Recent Research Suggests that Global Warming Poses a Significant Threat to U.S. Agriculture

Most scientists initially believed that global warming does not pose a significant threat to U.S. agriculture. Scientists hypothesized that U.S. farmers would be able to adapt to increased temperatures. Furthermore, early research suggested that rising atmospheric carbon dioxide levels would boost crop yields. Conventional wisdom held that while global warming was likely to harm natural systems (arctic tundra, forests, remnant prairies etc.), global warming was unlikely to harm agricultural systems.

Recent research suggests that global warming does pose a significant threat to U.S. agriculture. First, the newest generation of climate models project larger temperature increases than past climate models projected. Second, in contrast to initial expectations, recent experimental research suggests that elevated carbon dioxide concentrations will not boost yields of corn, sorghum, and other C4 crops. Third, observational studies indicate that global warming may already be increasing flood frequencies in the Midwest - one of the primary agricultural regions in the United States.

The newest generation of climate models project larger temperature increases than past climate models projected.

In 2001, the Intergovernmental Panel on Climate Change (IPCC), a very reputable group of 200+ scientists from around the world, predicted that the global mean surface temperature will increase by approximately 2.5 degrees Celsius by 2100. The IPCC's prediction was based on projections from the best global climate models in existence at the time.

Projections by the newest generation of global climate models suggest that the global mean surface temperature will increase by more than 2.5 degrees Celsius by 2100. The HadCM3 Model projects that the global mean surface temperature will increase by 2.9 to 3.3 degrees Celsius within the coming century. T. Johns et al. (2003). The HadGEM1 Model projects that the global mean surface temperature will increase by 3.4 to 3.8 degrees Celsius within the coming century. P. Stott et al. (2006). Assuming moderate future carbon dioxide emissions, the surface temperature over the United States is projected to increase by 4.0 to 8.0 degrees Celsius by the HadCM3 Model, by 3.0 to 5.0 degrees Celsius by the HadGEM1 Model, and by 2.0 to 4.5 degrees Celsius by the CCSM3 Model within the coming century. Id.

The magnitude of these projected temperature increases is alarming. To put these projections in context, during the 20th Century the global mean surface temperature increased by 0.6 degrees Celsius. This 0.6 degree Celsius increase was unprecedented - a substantially greater increase than the world had experienced during the previous 900 years. P. Stott et al., figures 4 & 5 (2006); M. Salinger, figure 1 (2005).

During the coming century, warmer temperatures will cause heat stress and likely decrease crop yields. A. Thomson et al. (2005); J. Antle et al. (2001). Global warming will likely cause less harm to livestock yields than to crop yields, but will increase the amount that livestock farmers have to spend to air condition confinement buildings.

Recent experimental research suggests that elevated carbon dioxide concentrations will not boost yields of corn, sorghum, and other C4 crops.

Initial experiments to determine the effects of elevated atmospheric carbon dioxide on crop growth were conducted in enclosed chambers. These initial experiments suggested that rising atmospheric carbon dioxide levels would boost crop yields.

To more accurately simulate field conditions, University of Illinois scientists are using a method referred to as "free-air concentration enrichment" or "FACE" to study the effects of elevated carbon dioxide on corn yields. FACE technology involves rings of pipes that pump carbon dioxide into the open air surrounding crops grown in fields. Results from experimental research using FACE technology indicate that elevated carbon dioxide levels do not boost corn yields. A. Leakey et al. (2006); S. Long et al. (2006). This research suggests that elevated carbon dioxide may not boost C4 crop yields (e.g. corn and sorghum yields). (Elevated carbon dioxide does boost C3 crop yields (e.g. soybean yields).)

This recent research has important implications. Scientists who projected that global warming would not harm U.S. agriculture assumed that elevated carbon dioxide levels would boost corn yields, offsetting the negative impacts of heat stress. This recent research indicates that global warming will likely have a net negative impact on U.S. corn yields; harm to corn yields from heat stress will not be offset by benefits from elevated atmospheric carbon dioxide levels.

Observational studies indicate that global warming may already be increasing the frequency of heavy rainfall events and flooding in the Midwest.

Recent studies indicate that the frequency of heavy precipitation events has been increasing in recent decades, paralleling increases in atmospheric carbon dioxide concentrations. Kenneth Kunkel et al. (1999) found that the frequency of 7-day heavy precipitation events has increased in the upper midwest (Iowa, Michigan, Minnesota, and Wisconsin) during the 20th Century. Pavel Groisman et al. (2004) found that the frequency of 1-day heavy precipitation events has increased significantly over the entire central United States, from Minnesota and Michigan to Texas and Louisiana during the 20th Century.

Kunkel's and Groisman's studies are consistent with theoretical evidence that suggests that warmer conditions increase the frequency of heavy precipitation events and projections from global climate models that project increased flooding in the future. These observational studies are important because they indicate that global warming may already be causing flood frequency to increase. Pavel Groisman et al. concluded: "Clearly more work is needed, but the evidence is growing that the observed historical trends of increasing very heavy precipitation are linked to global warming." P. Groisman et al. (2005). In a 2000 Science article, David Easterling et al. asserted that global warming has "likely" (67 to 90% certainty) caused increases in the frequencies of heavy precipitation events in the United States during the 20th Century. D. Easterling et al. (2000).


Substantial uncertainty remains regarding how effectively U.S. farmers will be able to adapt to global warming. On one hand, scientific innovation has generated substantial yield increases during the past 150 years, unimaginable 150 years ago. On the other hand, it is difficult to believe that scientific innovation will be sufficient to counter the negative impacts of 2 to 8 degree Celsius temperature increases in combination with increases in spring flood, summer drought, and heat wave frequencies. Recent scientific research suggests that persons concerned about the future of U.S. agriculture should take the threat posed by global warming seriously.