Agricultural land represents about 12% of the land area of North America. Approximately 3% of the population and 1.7% of the annual growth in gross national product (GNP) are related to agriculture. Agricultural land use comprises a total of approximately 233 million ha. Irrigated farmland represents 21 million ha in the United States, with much of this along the Mississippi River, the central Great Plains, and the western states. North America is characterized by an abundance of fertile soils and a highly productive agricultural sector that leads the world in the production of small grains. Within the United States, there are 10 farm production regions, with 6 corresponding regions in Canada (Adams et al., 1995b; Brklacich et al., 1997a).
Agriculture in North America has a long history of sensitivity to climate variability (e.g., the timing and magnitude of droughts and floods, extremes in heat and cold) and is subject to a wide array of other factors that can limit potential productivity (e.g., tropospheric ozone, pests, diseases, and weeds). Agriculture has an equally long history of developing strategies to cope with the many factors capable of limiting production. Climate change is an additional factor that could enhance or reduce the sensitivity of the agricultural sector to these current stress factors. As world population grows, the demand for North American agricultural products is expected to increase, with possible increases in agricultural commodity prices (IPCC 1996, WG II, Section 13.6.8). Should increased demand lead to further intensification of agriculture in North America, increased emphasis on sustainable agriculture is likely (Matson et al., 1997).
Potential impacts of climate change on agriculture will be reflected most directly through the response of crops, livestock, soils, weeds, and insects and diseases to the elements of climate to which they are most sensitive. Soil moisture and temperature are the climate factors likely to be most sensitive to change across large agricultural areas of North America. The differential response of species to elevated CO2 concentrations is expected to show a generally positive but variable increase in productivity and WUE for annual crops; limited evidence suggests less of a growth response for perennial crop species. Many weed species are expected to benefit from CO2 "fertilization" and increased WUE, and increased temperatures may facilitate the expansion of warm-season weed species to more northerly latitudes (IPCC 1996, WG II, Section 13.2). Insect pests and fungal and bacterial pathogens of importance to agricultural production are sensitive to climate change through the direct effects of changes of temperature and moisture on the pest or pathogen, on host susceptibility, and on the host-parasite interrelation (IPCC 1996, WG II, Section 13.4). Livestock is sensitive to climate through impacts on feed and forage crops, through the direct effects of weather and extreme events on animal health, and through changes in livestock diseases (IPCC 1996, WG II, Section 13.5).
Long-term crop management strategies that increase soil organic matter will benefit agricultural lands by increasing soil nutrient status and water-holding capacity while increasing soil carbon storage (Matson et al., 1997).
Changes in mean temperature and precipitation will likely affect agricultural crop and livestock production. Climate modifications that lead to changes in daily and interannual variability in temperatures and, in particular, precipitation also will impact crop yields.
Mearns et al. (1996) used the Clouds and Earth's Radiant Energy System (CERES)-Wheat model to demonstrate the impact of daily temperature variability on simulated wheat yields at two sites in Kansas. A doubling of daily temperature variability contributed to increased crop failures and lower yields as a consequence of cold damage and winter kill. Simulated wheat yields also decreased as variability in precipitation increased, although absolute reductions in yield were dependent on soil type and associated moisture-holding capacity. Although these simulations illustrate the potential sensitivity of wheat production to increased variability in temperature and precipitation, they do not incorporate the beneficial role that elevated CO2 may play in modifying these responses, nor are extreme events considered in these analyses. Extreme events like drought, flooding, hail, hurricanes, and tornadoes also will impact agriculture, but reliable forecasts of such occurrences are not yet regionally available.
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