In general, higher temperatures are associated with higher radiation and higher water use. Two effects of temperature are distinguished: (i) the physiological effects (at the level of plants and plant organs); and (ii) the crop-ecosystem effects (at the level of the field or at the region). These effects have both positive and negative impacts, largely depending on local conditions and plant species (Bazzaz and Sombroek, 1996).
At the plant level, it is expected that rising temperatures would diminish the yield of some crops, especially if night temperatures are higher (Kukla and Karl, 1993). However, as long as plants do not get overheated, some positive effects on crop growth can also be expected. A negative aspect is that higher cold-season temperatures may lead to earlier ripening of annual crops, diminishing yield per crop. An adaptation option to cope with this effect is to allow locally for the growth of more crops per year due to the lengthening of the growing season.
At the crop-ecosystem level, it is expected that crop yields will be enhanced in the northern regions of the former Soviet Union, Canada and Europe. The cereal-growing belts of North America might shift northwards by several hundred kilometres for every degree Celsius rise in temperature. The predicted yield increases in the higher latitude regions are primarily due to a lengthening of the growing season and the mitigation of negative cold weather effects on plant growth (Parry and Rosenzweig, 1993). Negative effects on crop and livestock productivity are expected in northern middle latitude countries like the USA, Western Europe and most of Canada's currently productive agricultural regions. This is due to a shortening of the growing period caused by increased temperatures and evapotranspiration rates (Tobey et al., 1992; see also Chapter 4).
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