Geography Reference
In-Depth Information
During the same period, the Southern
Hemisphere showed no coherent trend but the
1970s were marked by warming, and this tendency
continued through to 1990 in the eastern sector
(0-180° E). Renewed warming is in evidence
throughout the Northern Hemisphere since the
early 1970s.
climatic anomalies. Pronouncements by
geographers have
tended to reiterate the broad scientific opinion of
the time (see Goudie 1990). However, by the end
of 1995, the eight warmest years of the global
record (1860-1995) had all occurred within the
thirteen-year period 1983-95 (see Figure 2.4).
This finally led to the first clear statement of
attribution by the IPCC Scientific Working
Group at its meeting late in 1995 in Madrid. Even
in late 1997—the warmest year yet in the global
instrumental record—there was still some caution
in British government publications about
admitting attribution (see May 1997; Box 2.1).
Attribution
The question then arises as to whether the
general rise in global temperatures may be
directly attributed to the greenhouse effect.
Opinion has changed over the past two decades,
as is evident from the following quotations.
Gribbin (1978) reflected the uncertainty in the
1970s of not knowing whether the climate was
cooling down or warming up, while the British
government Cabinet Office statement was, in
part, a reaction to the unexpectedly long Western
European drought of 1975-6 and the wish to
allay public fears of the impact of even greater
MODELLING CLIMATE CHANGE
Modelling now lies at the centre of enquiries into
global warming. The demand of governments for
climatic forecasts has led to models being used
inter
alia
to simulate the effects of doubling CO2 on
future climate. Each set of assumptions within a
model run produces what has come to be known
as a 'scenario' exemplified by the IS 92a business-
as-usual scenario (see Box 2.3 on p. 32). Since about
1994, the more advanced equilibrium models have
become 'full' in that they comprise not just the
physical atmosphere but the ocean, cryosphere, land
vegetation surface and chemical composition of the
atmosphere. Despite increasing ability to model
complex components, some fundamental problems
remain. For example, in order to achieve linkage
between atmosphere and ocean models, it is
customary to 'spin up' the climate and the ocean
component separately before coupling them. An
example of a latitudinal performance envelope of
nine atmosphere-ocean coupled models is shown
in Figure 2.5 for the months December to February.
It reveals that some models have rather large
'climatic drifts' (poor agreement) in the higher
latitudes, as shown by the wider shaded area. This is
mostly due to problems of sensitivity to radiative
factors in the models.
Unlike the equilibrium models outlined above,
transient models allow for annual adjustments in
greenhouse gas concentrations and could be
Box 2.1
Changing attitudes towards global
warming
Since the early 1960s, we have seen only too clearly,
the shift towards…a slight cooling of the Northern
Hemisphere that we now know signals a return towards
the expanded circumpolar vortex conditions of the Little
Ice Age.
(Gribbin 1978: p. 54)
Meteorological Office scientists take the view that the
variations in weather in recent years are compatible
with established climatic patterns. They see no reason
to conclude from the historical record that especially
large changes are likely in the next few decades.
(Cabinet Office 1980)
The balance of evidence suggests a discernible human
influence on global climate.
(IPCC WGI 1996)
Although we do not have data reaching back many
hundreds of years, by comparing observations of global
mean temperatures with natural variability estimated
from climate models, we find the warming has, over
the past couple of decades, extended beyond the
bounds of our estimates of natural variability.
(May 1997)
