Determining what the net effect of a changing climate may be on an agro-ecosystem is complicated due to the interactions of several simultaneous biophysical processes. In some cases, changes in climate may be beneficial, while in others they may be detrimental (Figure 10.2). On the beneficial side,
increasing levels of atmospheric CO2 have been shown to increase photosynthesis rates and to increase stomatal resistance in crops, leading to overall increased water-use efficiency (Kimball et al., 2002). These processes have been called "CO2 fertilization."
Another beneficial impact would be the prolongation of crop growing seasons in areas where they are now limited by cold temperature, that is, at high latitudes and high elevations. A further benefit for crops may accrue in some semiarid locations from increased precipitation, since a warmer atmosphere can hold more water vapor. However, the location and extent of any such regions of enhanced precipitation is not known precisely, due primarily to the difficulty of simulating the regional-scale hydrological cycle in global climate models.
A warmer and more variable climate is likely to have negative as well as positive effects on agricultural regions around the world. Potential negative effects include more frequent droughts and floods, heat stress, increased outbreaks of diseases and pests, shortening of crop growing periods, and — in coastal regions — increased flooding and salination due to sea-level rise and impeded drainage. While the absolute magnitude of precipitation change in any one region or decade is not predictable, global climate models project that hydro-logical regimes are likely to become more intense as well as more variable (IPCC, 2001). Episodes of heat stress are known to be detrimental to crops, especially during critical growth stages, and such episodes are likely to be more frequent and prolonged in the future.
An important, albeit counterintuitive, negative effect that warming has on crops is the shortening of their growing period (not their overall growing season). Warmer temperatures speed crops through their growing cycle, especially the grain-filling stage. Total yield is a product of the rate and duration of grain filling, which is determined by accumulated temperature. Since higher temperatures shorten the duration of grain filling, higher temperature tends to exert a negative pressure on the yield of most annual crops.
Finally, in agricultural regions close to the ocean, sea-level rise and associated saltwater intrusion and flooding can harm crops through impeded soil aeration and salination. This is likely to be most serious in countries such as Egypt and Bangladesh, which have major crop-growing areas in low-lying coastal regions.
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