The United States dominates international trade of several agricultural commodities. It is the leading exporter of maize, soybeans, and wheat flour, and therefore production of these crops in the United States has an important influence on food prices throughout the world and thus on food security. The United States is also one of the most extensively studied regions in terms of climate change impacts. An early analysis based on crop models indicated that some Northern regions would gain from temperature increases, but that in most important production regions, such as the Corn Belt and Southern Great Plains, warming accompanying a doubling of CO2 would reduce yields by an amount roughly equal to the fertilization effect of CO2 (Adams et al. 1990). Thus, it was concluded that a doubling of CO2 would result in small net changes to major commodities in the United States, and that adaptation to warming could even result in net benefits.
These conclusions have been generally supported by many subsequent studies, many of which are reviewed in a recent national summary report (CCSP 2008). This report also highlights some factors that have not been considered in most modeling studies, such as the likely northward migration of weeds. The range of several important pests, such as corn earworm, are also currently constrained by winter temperatures and can be expected to expand greatly in future climates (Diffenbaugh et al. 2008). There may also be smaller than expected CO2 benefits for the most widely grown crop, maize (see Chapter 7). Finally, there might also be greater impacts from extreme heat episodes than most currently used models anticipate. For instance, Schlenker and Roberts (2008) find that yields of maize, soybeans, and cotton in the United States are very sensitive to extreme heat (see Chapter 6). Such results raise important questions about the generally low sensitivity of process-based crop models to warming in these systems.
As some key regions such as California and Nebraska are heavily dependent on irrigation, changes in water resources will also be important. For much of the country, the future direction of rainfall trends remains ambiguous and therefore water availability may increase or decrease (Thomson et al. 2005a). In the West, heavy dependence on snowpack combined with anticipated higher temperatures will very likely result in reduced water availability during the growing season months (Maurer and Duffy 2005). Interestingly, total national irrigated area was projected to decrease across a range of climate scenarios, because rainfed crops became more competitive in scenarios of rainfall increases while irrigation water became limiting in scenarios of drying (Thomson et al. 2005b).
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.