Encyclopedia of Environmental Science and Engineering, Volume I and II

427

GREENHOUSE GASES EFFECTS

INTRODUCTION

The possibility that man-made emissions of carbon dioxide

and other infra-red absorbing gases may enhance the natural

greenhouse effect and lead to a warming of the atmosphere

and attendant changes in other climate parameters such as pre-

cipitation, snow and ice cover, soil moisture and sea-level rise,

constitutes perhaps the most complex and controversial of all

environmental issues and one that is likely to remain high on

both the scientific and political agenda for a decade or more.

The issues have been obscured by a good deal of exaggeration

and distortion by the media, and by some scientists, so that

governments, the public, and scientists in other disciplines are

confused and sceptical about the evidence for global warming

and the credibility of the predictions for the future.

Until very recently the atmospheric concentrations of

carbon dioxide had been increasing and accelerating since

regular measurements began in 1958. Only during the last

few years has there been a levelling off, probably because

of the world-wide recession, the run-down of industry in the

former Soviet bloc, and the substitution of gas for coal. This

pause is likely to be only temporary, and if the concentrations

resume their upward trend, they will eventually lead to sig-

nificant climate changes. The important questions concern

the likely magnitude and timing of these events. Are they

likely to be so large and imminent as to warrant immediate

remedial action, or are they likely to be sufficiently small

and delayed that we can live with them or adapt to them?

Careful reconstruction of historical records of near-surface

air temperatures and sea-surface temperatures has revealed

that globally-averaged annual mean temperatures have risen

about 0.6
C, since 1860 (see Figure 1). There is general con-

sensus among climatolologists that this can now confidently

be ascribed to enhanced greenhouse warming rather than to

natural fluctuations. The last decade has been the warmest

of this century and 9 of the 10 warmest years have occurred

since 1990. Moreover, as described later, any temperature

rise due to accumulated concentrations of greenhouse gases

may have been masked by a concomitant increase in concen-

trations of aerosols and by delay in the oceans.

ROLE OF CARBON DIOXIDE IN CLIMATE

Carbon dioxide, together with water vapor, are the two main

greenhouse gases which regulate the temperature of the Earth

of its atmosphere. In the absence of these gases, the average

surface temperature would be −19
C instead of the present

value of +15
C, and the Earth would be a frozen, lifeless

planet. The greenhouse gases act by absorbing much of the

infrared radiation emitted by the Earth that would otherwise

escape to outer space, and re-radiate it back to the Earth to

keep it warm. This total net absorption over the whole globe

is about 75 PW an average of 150 Wm −2 , roughly one-third

by carbon dioxide and two-thirds by water vapour.

There is now concern that atmospheric and surface tem-

peratures will rise further, owing to the steadily increasing

concentration of carbon dioxide resulting largely from the

burning of fossil fuels. The concentration is now 367 ppmv,

31% higher than the 280 ppm which prevailed before the

industrial revolution and, until very recently was increasing

at 0.5% p.a. If this were to continue, it would double its pre-

industrial value by 2085 AD and double its present value

by 2135. However, if the world’s population continues to

increase at the present rate, the concentration of carbon diox-

ide may well reach double the present value in the second

half of this century.

Future concentrations of atmospheric carbon dioxide

will be determined not only by future rates of emissions,

which can only be guessed at, but also by how the added

CO 2 is partitioned between the atmosphere, ocean and bio-

sphere. During the decade 1940–9, the rate of emission

from the burning of fossil fuels and wood is estimated at

63 ± 0.5 GtC/yr (gigatonnes of carbon per year). The atmo-

sphere retained 3.2 GtC (about half of that emitted), leaving

3.4 GtC/yr to be taken up by the oceans and terrestrial bio-

sphere. Models of the ocean carbon balance suggest that it

can take up only 1.7 ± 0.5 GtC/yr so that there is an apparent

imbalance of 1.4 ± 0.7 GtC/yr. Some scientists believe that

this difference can be accounted for by additional uptake by

newly growing forests and by the soil, but this is doubtful,

and the gap is a measure of the uncertainty in current under-

standing of the complete carbon cycle. Reliable quantitative

estimates of the combined effects of the physical, chemical

and biological processes involved, and hence of the mag-

nitude and timing of enhanced greenhouse warming await

further research.

Nevertheless, very large and complex computer models

of the climate system have been developed to simulate the

present climate and to predict the likely effects of, say, dou-

bling to atmospheric concentration of CO 2 , or of increasing it

at an arbitrary rate. This approach bypasses the uncertainties

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