Encyclopedia of Environmental Science and Engineering, Volume I and II

1840 1860 1880 1900 1920 1940 1960 1980 2000

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°C COMBINED LAND, AIR AND SEA SURFACE TEMPERATURES

RELATIVE TO 1951–80 AVERAGES

FIGURE 1 Observed changes in the globally-averaged surface temperatures from 1860–1991

relative to the 30-year mean for 1951–1980.

in future emissions and the natural regulation of atmospheric

concentrations and is therefore unable to predict when the cli-

mate changes are likely to happen.

MODEL SIMULATIONS AND PREDICTIONS OF

CLIMATE CHANGE

Introduction

Since changes in global and regional climates due to anthro-

pogenic emissions of greenhouse gases will be small, slow

and difficult to detect above natural fluctuations during the

next 10 to 20 years, we have to rely heavily on model predic-

tions of changes in temperature, rainfall, soil moisture, ice

cover, sea level, etc. Indeed, in the absence of convincing

direct evidence, concern over an enhanced greenhouse effect

is based almost entirely on model predictions, the credibility

of which must be largely judged on the ability of the models

to simulate the present observed climate and its variability

on seasonal, inter-annual, decadal and longer time scales.

Climate models, ranging from simple one-dimensional

energy-balance models to enormously complex three-

dimensional global models requiring years of scientific

development and vast computing power, have been devel-

oped during the last 25 years, the most advanced at three

centres in the USA and at the UK Meterological Office and,

recently, at centers in Canada, France and Germany.

Until very recently, effort was concentrated on develop-

ing models (that evolved from weather prediction models) of

the global atmosphere coupled to the oceans and cryosphere

(sea and land ice) only through prescribing and up-dating sur-

face parameters such as temperature and albedo, from obser-

vations. However, realistic predictions of long-term changes

in climate, natural or man-made, must involve the atmosphere,

ocean play, cryosphere and, eventually, the biosphere, treated

as a single, strongly coupled and highly interactive system.

The oceans play a major stabilizing role in global climate

because of their inertia and heat storage capacity. Moreover,

they transport nearly as much heat between the equator and

the poles as does the atmosphere. The oceans absorb about

half of the carbon dioxide emitted by fossil fuels and also

absorb and transport a good deal of the associated additional

heat flux and hence will delay warming of the atmosphere.

During the 1980s the UK Meterological Office (UKMO)

developed one of the most advanced models of the global

atmosphere coupled to a shallow mixed-layer ocean and

used this to simulate the present climate and to study the

effects of nearly doubling the present level of carbon dioxide

to 600 ppmv. A general description of the physical basis,

structure and operation of the model, of its simulations and

predictions may be found in Mason (1989).

Simulation of the Present Climate

Models of the type just mentioned, the most important com-

puted variables of which are:

E–W and N–S components of the wind

Vertical motion

Air temperatures and humidity

428 GREENHOUSE GASES EFFECTS

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