Since the 1940s, harvests across the United States have become ever more bountiful as farming technology has improved. But over the past 2 decades, farmers have had more than a little help: A new study shows that a surprisingly high percentage of the improvement in yield was due not to farm management but to climate change. The finding suggests that food production in the United States may be more sensitive to shifts in climate than was previously suspected, a fact that could affect global food security.
Graduate student David Lobell and Gregory Asner, both of the Carnegie Institution of Washington and Stanford University, investigated the interplay among temperature, rainfall, amount of sunshine, and corn and soybeans yields from 1982 to 1998. During this time, summers in a large swath of the Midwest became slightly cooler. Lower temperatures in the region are known to boost corn and soybeans yields, which rose about 30% over the study period. The United States leads the world in production of the two crops.
Lobell and Asner wanted to tease out the impact of those gradual climate shifts relative to other influences on yield, such as farming practices. To reduce the statistical noise, they picked counties throughout the United States where yields had responded to climate in the same way, either rising in cooler summers or falling in warmer ones. Using a statistical model to compare these climate variations among counties with changes in yield, the researchers found that the cooling climate was responsible for about 20% of the gains over the 17 years. The remainder they credit to management and other factors, such as increased carbon dioxide in the atmosphere.
"This points out that our food production may be more vulnerable to shifts in climate than we thought," says Jonathan Foley, a climatologist at the University of Wisconsin, Madison. Lobell and Asner's analysis, reported in the 14 February issue of Science, indicates that yields would drop by 17% for each degree that the growing season warms. That's three times as much as other studies have suggested. Most climate models predict that the Corn Belt of the U.S. Midwest will warm over the next few decades. Other experts point out, however, that many other aspects of climate could have an impact too.