Environmental Engineering Reference
In-Depth Information
understood, for example. The former are difficult
to simulate in GCMs, in part because they
develop at the regional level, whereas GCMs are
global in scale. Parameterization provides only a
partial solution (see Chapter 2), and the IPCC
supplement has identified the problem of dealing
with clouds, and other elements of the
atmospheric water budget, as one of the main
limitations to a better understanding of climate
(Houghton
et al.
1992).
Oceans create uncertainty in climate models
mainly as a result of their thermal characteristics.
They have a greater heat capacity than the
atmosphere and a built-in thermal inertia which
slows their rate of response to any change in the
system. Thus, models which involve both
atmosphere and ocean have to include some
concessions to accommodate these different
response rates. In representing the oceans it is
not enough to deal only with surface conditions,
yet incorporating other elements—such as the
deep ocean circulation—is both complex and
costly. As a compromise, many coupled ocean/
atmosphere climate models include only the
upper, well-mixed layer of the oceans. These are
the so-called 'slab' models in which the ocean is
represented by a layer or slab of water about 70
m deep. While of limited utility in dealing with
long-term change, this approach at least allows
the seasonal variation in ocean surface
temperatures to be represented (Gadd 1992). A
more realistic representation of the system would
require coupled, deep-ocean/atmosphere models,
but their development is constrained by the
limited observational data available from the
world's oceans and the high demands that such
models place on computer capacity and costs.
Existing coupled models do provide results
consistent with current knowledge of the
circulation of the oceans, but they are simplified
representations of reality, lacking the detail
required to provide simulations that can be
accepted with a high level of confidence
(Bretherton
et al
. 1990).
GCMs also have difficulty dealing with
feedback mechanisms which act to enhance or
diminish the thermal response to increasing
greenhouse gas levels (Rowntree 1990).
Feedbacks are commonly classified as positive
or negative, but in the earth/atmosphere system
they may be so intimately interwoven that their
ultimate climatological impact might be difficult
to assess. For example, the higher temperatures
associated with an intensified greenhouse effect
would bring about more evaporation from the
earth's surface. Since water vapour is a very
effective greenhouse gas, this would create a
positive feedback to augment the initial rise in
temperature. With time, however, the rising water
vapour would condense, leading to increased
cloudiness. The clouds in turn would reduce the
amount of solar radiation reaching the surface,
and therefore cause a temperature reduction—a
negative feedback—which might moderate the
initial increase. Such complexities are difficult to
unravel in the real world. Their incorporation in
climate models is therefore not easy, but the
importance of atmospheric water vapour
feedback in climate change is well recognized by
researchers (Ramanathan
et al.
1983;
Ramanathan 1988), and at least some of the
mechanisms involved are represented in most
current models.
Feedbacks associated with global warming are
present in all sectors of the earth/atmosphere
system, and some have the potential to cause
major change. The colder northern waters of the
world's oceans, for example, act as an important
sink for CO
2
, but their ability to absorb the gas
decreases as temperatures rise (Bolin 1986). With
global warming expected to be significant in high
latitudes there would be a reduction in the ability
of the oceans to act as a sink. Instead of being
absorbed by the oceans, CO
2
would remain in
the atmosphere, thereby adding to the greenhouse
effect. On land the feedbacks often work through
soil and vegetation. Increased organic decay in
soils at higher temperatures would release
additional greenhouse gases—such as CO
2
and
CH
4
—into the atmosphere, producing a positive
feedback. This may be particularly effective in
higher latitudes where the tundra, currently a sink
for CO
2
, would begin to release the gas into the
atmosphere in response to rising temperatures
