Environmental Engineering Reference
In-Depth Information
BOX 5.1 HOW WILL MAIZE YIELDS IN THE UNITED STATES RESPOND TO CLIMATE
CHANGES?
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 35°C 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 associ-
ated 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 °C 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 °C of global warming (Figure 5.1).
Whether these losses are realized will depend in large part on the effectiveness of adapta-
tion 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 sus-
tained 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 CO 2 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
2°C of local warming (Easterling et al., 2007), and they will be less effec-
tive 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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