conclusion has to be that an understanding of the cycles of the non-CO 2 green-
house gases is in total as important as knowledge of the cycling of CO 2.
So far we have concentrated only on greenhouse-gas-induced temperature
changes. However, other climatological changes—for example, in the distribu-
tion of rainfall—may be more important in a practical sense than temperature
increase per se. Computer models of likely changes in climate as a result of
increase in CO 2 and other gases indicate that global average water vapour, evap-
oration and rainfall are projected to increase, whereas at the regional scale, both
increases and decreases in rainfall are likely. It is the net result of all these changes
in rainfall, atmospheric water vapour and evaporation that will determine the
agriculturally important property of soil-water content. Water is vital to crop
growth and so it is hugely important to be able to predict changes in amounts of
soil water, which can be either beneficial or harmful to crop yields. The social,
economic and political consequences of such changes and geographical shifts are
likely to be considerable.
Another potentially important consequence of global warming would be a
global rise in sealevel. This would come about in part due to thermal expansion
of seawater and also as a result of melting of glaciers and small ice-caps. Calcu-
lations of the magnitude of sealevel rise have considerable uncertainty, but a figure
of about half a metre for a doubling of atmospheric CO 2 is the current best esti-
mate. If it occurs, this would have very significant effects in many countries that
have centres of population close to the sea or on low-lying land. Further, there
is a possibility that warming might eventually lead to the melting of a large mass
of grounded ice, for example, the west Antarctic ice sheet. Such an event could
produce a more substantial rise in sealevel (several metres), but, even if the tem-
perature rise is great enough to melt the ice, it is estimated that it would take
several hundred years for this to occur.
A further impact of rising levels of CO 2 is on the chemistry of the oceans, in
particular their pH. Increase in atmospheric CO 2 will lead to a slight lowering of
pH of the surface oceans (Section 6.4.4) as a result of extra amounts of the gas
crossing from the atmosphere into the oceans. The resulting lowering in pH can
be calculated, as can the potential for this increased acidity leading to enhanced
calcium carbonate dissolution, for example in corals. Figure. 7.15 shows the pH
of surface seawater as a function of temperature calculated for three different con-
centrations of atmospheric CO 2. A pCO 2 of 280 ppm corresponds to the situa-
tion in pre-industrial times with a seawater pH value of just below 8.2. A pCO 2
of 354 ppm, corresponds to 1992 and a seawater pH of just under 8.1 (i.e. a drop
in pH of about 0.1 unit from the pre-industrial value). Finally, a pCO 2 of 750
ppm is given as a reasonable estimate of the likely concentration at the close of
this century, with a corresponding seawater pH of just under 7.8 (i.e. a drop of
0.4 units from the pre-industrial level). Also shown are pH values derived from
measurements made in the Atlantic Ocean surface waters in summer; they cluster
in the range 8.0–8.3. What is clear is that by 2100, or whenever the atmospheric
pCO 2 reaches double its current value, the pH of surface seawater will be far
outside the range of values currently experienced by organisms living in the
surface oceans. The effects of such a change on the biology of the oceans and on260 Chapter Seven