Environmental Engineering Reference
In-Depth Information
Figure 7.13
Changing
global annual
surface
temperature
Source:
After
Schneider and Mass
(1975)
Note:
The lower line
between 1900 and
1990 indicates actual
change, whereas the
upper line indicates
the estimated
temperature if the
enhanced
greenhouse effect is
included. The
temperature
expressed in K is
equivalent to
temperature in
degrees Celsius plus
273.15°C. It is
possible that the
difference has been
caused by the
cooling effects of
increased
atmospheric
turbidity, which have
prevented the full
impact of the
enhanced
greenhouse effect
from being realized
(Webb and Overpeck 1993). The additional flux
of carbon from terrestrial storage might add as
much as 200 billion tonnes of carbon to the
atmosphere in the next century (Smith and
Shugart 1993). In contrast, higher temperatures
and more efficient photosynthesis in low and
middle latitudes would initiate a negative
feedback in which increased vegetation growth
would cause more CO
2
to be recycled and stored
(Webb and Overpeck 1993).
Feedback mechanisms are incorporated in
some form in most GCMs. However, the
number and complexity of the feedbacks
included, varies from model to model, and
current modelling techniques continue to have
difficulty dealing with them. Such constraints in
the existing models must be recognized, and
appropriate allowances made when predictions
of global warming are used.
A basic concern among some researchers is
the concentration on one variable—greenhouse
gas levels—which has allowed the role of other
elements in the system to be ignored. It is well-
known that the earth's climate is not static, but
has varied over the years (see e.g. Lamb 1977).
Some of the variations have been major, such as
