The carbon dioxide fertilisation effect

An important positive effect of increased carbon dioxide concentrations in the atmosphere is the boost to growth in plants given by the additional carbon dioxide. Higher carbon dioxide concentrations stimulate photosynthesis, enabling plants to fix carbon at a higher rate. This is why in glasshouses additional carbon dioxide may be introduced artificially to increase productivity. The effect is particularly applicable to what are called C3 plants (such as wheat, rice and soya bean), but less so to C4 plants (for example, maize, sorghum, sugar cane, millet and many pasture and forage grasses). Under ideal conditions it can be a large effect; for C3 crops under doubled carbon dioxide, an average of +30%,40 although grain and forage quality tends to decline with carbon dioxide enrichment and higher temperatures. However, under real conditions on the large scale where water and nutrient availability are also important factors influencing plant growth, experiments show increases under unstressed conditions in the range 10-25% for C3 crops and 0-10% for C4 crops. Enhanced growth has been observed for young tree stands but no significant response has been measured for mature forest stands. Ozone exposure limits carbon dioxide response in both crops and forests.41 More research is required especially for many tropical crop species and for crops grown under suboptimal conditions (low nutrients, weeds, pests and diseases). More information is also needed about possible effects on the nutritional value of the crops with carbon dioxide fertilisation.42

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 carbon dioxide 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 increases in temperature (2-3 °C) but decrease for larger temperature rises (Figure 7.11).43 In most tropical and subtropical regions, potential yields are projected to decrease for most increases in temperature; this is because such crops are near their maximum temperature tolerance. Where there is a large decrease in rainfall, tropical crop yields would be even more adversely affected.

Taking the supply of food for the world as a whole, studies tend to show that, with appropriate adaptation, the effect of climate change on total global food supply is not likely to be large. However, none of them has adequately taken into account the likely effect on food production of climate extremes (especially of the incidence of drought), of increasingly limited water availability or of other factors such as the integrity of the world's soils, which are currently being degraded at an alarming rate.44 A serious issue exposed by the studies is that climate change is likely to affect countries very differently. Production in developed countries with relatively stable populations may increase, whereas

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