BOX 5.1


Nearly 40% of global maize (or corn) production occurs in the United States, much of which is exported to other nations. The future yield of U.S. maize is therefore important for nearly all aspects of domestic and international agriculture. Higher temperatures speed development of maize, increase soil evaporation rates, and above 35ºC can compromise pollen viability, all of which reduce final yields. High temperatures and low soil moisture during the flowering stage are especially harmful as they can inhibit successful formation of kernels. In northern states, warmer years generally improve yields as they extend the frost-free growing season and bring temperature closer to optimum levels for photosynthesis. The majority of production, however, occurs in areas where yields are favored by cooler than normal years, so that warming associated with climate change would lower average national yields. The most robust studies, based on analysis of thousands of weather station and harvest statistics for rainfed maize (>80% of U.S. production), suggest a roughly 7% yield loss per ºC of local warming, which is in line with previous estimates (USCCSP, 2008b). Given the rate of local warming in the Corn Belt relative to global average, this implies an 11% yield loss per ºC of global warming (Figure 5.1).

Whether these losses are realized will depend in large part on the effectiveness of adaptation strategies, which include shifts in sowing dates, switches to longer maturing varieties, and

increased flood frequencies; and responses to extremely high temperatures. Moreover, most crop modeling studies have not considered changes in sustained droughts, which are likely to increase in many regions (Wang, 2005; Sheffield and Wood, 2008), or potential changes in year-to-year variability of yields. The net effect of these and other factors remains an elusive goal, but these are likely to push yields in a negative direction. For example, recent observations have shown that kudzu (Pueraria lobata), an invasive weed favored by high CO2 and warm winters, has expanded over the past few decades into the Midwest Corn Belt (Ziska et al., 2010).

Adaptation responses by growers are also poorly understood and could, in contrast, reduce yield losses. For example, temperate growers are likely to shift to earlier planting and longer maturing varieties as climate warms, and models suggest this response could entirely offset losses in certain situations. More commonly, however, these adaptations will at best be able to offset 2ºC of local warming (Easterling et al., 2007), and they will be less effective in tropical regions where soil moisture, rather than cold temperatures, limits the length of the growing season. Very few studies have considered the evidence for ongoing adaptations to existing climate trends and quantified the benefits of these adaptations.

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