While food production may benefit from a warmer climate, the increased potential for droughts, floods and heat waves will pose challenges for farmers. Additionally, the enduring changes in climate, water supply and soil moisture could make it less feasible to continue crop production in certain regions.

The National Research Council (NRC, 2001) concluded:

In the near term, agriculture and forestry are likely to benefit from CO2 fertilization effects and the increased water efficiency of many plants at higher atmospheric CO2 concentrations. Many crop distributions will change, thus requiring significant regional adaptations. Given their resource base, the Assessment concludes that such changes will be costlier for small farmers than for large corporate farms. However, the combination of the geographic and climatic breadth of the United States, possibly augmented by advances in genetics, increases the nation's robustness to climate change. These conclusions depend on the climate scenario, with hotter and drier conditions increasing the potential for declines in both agriculture and forestry. In addition, the response of insects and plant diseases to warming is poorly understood. On the regional scale and in the longer term, there is much more uncertainty.

Climate Factors

Several factors directly connect climate change and agricultural productivity:

Most agricultural impact studies have considered the effects of one or two aspects of climate change on a particular farming activity. Few, however, have considered the full set of anticipated shifts and their impact on agricultural production across the country.

Temperature: An increase in average temperature can 1) lengthen the growing season in regions with a relatively cool spring and fall; 2) adversely affect crops in regions where summer heat already limits production; 3) increase soil evaporation rates, and 4) increase the chances of severe droughts.

Rainfall: Changes in rainfall can affect soil erosion rates and soil moisture, both of which are important for crop yields. Predicting future changes in rainfall, especially at regional scales, remains a challenge. However, the most widely used global climate models tend to forecast not only changes in the amount of precipitation, but increased intensity of rainfall events (IPCC, 2001).

CO2 fertilization: Increasing atmospheric CO2 levels, driven by emissions from human activities, can act as a fertilizer and enhance the growth of some crops such as wheat, rice and soybeans. CO2 can be one of a number of limiting factors that, when increased, can enhance crop growth. Other limiting factors include water and nutrient availability. The strength of a CO2 fertilization effect, therefore, can either be strengthened or weakened depending on temperature effects, nutrient availability and the harmful effects of tropospheric ozone (IPCC, 2001).

Tropospheric ozone: Higher levels of ground level ozone limit the growth of crops. Since ozone levels in the lower atmosphere are shaped by both emissions and temperature, climate change will most likely increase ozone concentrations. Such changes may offset any beneficial yield effects that result from elevated CO2 levels.

Climatic variability and extreme events: Changes in the frequency and severity of heat waves, drought, floods and hurricanes, remain a key uncertainty about future climate change. Such changes are anticipated by global climate models, but regional changes and the potential affects on agriculture are more difficult to forecast.

Implications for North America

The National Research Council¹⁰ concluded that there may be significant regional transitions associated with shifts in agriculture as a result of climate change (NRC, 2001). Similarly, the IPCC concluded¹¹ that, for North America as a whole (IPCC, 2001):

Agriculture in the U.S. and other industrialized countries is expected to be less vulnerable to climate change than agriculture in developing nations, especially in the tropics, where farmers may have a limited ability to adapt. In addition, the effects of climate change on U.S. and world agriculture will depend not only on changing climatic conditions but also on changes in agriculture's ability to be productive and adapt through future changes in technology, demand for food, and environmental conditions, such as water availability and soil quality. Management practices, the opportunity to switch management and crop selection from season to season, and technology can help the agricultural sector cope with and adapt to climatic variability and change.

References