2.6 FINDINGS AND CONCLUSIONS2.6.1 Crops22.214.171.124 GRAINANDOILSEEDCROPSCrop yield response to temperature and CO2for maize, soybean, wheat, rice, sorghum, cotton, peanut, and dry bean in the United States was assembled from the scientific literature. Cardinal base, optimum, and upper failure-point tem-peratures for crop development, vegetative, and reproductive growth and slopes-of-yield decline with increase in temperature were reviewed. In general, the optimum temperature for reproduc-tive growth and development is lower than that for vegetative growth. Consequently, life cyclewill progress more rapidly, especially given a shortened grain-filling duration and reduced yield as temperature rises. Furthermore, these crops are characterized by an upper failure-point temperature at which pollination and grain-set processes fail. Considering these aspects, the optimum mean temperature for grain yield is fairly low for the major agronomic crops: 18-22ºC for maize, 22-24ºC for soybean, 15ºC for wheat, 23-26ºC for rice, 25ºC for sorghum, 25-26ºC for cotton, 20-26ºC for peanut, 23-24ºC for dry bean, and 22-25ºC for tomato.Without the benefit of CO2, the anticipated 1.2ºC rise in temperature over the next 30 years is projected to decrease maize, wheat, sorghum, and dry bean yields by 4.0, 6.7, 9.4, and 8.6 percent, respectively, in their major production regions. For soybean, the 1.2ºC temperature rise will increase yield 2.5 percent in the Mid-west where temperatures during July, August, September average 22.5ºC, but will decrease yield 3.5 percent in the South, where mean temperature during July, August, and September averages 26.7ºC. Likewise, in the South, that same mean temperature will result in reduced rice, cotton, and peanut yields, which will de-crease 12.0, 5.7, and 5.4 percent, respectively. An anticipated CO2increase from 380 to 440 ppm will increase maize and sorghum yield by only 1 percent, whereas the listed C3crops will increase yield by 6.1 to 7.4 percent, except for cotton, which shows a 9.2 percent increase. The response to CO2was developed from interpola-tion of extensive literature summarization of Diversifying crops also reduces incidence of pests, diseases, and weeds, imparting resilience to the agro-ecosystem. This resilience will become increasingly important as a component of farm adaptation to climate change.
The U.S. Climate Change Science Program70Chapter 2response to ambient versus doubled CO2. The net effect of rising temperature and CO2on yield will be maize (-3.0 percent), soybean (Midwest, +9.9 percent; South, +3.9 percent), wheat (+0.1 percent), rice (-5.6 percent), sorghum (-8.4 percent), cotton (+3.5 percent), peanut (+1.3 percent), and dry bean (-2.5 percent). The CO2-induced decrease in measured ET summarized from chamber and FACE studies, from 380 to 440 ppm, gives a fairly repeatable reduction in ET of 1.4 to 2.1 percent, although the 1.2ºC rise in temperature would increase ET by 1.8 percent, giving an unimportant net -0.4 to +0.3 percent reduction in ET. This effect could lead to a further small -0.4 to +0.3 percent change in yield under rainfed production. A similar small