8.3.4. Food and Fiber: Agriculture
8.3.4.1. Description of the Resource
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).
8.3.4.2. Potential Impacts of Climate Change on Agriculture
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).
8.3.4.3. Climate Variability and Extreme Events
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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