Agriculture & Global Climate Change: A Review of Impacts to U.S. Agricultural Resources


In order to intelligently respond to climate change, we must first understand the likely consequences on our environment and health. This report, the first in a series of environmental impact reports, will explore anticipated effects of climate change on U.S. agriculture. Other reports in this series will assess what is known about the impact of climate change on weather and include analyses of its impact on water resources, coastal areas, human health, ecosystems, and forests. In evaluating the current state of scientific knowledge regarding the anticipated effects of climate change on U.S. agriculture, this report yields several key observations:

Climate change will result in agricultural shifts and changes across the United States. Given the requisite time and resources to adapt, the United States is likely to continue to be able to feed itself; however, there will clearly be regional winners and losers.

If the rate of greenhouse gas emissions exceeds projected levels or if unanticipated or more frequent extreme events accompany this change, the outlook for the United States would likely worsen. The projections in this report, for example, are based on a doubling of carbon dioxide (CO2) in the atmosphere which could understate the severity of climate change impacts over the long-term.

Some countries will experience more negative effects on agriculture associated with climate change. The situation will be particularly acute in developing nations that do not have the same resources as the United States to respond to the agricultural changes projected.

This report broadly outlines projected effects on U.S. agricultural regions. The complexity of the climate system itself and its relationship to agricultural resources make it difficult to project specific effects on individual states or communities. More research is needed to better understand this complex system and to incorporate relevant factors into future climate models and assessments. The report does, however, provide an objective foundation upon which to build and clearly demonstrates the impact climate change will have, both direct and indirect, on U.S. agricultural systems.

In addition to reporting on the environmental impacts of climate change, the Pew Center undertakes analyses on domestic and international policy matters and economics. The Center was established in 1998 by the Pew Charitable Trusts to bring a new, cooperative approach and critical scientific, economic and technological expertise to the global climate change debate.

A number of major corporations have taken a bold and historic step in joining the Center’s Business Environmental Leadership Council. In doing so, they have accepted “the views of most scientists that enough is known about the science and environmental impacts of climate change for us to take actions to address its consequences.” Understanding the potential environmental impacts of climate change, as this report illustrates, is an important step toward promoting informed action.

Executive Summary

This paper analyzes the current state of knowledge about the effects of climate change on U.S. food production and agricultural resources. The paper also considers regional changes in agricultural production, including distributional impacts.

The linkages between agriculture and climate are pronounced, often complex, and not always well understood. Temperature increases can have both positive and negative effects on crop yields, with the difference depending in part on location and on the magnitude of the increase. Crop yields in the northern United States and Canada may increase, but yields in the already warm, low-latitude regions of the southern United States are likely to decline. Evidence also suggests positive crop yield effects for mild to moderate temperature increases such as 2°C to 3°C (3.6°F to 5.4°F). However, once average global temperatures rise beyond about 4°C (7.2°F), yields begin to fall. Increases in precipitation level, timing, and variability may benefit semi-arid and other water-short areas by increasing soil moisture, but could aggravate problems in regions with excess water. Although most climate models predict precipitation increases, some regions will experience decreased precipitation, which could exacerbate water shortages and droughts. Higher carbon dioxide (CO2) levels in controlled experiments increase crop growth and decrease water use. However, these experiments often have demonstrated a more positive response than observed under actual field conditions.

Agricultural systems are most sensitive to extreme climatic events such as floods, wind storms, and droughts, and to seasonal variability such as periods of frost, cold temperatures, and changing rainfall patterns. Climate change could alter the frequency and magnitude of extreme events and could change seasonal patterns in both favorable and unfavorable ways, depending on regional conditions. Increases in rainfall intensity pose a threat to agriculture and the environment because heavy rainfall is primarily responsible for soil erosion, leaching of agricultural chemicals, and run off that carries livestock waste and nutrients into water bodies. Currently available climate forecasts cannot resolve how extreme events and variability will change; however, both are potential risks to agriculture. The rate of change is also uncertain. Adjustment costs are likely to be higher with greater rates of change.

Agricultural systems are managed. Farmers have a number of adaptation options open to them, such as changing planting and harvest dates, rotating crops, selecting crops and crop varieties for cultivation, consuming water for irrigation, using fertilizers, and choosing tillage practices. These adaptation strategies can lessen potential yield losses from climate change and improve yields in regions where climate change has beneficial effects. At the market level, price and other changes can signal further opportunities to adapt as farmers make decisions about land use and which crops to grow. Thus, patterns of food production respond not only to biophysical changes in crop and livestock productivity brought about by climate change or technological change, but also to changes in agricultural management practices, crop and livestock prices, the cost and availability of inputs, and government policies. In the longer term, adaptations include the development and use of new crop varieties that offer advantages under changed climates, or investments in new irrigation infrastructure as insurance against potentially less reliable rainfall. The extent to which opportunities for adaptation are realized depends upon a variety of factors such as information flow, access to capital, and the flexibility of government programs and policies.

Climate change can also have a number of negative indirect effects on agro-environmental systems effects that have been largely ignored in climate change assessments. These indirect effects include changes in the incidence and distribution of pests and pathogens, increased rates of soil erosion and degradation, and increased tropospheric ozone levels from rising temperatures. Regional shifts in crop production and expansion of irrigated acreage may stress environmental and natural resources, including water quantity and quality, wetlands, soil, fish, and wildlife.

The focus of this paper is on the impacts of climate change on agriculture. However, agriculture is also a potential source of greenhouse gas (GHG) emissions, and it can play an important role in mitigating these emissions. Methane from rice paddies and livestock, nitrous oxide (N2O) from cultivated soils and feedlots, and CO2 from the cultivation of virgin agricultural lands and intensive production methods contribute to global warming. Changes in management can reduce emissions from these sources. Agriculture can reduce atmospheric CO2 through tree-planting and similar programs that sequester significant amounts of carbon and through increased planting of biofuel crops that could replace fossil fuels.

The following describes the current understanding regarding the potential impacts of climate change on U.S. agriculture:

CROPS AND LIVESTOCK ARE SENSITIVE TO CLIMATE CHANGES IN BOTH POSITIVE AND NEGATIVE WAYS. Understanding the direct biophysical and economic responses to these changes is complicated and requires more research. In addition, indirect effects – such as changes in pests and water quality and changes in extreme climate events – are not well understood.


THE IMPACT OF CLIMATE CHANGE ON U.S. AGRICULTURE IS MIXED. Climate change is not expected to threaten the ability of the United States to produce enough food to feed itself through the next century; however, regional patterns of production are likely to change. Regions such as the Northern Great Plains and Great Lakes may have increased productivity while the Southern Plains, Delta states, and possibly the Southeast and portions of the Corn Belt could see agricultural productivity fall. However, the form and pattern of change are uncertain because changes in regional climate cannot be predicted with a high degree of confidence.

CONSIDERATION OF ADAPTATION AND HUMAN RESPONSE IS CRITICAL TO THE ACCURATE AND CREDIBLE ASSESSMENT OF CLIMATE CHANGE IMPACTS.However, because of the long time horizons involved in climate change assessments and uncertainties concerning the rate at which climate will change, it is difficult to predict accurately what adaptations people will make. This is particularly challenging since adaptations are influenced by many factors, including government policy, prices, technology research and development, and agricultural extension services.

BETTER CLIMATE CHANGE FORECASTS ARE KEY TO IMPROVE DASSESSMENTS OF THE IMPACTS OF CLIMATE CHANGE. In the meantime, farmers and the agricultural community must consider strategies that are economically and environmentally viable in the face of uncertainty about the course of climate change.

AGRICULTURE IS A SECTOR THAT CAN ADAPT, BUT THERE ARE SOME FACTORS NOT INCLUDED IN ASSESSMENTS THAT COULD CHANGE THIS CONCLUSION.Changes in the incidence and severity of agricultural pests, diseases, soil erosion, and tropospheric ozone levels, as well as changes in extreme events such as droughts and floods, are largely unmeasured or uncertain and have not been incorporated into estimates of impacts. These omitted effects could result in a very different assessment of the true impacts of climate change on agriculture. If the rate or magnitude of climate change is much greater than anticipated, adaptation could be more difficult and impacts could be greater than currently expected.
Overall, the consensus of economic assessments is that global climate change of the magnitudes currently being discussed by the Intergovernmental Panel on Climate Change (IPCC) and other organizations (i.e., +0.8°C to +4.5°C or +1.4°F to +8.1°F) could result in some lowering of global production but will have only a small overall effect on U.S. agriculture and its ability to provide sufficient food and fiber to both domestic and global customers over the next 100 years. However, distributional effects within the United States can be significant because consumers, producers, and local economies will gain in some regions and lose in others.

Warming beyond that reflected in current studies (i.e., associated with a continued rise in CO2 beyond the doubling that has been commonly investigated) is expected to impose greater costs, decreasing agricultural production in most areas of the United States and substantially limiting global production. This reinforces the need to determine the magnitude and rate of warming that may accompany the CO2 and greenhouse gas build-up currently underway in the atmosphere.