Page 60 - Earth's Climate Past and Future

36 PART I • Framework of Climate Science

FIGURE 2-22 Ocean GCMs Ocean
Air-sea interactions
models use 3-D grid boxes that
Water vapor Heat Momentum (winds) represent the shapes of ocean
basins. Exchanges of water, heat,
and momentum between the ocean
and the atmosphere occur at the sea
surface.
Ocean Continental shelf
grid
box

Deep-sea floor

would have slowly melted or grown or remained at con- simulates the resulting changes in vegetation. Then the
stant size under the climate conditions simulated. simulated changes in vegetation are used as input to
A-GCMs can reproduce only short-lived snapshots another GCM experiment that simulates the additional
of the circulation of the atmosphere, and as a result they climatic feedback effects caused by the changes in vege-
cannot simulate the slow evolution of ice sheets over tation (primarily increases or decreases in recycling
long intervals of time. To learn about this longer-term of water vapor and in reflectivity of Earth’s surface).
response, climate scientists create physical models of Another approach embeds a vegetation submodel more
the ice sheets. One simplified type of ice-sheet model directly in the main model.
has two dimensions, one vertical and the other showing
average variations with latitude but omitting any repre- 2-7 Geochemical Models
sentation of longitude. These 2-D ice sheet models
have been used to simulate the growth and decay of ice Geochemical models are used to follow the movements
sheets in the northern hemisphere over tens of thou- of Earth’s materials (called geochemical tracers)
sands of years in response to changes in solar radiation through the climate system. Unlike physical circulation
caused by changes in Earth’s orbit. The models simulate models, most geochemical models do not reproduce the
features such as changes in ice accumulation and melt- physical processes that govern the flow of air and water.
ing with ice elevation, flow within the ice, and depres- Instead, the models trace the sources, rates of transfer,
sion of underlying bedrock by the weight of the ice. and ultimate depositional fate of two major compo-
The 2-D ice sheet models can also be linked to 2-D nents: sediment particles that result from physical
atmospheric circulation models to simulate interactions weathering (wind, water, and ice), and dissolved ions
among the ice sheets, atmosphere, and land surface. produced by chemical weathering (dissolution or
Some ice sheet models are three-dimensional, with the hydrolysis). Movements of tracers can be evaluated if
ice accumulating on a specified land surface (such as they are not created or destroyed by radioactive decay
Antarctica) divided into grid boxes 50 to 100 kilometers along the way. Geochemical models can also trace
on a side. exchanges of biogeochemical materials such as carbon
Vegetation Models Vegetation is an active compo- or oxygen isotopes that cycle back and forth among the
nent in the climate system, and the representation of atmosphere, ocean, ice, and vegetation.
vegetation in climate models has progressed through One-Way Transfer Models The most basic kind
several stages. Early A-GCMs either ignored vegetation of model tracks transfers of material from its source or
entirely or specified a representation of modern vegeta- sources to the ultimate sites of deposition, such as
tion that did not interact with the changes in climate debris eroded from the land and deposited in ocean
simulated by the model. sediments. If the material deposited has distinctive
More recent models incorporate vegetation in an geochemical characteristics, it can be analyzed and its
interactive way. One such modeling approach works in abundance quantified in terms of a flux rate—its rate of
two steps. First, climate data derived as output from a burial in that sedimentary archive (Figure 2–23). For
GCM experiment (changes in temperature and precipi- example, scientists can quantify the rate of influx of ice-
tation) are used as input to a vegetation model that rafted debris to high-latitude polar oceans by extracting