166 The impactsof climate change
of water is the most important of the factors. The vulnerability of water
supplies to climate change carries over into a vulnerability in the growing
of crops and the production of food. Thus the arid or semi-arid areas,
mostly in developing countries, are most at risk. A second factor, which
tends to lead to increased production as a result of climate change, is the
boost to growth that is given, particularly to some crops, by increased
atmospheric carbon dioxide (see box below). A third factor is the effect of
temperature changes; in particular, under very high temperatures, yields
of some crops are substantially reduced.The carbon dioxide ‘fertilisation’ effect
An important positive effect of increased carbon dioxide(CO 2 ) con-
centrations in the atmosphere is the boost to growth in plants given by
the additional CO 2. Higher CO 2 concentrations stimulate photosynthe-
sis, enabling the plants to fix carbon at a higher rate. This is why in
glasshousesadditional CO 2 may be introduced artificially to increase
productivity. The effect is particularly applicable to what are called C3
plants (such as wheat, riceand soya bean), but less so toC4 plants (for
example, maize, sorghum, sugar-cane, millet and many pasture and for-
age grasses). Under ideal conditions it can be a large effect; for C3 crops
under doubled CO 2 , an average of+30%.^37 However, under real con-
ditions on the large scale where water and nutrient availability are also
important factors influencingplant growth, experiments show that the
increases, although difficult to measure accurately, tend to be substan-
tially less than the ideal.^38 In experimental work, grain and forage quality
declines with CO 2 enrichment and higher temperatures. More research
is required especially for many tropical crop species and for crops grown
under suboptimal conditions (low nutrients, weeds, pests and diseases).Detailed studies have been carried out of the sensitivity to climate
change during the twenty-first century of the major crops which make up
a large proportion of the world’s food supply (see box below). They have
used the results of climate models to estimate changes in temperature
and precipitation. Many of them study the effect of CO 2 fertilisation and
some also model the effects of climate variability as well as changes in
the means. Some also include the possible effects of economic factors
and of modest levels of adaptation. These studies in general indicate that
the benefit of increased CO 2 concentration on crop growth and yield does
not always overcome the effects of excessive heat and drought. For cereal
crops in mid latitudes, potential yields are projected to increase for small