Geoscience Reference
In-Depth Information
temperate forests in Europe, and currently of tropical
forest in South America and East Asia, are believed to have
the potential to modify climate. Some model estimates
predict that a change of Brazilian rain forest to savannah
would lead to a decrease of evapotranspiration of up to
40 per cent, an increase of run-off from 14 per cent of
rainfall to 43 per cent, and an average increase in soil
temperature from 27
C to 32
C, but the precise figures
depend upon the assumptions embodied in the models.
Degradation of vegetation has also been given as a factor
in influencing the climatic change in the Sahel.
Another surface change which would have clearer
effects is when snow or ice melts. If a surface is de-
glacierized, its albedo will decrease from relatively high
values to much lower ones; as a result more solar energy
will be available to warm the surface. With snow or ice on
the ground much energy will be reflected or consumed in
melting or ablating. The recent break-up of some of the
Antarctic ice shelves such as Larsen B will produce a
dramatic decrease in albedo. This energy will become
available for heating when the surface changes. All these
factors potentially produce a marked increase of tempera-
ture at the surface. Conversely, a change to snow and ice
would trigger a positive feedback to enhance cooling. It
has been suggested that the regional cooling over Europe
between 12,800 and 11,600 years
BP
may have resulted
from a breakdown of the North Atlantic Gulf Stream due
to a sudden massive surge of fresh pro-glacial meltwater
from the St Lawrence and the break-up of sea ice from
northern Canada.
It is well known that atmospheric composition can and
does change through time, though precise levels of
measurement may not be available. One major influence
is volcanic activity. When a volcano erupts it may expel
vast quantities of dust and gases such as carbon dioxide
and sulphur dioxide into the atmosphere. How significant
the eruption is for climatic conditions depends particu-
larly on how much material is ejected into the stratosphere
and how long it is able to survive there as well as the
latitude of the eruption. If dust and sulphate particles can
survive in the stratosphere they are able to reduce by
reflection the amount of solar radiation reaching the
ground. The longer they survive the greater will be
the impact of the eruption. Major eruptions can result in
surface cooling of about 0ยท2
C for a few years after the
event. The eruption of Mount Pinatubo in the Philippines
in 1991 led to a brief reduction in the recent trend
of increasing global mean temperatures (
Figure 9.6
).
Although large amounts of smoke were released from the
Kuwaiti oilfield fires in the First Gulf War, none of it was
able to penetrate into the stratosphere and its climatic
1.5
TI
TII
TV
TIX
1.0
TVII
0.5
0.0
-0.5
-1.0
-1.5
0
100
200
300
400
500
600
700
800
Age (kyr)
Figure 9.13
Sedimentary evidence for the sudden termina-
tion of glaciations. Data show variations in
18
O ratio from
Source: After Muller and Macdonald (2000)
the orbital variations could have been responsible for
such sharp climatic fluctuations as described here. In
some cases this sudden termination preceded the orbit-
induced warming that was supposed to cause it. For such
changes we must look to other mechanisms.
Internal forcing
Internal mechanisms which can also generate climate
change are illustrated in
Figure 9.9
.
Some are likely to
operate only very slowly. For example, changes in Earth's
orography take millions of years to become significant in
terms of their effects on the atmosphere, though they can
be extremely important. It is interesting to speculate what
the climate of the northern hemisphere would be like
without the western cordillera of the United States or the
Tibetan plateau. The westerly circulation would certainly
be different, probably with less meridional exchange and
a different pattern of precipitation as areas favouring
convergence and divergence changed. The ideas can be
tested by using climate models with different surface
topography, though the validity of the conclusions
would depend upon the reliability of the model. Similarly,
ocean basin shape may vary, as it did to some extent
when sea level dropped markedly during glacial phases.
Shape changes may affect ocean current patterns and -
something which has been appreciated only recently -
they may affect salinity levels and interactions between
surface and deep-sea waters (
Figure 11.9
).
Changes in surface features may have drastic effects on
climate over rather shorter time periods than orography
and ocean shape. Deforestation is a clear case of a change
in surface properties which, by changing surface albedo
and heat budget, could affect climate. Clearance of
