Agriculture

-- An overview prepared by Janine Bloomfield, Ph.D. (Environmental Defense) and
Francesco Tubiello, Ph.D. (Columbia University) --

Climate and changes in it – regardless of their cause – matter to people, communities and businesses. Global warming is likely to bring many changes to the nation. The United States as a whole is in a strong economic position to adapt to many of these changes, but adaptation is often expensive, not always possible or successful, and during transitions ecosystems, communities, and individuals could suffer. Moreover, national impact summaries disguise local dislocations and disruptions to the ways we live, work and recreate. Climate change adds a serious stress to our already threatened resources and treasured places. Overall impact statements also mask significant opportunities. To minimize the negative changes and make the most of the positive changes we need to take a close look at how climate change will affect each region. How will U.S. agriculture experience the effects of global warming? And how can we respond?

Key Findings

Climate change models project that much of America's agricultural land will experience warming of 5 to 10° F by 2100. Droughts could increase in some areas, while others might experience substantially more precipitation than today. The US National Assessment investigated the potential consequences of climate variability and change to agricultural production. Its report suggests that future climate change, higher concentrations of atmospheric CO2, and appropriate farmer adaptation, could result in higher crop yields for the US as a whole. Nonetheless, projections at the regional level are mixed, with winners and losers. In general, northern production areas are expected to gain from an increased length of the growing seasons and warmer temperatures, while southern areas might be at risk from increased temperature and/or water stress. It is therefore reasonable to expect disruptions for individual farmers and some rural communities, as the geographical distribution of production areas will likely change.

The scenarios of higher temperatures and considerably more rainfall employed in the National Assessment suggest that heat waves, droughts, and especially floods, may occur more often, last longer, and inflict greater damage to crops than they do today. Production risks include direct physical plant damage by flooding and water-logging, as well as related problems such as increased pest and pathogen outbreaks, enhanced soil erosion, and threatened groundwater quality from increased pesticide and herbicide runoff. Current state-of-the-art crop models like those used in the National Assessment do not fully capture yield reductions due to increased climate variability and related disturbances. Once these effects are fully incorporated into the models, it is expected that the projections of future US crop yields under climate change could be significantly lower than currently estimated.

Additional Potential Impacts and Considerations

The consequences of higher temperatures on crop yields and livestock
Warmer temperatures have negative effects on crop yields except at northernmost latitudes, where a lengthening of the growing season is beneficial to crop growth. For example, high temperatures (over 90°F) can significantly decrease corn yield, and very high temperatures (over 100°F) can cause severe damage to this crop. The National Assessment reports that in several regions, particularly the Southeast, Gulf states and other southern production areas where current temperatures are already high, warmer climates generally lead to decreased yields of irrigated crops. Adaptations, including early sowing and using crop varieties better suited to higher temperatures could lower yield losses.

Livestock are also sensitive to high heat, leading to lowered animal productivity and dairy production. Specific adaptations, including heat control in large covered operations and use of heat-tolerant livestock species, could help mitigate these problems.

Pests, diseases, and weeds
Crop growth models used in the National Assessment do not include the effects of global warming on pests, diseases and weeds. If implemented in the models, the positive outcome on yields predicted would likely have been less. Warmer conditions allow more rapid maturation and reproduction of many insects and may also allow the northward migration of warm-weather species. Wetter conditions favor soil pathogens. Warmer conditions and higher CO2 concentrations will favor many weed species, increasing their impacts on crop yields, and/or increasing requirements for agricultural chemicals or integrated pest management.

A range of consequences depending on precipitation changes
All climate models predict a generalized increase in surface temperature for the next century. However, regional precipitation is predicted poorly, and the models often disagree even on the direction of precipitation change over most areas. Within the National Assessment, which considers one “wetter” and one “drier” scenario of climate change, there are some regional disagreements. Perhaps the most important of such cases is rain-fed hard red winter wheat production in western Kansas, a key US breadbasket region. There, the generally wetter model (Hadley climate scenario) predicts higher annual precipitation than at present and projects an increase of rain-fed wheat production on average by 30%. Under the generally hotter and drier model (the Canadian climate scenario), rain-fed wheat production would be severely affected by a marked decrease in precipitation, and future average yields could decrease by 30% or more and farms could experience an increased risk of crop failures. Similar situations were found for rice, sorghum and soybean production in the Southeast coastal regions of Louisiana (the Mississippi delta), Alabama, and Florida, which were severely affected under the Canadian climate projections, but not under the Hadley scenario.

Positive crop yields are strongly linked to CO2 fertilization
The positive effects of elevated CO2 (fertilization) on crop growth, implemented in today’s crop models using data from controlled-environment studies, significantly contribute to the projected yield increases found in the National Assessment. If CO2 effects on plant growth were smaller in the field than found in the laboratory, the potential for negative impacts of climate change on crop yields would be greater than currently suggested by the assessment.

How American Farmers Can Adapt

Climate scientists agree that further climate change may be inevitable and will therefore require adaptation, although most scientists also think that the pace of climate change can be slowed by substantially reducing greenhouse gas emissions. This would give governments, businesses, and ecosystems around the world more time to respond and adapt to climate change as well as reducing the overall severity of climate change-related impacts, thereby buying "insurance" for an uncertain future. Another way to buy insurance now is to incorporate climate change into all long-term decisions about natural resources, thereby providing greater resilience.

The scenarios analyzed in the National Assessment represent a range of outcomes, all of which are plausible physical representations of a possible future. Because we can't be sure how the future will unfold, there is much value in developing response strategies that minimize risk under the potentially negative cases, while researching methods that maximize gains under the potentially positive ones.
Farmers and ranchers can adopt a number of strategies to reduce the potential negative impacts and to take advantage of possible benefits of global warming. These include:

• changing sowing dates to exploit a lengthening of the growing seasons and to avoid excessive heat stress in the summer;


• adopting different cropping systems that take advantage of warmer climates;


• adopting new crop varieties better suited to the changed climate (warmer temperatures, increased water stresses, etc.), as they become available through either genetic engineering or standard plant breeding techniques;


• increasing the use of conservation tillage to better retain soil organic matter in the face of high temperatures and increased flooding;


• implementing the use of short-term climate prediction to reduce losses due to weather variability.

Some consequences of global warming could be hard to adapt to. These include extreme events such as floods, prolonged or repeated droughts and heavy downpours. Farmers can play an important role in mitigating climate change by using many practices that make sense in terms of good farm management as well as the bottom-line. Such practices include:

• storing or sequestering carbon in soil through the use of cover crops, improved fertilization techniques and conservation tillage;


• installing permanently vegetated conservation buffers such as riparian strips;


• restoring marginal agricultural lands to grasslands, forest lands, or wetlands;


• avoiding emissions of carbon through decreased fossil fuel use and using methane-recovery systems for liquid manure, such as digesters or covered lagoons, to reduce methane emissions and provide on-farm sources of biogas fuel for large livestock operations.

Additional Information
For more detailed information on the potential impacts of global warming on agriculture, several experts are available to answer your questions:

Dr. Francesco Tubiello, Columbia University and NASA-GISS
 Tel.: 212-678-5585; e-mail: franci@giss.nasa.gov
Dr. Cynthia Rosenzweig, NASA-GISS
 Tel.: 212-678-5562; e-mail: crosenzweig@giss.nasa.gov
Dr. Janine Bloomfield, Environmental Defense
 Tel: 212-505-2100; e-mail: jbloomfield@environmentaldefense.org