Potential Interactions with Agriculture

U.S. croplands, grassland pasture, and range occupy about 420 million hectares (about 1,030 million acres), or nearly 55 percent of the U.S. land area, excluding Alaska and Hawaii (USDA/ERS 2000). Throughout the 20th century, agricultural production shifted toward the West and Southwest. This trend allowed regrowth of some forests and grasslands, generally enhancing wildlife habitats, especially in the Northeast, and contributing to sequestration of carbon in these regions.

U.S. food production and distribution comprise about 10 percent of the U.S. economy. The value of U.S. agricultural commodities (food and fiber) exceeds $165 billion at the farm level and over $500 billion after processing and marketing, Because of the productivity of U.S. agriculture, the United States is a major supplier of food and fiber for the world, accounting for more than 25 percent of total global trade in wheat, corn, soybeans, and cotton.

Changes in Agricultural Productivity

U.S. agricultural productivity has improved by over 1 percent a year since 1950, resulting in a decline in both production costs and commodity prices, limiting the net conversion of natural habitat to cropland, and freeing up land for the Conservation Reserve Program. Although the increased production and the two-thirds drop in real commodity prices have been particularly beneficial to consumers inside and outside the United States and have helped to reduce hunger and malnourishment around the world, the lower prices have become a major concern for producers and have contributed to the continuing decline in the number of small farmers across the country. Continuation of these trends is expected, regardless of whether climate changes, with continuing pressures on individual producers to further increase productivity and reduce production costs.

On the other hand, producers consider anything that might increase their costs relative to other producers or that might limit their markets as a threat to their economic well-being. Issues of concern include regulatory actions, such as efforts to control the off-site consequences of soil erosion, agricultural chemicals, and livestock wastes; extreme weather or climate events; new pests; and the development of pest resistance to existing pest control strategies.

Future changes in climate are expected to interact with all of these issues. In particular, although some factors may tend to limit growth in yields, rising CO₂ concentrations and continuing climate change are projected, on average, to contribute to extending the persistent upward trend in crop yields that has been evident during the second half of the 20th century. In addition, if all else remains equal, these changes could change supplies of and requirements for irrigation water, increase the need for fertilizers to sustain the gain in carbon production, lead to changes in surface-water quality, necessitate increased use of pesticides or other means to limit damage from pests, and alter the variability of the climate to which the prevailing agricultural sector has become accustomed. However, agricultural technology is currently undergoing rapid change, and future production technologies and practices seem likely to be able to contain or reduce these impacts.

Assuming that technological advances continue at historical rates, that there are no dramatic changes in federal policies or in international markets, that adequate supplies of nutrients are available and can be applied without exacerbating pollution problems, and that no prolonged droughts occur in major agricultural regions, U.S. analyses indicate that it is unlikely that climate change will imperil the ability of the United States to feed its population and to export substantial amounts of foodstuffs (NAAG 2002). These studies indicate that, at the national level, overall agricultural productivity is likely to increase as a result of changes in the CO₂ concentration and in climate projected for at least the next several decades. The crop models used in these studies assume that the CO₂ fertilization effect will be strongly beneficial and will also allow for a limited set of on-farm adaptation options, including changing planting dates and varieties, in res-ponse to the changing conditions. These adaptation measures contribute small additional gains in yields of dry-land crops and greater gains in yields of irrigated crops. However, analyses performed to date have neither considered all of the consequences of possible changes in pests, diseases, insects, and extreme events that may result, nor been able to consider the full range of potential adaptation options (e.g., genetic modification of crops to enhance resistance to pests, insects, and diseases).

Recognizing these limitations, available evaluations of the effects of anticipated changes in the CO₂ concentration and climate on crop production and yield and the adaptive actions by farmers generally show positive results for cotton, corn for grain and silage, soybeans, sorghum, barley, sugar beets, and citrus fruits (Figure 6-7). The productivity of pastures may also increase as a result of these changes. For other crops, including wheat, rice, oats, hay, sugar cane, potatoes, and tomatoes, yields are projected to increase under some conditions and decrease under others, as explained more fully in the agriculture assessment (NAAG 2002).

The studies also indicate that not all U.S. agricultural regions are likely to be affected to the same degree by the projected changes in climate that have been investigated. In general, northern areas, such as the Midwest, West, and Pacific Northwest, are projected to show large gains in yields, while influences on crop yields in other regions vary more widely, depending on the climate scenario and time period. For example, projected wheat yields in the southern Great Plains could decline if the warming is not accompanied by sufficient precipitation.

These analyses used market-scale economic models to evaluate the overall economic implications for various crops. These models allow for a wide range of adaptations in response to changing productivity, prices, and resource use, including changes in irrigation, use of fertilizer and pesticides, crops grown and the location of cropping, and a variety of other farm management options. Based on studies to date, unless there is inadequate or poorly distributed precipitation, the net effects of climate change on the agricultural segment of the U.S. economy over the 21st century are generally projected to be positive. These studies indicate that, economically, consumers are likely to benefit more from lower prices than producers suffer from the decline in profits. Complicating the analyses, however, the studies indicate that producer versus consumer effects will depend on how climate change affects production of these crops elsewhere in the world. For example, for crops grown in the United States, economic losses to farmers due to lower commodity prices are offset under some conditions by an increased advantage of U.S. farmers over foreign competitors, leading to an increased volume of exports.

Because U.S. food variety and supplies depend not only on foodstuffs produced nationally, the net effect of climate change on foods available for U.S. consumers will also depend on the effects of climate change on global production of these foodstuffs. These effects will in turn depend not only on international markets, but also on how farmers around the world are able to adapt to climate change and other factors they will face. While there are likely to be many regional variations, experience indicates that research sponsored by the United States and other nations has played an important role in promoting the ongoing, long-term increase in global agricultural productivity. Further research, covering opportunities ranging from genetic design to improving the salt tolerance of key crops, is expected to continue to enhance overall global production of foodstuffs.

Changes in Water Demands by Agriculture

Within the United States, a key determinant of agricultural productivity will be the ongoing availability of sufficient water where and when it is needed. The variability of the U.S. climate has provided many opportunities for learning to deal with a wide range of climate conditions, and the U.S. regions where many crops are grown have changed over time without disrupting production. In addition, steps to build up the amount of carbon in soils -- which is likely to be one component of any carbon mitigation program -- will enhance the water-holding capacity of soils and decrease erosion and vulnerability to drought, thereby helping to improve overall agricultural productivity. For areas that are insufficiently moist, irrigation has been used to enhance crop productivity. In addition, about 27 percent of U.S. cultivated land is currently under reduced tillage. Several projects, such as the Iowa Soil Carbon Sequestration Project, that are underway to promote conservation tillage practices as a means to mitigate climate change will have the ancillary benefits of reducing soil erosion and runoff while increasing soil water and nitrogen retention.

Analyses conducted for the National Assessment project that climate change will lead to changes in the demand for irrigation water and, if water resources are insufficient, to changes in the crops being grown. Although regional differences will likely be substantial, model projections indicate that, on average for the nation, agriculture's need for irrigation water is likely to slowly decline. At least two factors are responsible for this projected reduction: (1) precipitation will increase in some agricultural areas, and (2) faster development of crops due to higher temperatures and an increased CO₂ concentration is likely to result in a shorter growing period and consequently a reduced demand for irrigation water. Moreover, a higher CO₂ concentration generally enhance a plant's water-use efficiency. These factors can combine to compensate for the increased transpiration and soil water loss due to higher air temperatures. However, a decreased period of crop growth also leads to decreased yields, although it may be possible to overcome this disadvantage through crop breeding.

Changes in Surface-Water Quality due to Agriculture

Potential changes in surface-water quality as a result of climate change is an issue that has only started to be investigated. For example, in recent decades, soil erosion and excess nutrient runoff from crop and livestock production have severely degraded Chesapeake Bay, a highly valuable natural resource. In simulations for the National Assessment, loading of excess nitrogen from corn production into Chesapeake Bay is projected to increase due to both the change in average climate conditions and the effects of projected changes in extreme weather events, such as floods or heavy downpours that wash large amounts of fertilizers and animal manure into surface waters. Across the country, changes in future farm practice (such as no-till or reduced-till agriculture) that enhance buildup and retention of soil moisture, and better matching of the timing of a crop's need for fertilizer with the timing of application are examples of approaches that could reduce projected adverse impacts on water quality. In addition, the potential for reducing adverse impacts of fertilizer application and soil erosion by using genetically modified crops has not yet been considered.

Changes in Pesticide Use by Agriculture

Climate change is projected to cause farmers in most regions to increase their use of pesticides to sustain the productivity of current crop strains. While this increase is expected to result in slightly poorer overall economic performance, this effect is minimal because pesticide expenditures are a relatively small share of production costs. Neither the potential changes in environmental impacts as a result of increased pesticide use nor the potential for genetic modification to enhance pest resistance have yet been evaluated.

Effects of Changes in Climate Variability on Agriculture

Based on experience, agriculture is also likely to be affected if the extent and occurrence of climate fluctuations and extreme events change. The vulnerability of agricultural systems to climate and weather extremes varies with location because of differences in soils, production systems, and other factors. Changes in the form (rain, snow, or hail), timing, frequency, and intensity of precipitation, and changes in wind-driven events (e.g., wind storms, hurricanes, and tornadoes) are likely to have significant consequences in particular regions. For example, in the absence of adaptive measures, an increase in heavy precipitation events seems likely in some areas to aggravate erosion, water-logging of soils, and leaching of animal wastes, pesticides, fertilizers, and other chemicals into surface and ground water. Conversely, lower precipitation in other areas may reduce some types of impacts.

A major source of U.S. climate variability is the El Niño -- Southern Oscillation (ENSO). The effects of ENSO events vary widely across the country, creating wet conditions in some areas and dry conditions in others that can have significant impacts on agricultural production. For example, over the past several decades, average corn yield has been reduced by about 15 -- 30 percent in years with widespread floods or drought. Better prediction of such variations is a major focus of U.S. and international research activities (e.g., through the International Research Institute for Climate Prediction) because, in part, such information could increase the range of adaptive responses available to farmers. For example, given sufficient warning of climate anomalies (e.g., of conditions being warm and dry, cool and moist, etc.), crop species and crop planting dates could be optimized for the predicted variation, helping to reduce the adverse impact on yields and overall production. Because long-term projections suggest that ENSO variations may become even stronger as global average temperature increases, achieving even better predictive skill in the future will be especially important to efforts to maximize production in the face of climate fluctuations.

Potential Adaptation Strategies for Agriculture

To ameliorate the deleterious effects of climate change generally, such adaptation strategies as changing planting dates and varieties are likely to help to significantly offset economic losses and increase relative yields. Adaptive measures are likely to be particularly critical for the Southeast because of the large reductions in yields projected for some crops if summer precipitation declines. With the wide range of growing conditions across the United States, specific breeding for response to CO₂ is likely to be required to more fully benefit from the CO₂ fertilization effect detected in experimental crop studies. Breeding for tolerance to climatic stress has already been exploited, and varieties that do best under ideal conditions usually also out-perform other varieties under stress conditions.

Although many types of changes can likely be adapted to, some adaptations to climate change and its impacts may have negative secondary effects. For example, an analysis of the potential effects of climate change on water use from the Edward's aquifer region near San Antonio, Texas, found increased demand for ground-water resources. Increased water use from this aquifer would threaten endangered species dependent on flows from springs supported by the aquifer.

In addition, in the absence of genetic modification of available crop species to counter these influences, pesticide and herbicide use is likely to increase with warming. Greater chemical inputs would be expected to increase the potential for chemically contaminated runoff reaching prairie wetlands and ground water, which, if not controlled by on-site measures, could pollute rivers and lakes, drinking-water supplies, coastal waters, recreation areas, and waterfowl habitat.

As in the past, farmers will need to continue to adapt to the changing conditions affecting agriculture, and changing climate is likely to become an increasingly influential factor. Presuming adaptation to changing climate conditions is successful, the U.S. agricultural sector should remain strong -- growing more on less land while continuing to lower prices for the consumer, exporting large amounts of food to help feed the world, and storing carbon to enhance resilience to drought and contribute to the slowing of climate change.