Page 340 - Climate Change and Food Systems

climate change and food systems: global assessments and implications for food security and trade
figure 4
Variation in GDP growth with total seasonal rainfall variation in Ethiopia
Source: de Jong (2005), World Bank (2005)
of CO2; yield gains in these plants grown under elevated CO2 are much more modest than for C3 plants – for example, no yield change is observed for maize (Long et al., 2006). There is a small improvement in the efficiency of water use for both C3 and C4 crops under enhanced CO2 conditions.
Warmer temperatures affect the rate at which crops grow and develop, and potentially affect
the survival of plants and seeds at extremes of temperature. The duration from sowing to flowering to crop harvest is determined by temperature regime and by day-length (Craufurd and Wheeler, 2009). In a warmer climate, we expect the areas where crops are grown to shift northwards in
the northern hemisphere and southwards in the southern hemisphere. Where the appropriate genetic material is available, farmers at a particular location can adapt to these changes using new varieties or crops with longer durations; that
is, with higher thermal requirements for crop development. Where longer-season varieties cannot be used, crop yields will decline with warmer temperatures because less radiation will be captured and used for crop yield in seasons of shorter duration. For example, an analysis of more than 20 000 variety trials of maize across Africa found that, for each degree day spent above 30oC, final maize yield was reduced by 1 percent under optimal rainfed conditions and by 1.7 percent
under drought conditions (Lobell et al., 2011). In the future, with a shift to adapted varieties, some of the negative impacts of warmer temperature can be partly offset, although there are important differences among the world’s major crops – such as between C3 and C4 crops, and between
crops grown in temperate and tropical latitudes. For example, a synthesis of adaptation studies of wheat yield found that adaptation counteracted the equivalent of 4.5 to 5oC of warming in the mid to high latitudes, but only 1.5 to 3oC at low latitudes (Easterling et al., 2007). Beyond these values
of temperature warming, the impacts of climate change exceed adaptive capacity.
Extremes of hot temperature will become
more frequent under climate change (Figure 5). Even without any changes in the distribution of daily temperature, a warmer mean distribution
will increase the frequency of extremely hot days. Increased climate variability, which is expected under climate change, will further increase the frequency of extreme temperatures. Where extremely hot days coincide with a sensitive stage of crop development, such as flowering, we find dramatic decreases in seed or grain yields (Wheeler et al., 2000). For example, an increase in maximum temperature above 30oC reduced the seed set of rice cultivar IR64 by 7 percent per degree increase in heat stress (Jagadish et al., 2007). What is
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