Geoscience Reference
In-Depth Information
Solar radiation
forcing
10-100,000+Yrs
Stratosphere 1 yr
Troposphere 1 week
Atmospheric boundary layer
1 day
Atmosphere subsystem
Upper mixed
layer 2-7 months
Sea ice
days - 100s yrs
Ice and snow
surface 1 day
Lakes, rivers, soil,
vegetation 11 days
Mountain glaciers
300 yrs
Lower mixed
layer 7 yrs
Ice sheets
3000 yrs
Deep ocean 300 yrs
Ocean subsystem
Earth subsystem
Earth/atmosphere/ocean subsystem
Techtonic and geothermal forcing 10-100,000 yrs
Figure 8.4
The earth-atmosphere-ocean system showing estimated equilibrium times, together with the wide
time variations involving the external solar, tectonic, geothermal and anthropogenic forcing mechanisms.
Source: After Saltzman (1983).
volcanic dust layer), surface boundary conditions
(orography, land surface albedo, continental ice
sheets) or in the model physics (modifying the
convective scheme or the treatment of biosphere
exchanges). In these simulations, the model is
allowed to reach a new equilibrium and the result
is compared with a control experiment. A second
approach is to conduct a genuine climate change
experiment where, for example, the climate is
allowed to evolve as atmospheric trace gas
concentrations are increased at a specified annual
rate (a transient experiment).
A key issue in assessments of greenhouse-gas-
induced warming is the sensitivity of global climate
to CO
2
doubling which is projected to occur in the
mid-twenty-first century extrapolating current
trends. Atmospheric GCM simulations for equilib-
rium condition changes, with a simple ocean
treatment, indicate an increase in global mean
surface air temperature of 2.5 to 5°C, comparing
1
CO
2
concentrations in the models.
The range is in part the result of a dependence of
the temperature change on the temperature level
simulated for the base state 1
×
CO
2
and 2
×
CO
2
, and in part
arises from the variations in the strength of
feedback mechanisms incorporated in the models,
particularly atmospheric water vapor, clouds, snow
cover and sea ice. Use of coupled atmosphere-
×
