438 GREENHOUSE GASES EFFECTS
development will require higher spatial resolution, especially
in the oceans, better model physics, much faster computers
and, above, all, an adequate supply of global observations
from both the atmosphere and the oceans, to feed and vali-
date the models, and to monitor the actual changes in climate
that may eventually become evident.
The need for observations from both the surface and the
interior of the oceans, and how they might be provided by new
and advanced technology, are discussed by Mason (1993).
Despite these uncertainties and the fact that a doubling
of CO 2 will cause an increase of only 3% in the downward
flux of infra-red radiation from the greenhouse gases, future
predictions of the resulting globally-averaged tempera-
ture rise are unlikely to lie outside the range 1C to 2.5C.
However, the models provide little guidance as to when
these events are likely to occur. Their timing will be deter-
mined largely by the very uncertain future global emissions
of greenhouse gases and their retention in the atmosphere.
We must also realise that no existing climate model incorpo-
rates the carbon cycle in which exchanges of CO 2 between
the earth’s surface and the atmosphere are dominated by ter-
restrial and especially marine biology, man-made emissions
being only about 3% of the natural two-way exchanges. We
are always faced with having to compute small differences
between large quantities whose magnitudes are uncertain.
Given this and the complexity of the models, it is remark-
able that they simulate the climate and its variability as well
as they do, but there is a tendency to infer more from model
predictions than their input data, spatial resolution and sim-
plified physics can justify.
The latest version of the UKMO model has 38 pressure
levels in the atmosphere, 20 levels in the ocean, where the
horizontal grid spacing is reduced to 1.25 × 1.25, obvi-
ating the need for artificial flux corrections at the ocean/
atmosphere interface. Representations of the radiative
effects of clouds, of atmospheric convection and of the drag
exerted by mountain-induced gravity waves have all been
improved. The model now remains stable when run for 1,000
years and shows no long-term drift in the global climate.
Changes are calculated at about one million grid points sothat computation of one annual cycle of the global climate
involves about 10^15 numerical operations.
When estimates of the radiative effects of greenhouse
gases and aerosols prevailing in the years 1860 to 2000 were
inserted in the model the predicted increase in the mean
global near-surface air temperature T s was 0.7C in very
good agreement with observation. When the model run was
continued to Y2100, during which time CO 2 emissions were
assumed to increase from the present value of 6.3 GtC/yr
to 13.3 GtC/yr and the atmospheric concentration to almost
double from 365 to 620 ppm, T s was predicted to rise by
3.0C relative to 1860, or 2.3C relative to Y2000. The cor-
responding rise in global sea-level was 34 cm.
If the CO 2 concentration was assumed to increase at 1%
pa to double after 70 years to 730 ppm, the predicted rise in
T s was 1.9C. Similar experiments elsewhere with compa-
rable models gave values between 1.1 and 3.1C with an
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Imperial CollegeHAZARDOUS GASES: see PREVENTION OF TOXIC CHEMICAL RELEASE
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