Potential mechanisms include changes
in ocean chemistry, increased plankton growth
acting to sequester carbon dioxide, suppression of
air-sea gas exchange by sea ice, changes in ocean
temperature that affect the solubility of carbon
dioxide, and altered ocean circulation. Most
likely a suite of processes worked in concert.
Negative (positive) excursions in greenhouse gas
concentrations are associated with cold (warm)
intervals, as illustrated in Figure 2.6 .
atmosphere radiation imbalance) of about
4W m -2 . In response to this doubling the surface
and atmosphere would warm up. Eventually,
radiative balance would be restored again with a
new and higher surface temperature. Estimates of
equilibrium climate sensitivity obtained from the
current generation of global climate models range
from 2-4.5 o C, with a best estimate of 3.0 o C. The
uncertainly lies largely in the spread of model
estimates of the climate feedbacks, particularly in
the cloud feedbacks. Cloud feedbacks are complex
and hard to model. Negative feedbacks may
operate when increased global heating leads to
greater evaporation and greater amounts of high-
altitude cloud cover, which reflect more incoming
solar radiation. However, other types of clouds,
and clouds in the polar regions, can induce surface
warming
Expressed in a more convenient fashion, the
best estimate of 3 o C for carbon dioxide doubling
equates to 0.75 o C global mean surface tempera-
ture increase per W m -2 of forcing. It is stressed
that the climate simulations used to obtain these
sensitivity numbers only deal with the fast
feedbacks. If there were no feedbacks present in
the climate system, the climate sensitivity would
be only about 0.30 o C per W m -2 . While equilib-
rium climate sensitivity in the IPCC framework is
based on a doubling of atmospheric equivalent
carbon dioxide, it appears that the equilibrium
temperature response to any radiative forcing is
roughly the same. This is an important concept,
since it means that to a first approximation, one
can linearly add different forcings to obtain a net
value from which an equilibrium temperature
change can be estimated.
It also appears that most of the equilibrium
temperature response to a radiative forcing with
the fast feedbacks at work occurs over a time span
of 30 to 50 years. Most of the time lag is due to
the large thermal inertia of the oceans. The basic
issue is that the oceans can absorb and store a
great deal of heat without a large rise in the surface
(radiating) temperature. Consider what is hap-
pening in response to the current radiative forcing
from human activities of 1.6Wm -2 . Using the
3 Climate response
How much does the global mean surface tempera-
ture change in response to a radiative forcing of
a given magnitude? How long does it take for
the change to occur? These are among the most
important, pressing questions in climate change
science.
The first question deals with the issue of
equilibrium climate sensitivity. In the IPCC
framework, equilibrium climate sensitivity is
the equilibrium change in annual mean global
averaged surface air temperature following a
doubling of the atmospheric equivalent carbon
dioxide. Doubling the carbon dioxide concentra-
tion equates to a radiative forcing (top of
 
 
Search WWH ::




Custom Search