Science, Feb. 14, 2003
Study Shows Richer Harvests Owe Much to Climate
Since the 1940s, harvests across the United States have become ever more bountiful as farmers have planted better varieties of crops, generously fertilized them, and gained the upper hand against pests and weeds. But over the past 2 decades, they have had 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 vulnerable to shifts in climate than was previously suspected, a fact that could affect global food security. "It's an eye opener," says agricultural meteorologist Gene Takle of Iowa State University, Ames.
On page 1032, graduate student David Lobell and Gregory Asner, both of the Carnegie Institution of Washington and Stanford University, report an analysis of the role of climate and other factors in U.S. agriculture. They investigated the interplay among temperature, rainfall, amount of sunshine, and bushels of corn and soybeans per acre 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 the yields of corn and soybeans, 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 spot correlations amid 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. That was the case in about half of the counties where corn or soybeans were being grown--618 for corn, 444 for soybeans. (The amount of sunshine or rainfall was unrelated to changes in yield.)
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. "Gains from management have not been as high as we thought," Lobell says. "This has important implications for projections of future food production."Others agree. "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. "It is a little scary."
Lobell and Asner's analysis 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.
"These yield trends in the U.S. have global implications," says Kenneth Cassman, an agronomist at the University of Nebraska, Lincoln. For example, Cassman explains, a drop in U.S. production might stimulate more planting of soybeans in environmentally sensitive areas such as Amazonian watersheds in Brazil.
If climate in the United States should take an unfavorable turn, farmers might be hard pressed to compensate, says agronomist Cynthia Rosenzweig of Columbia University and NASA's Goddard Institute for Space Studies in New York City. That's because U.S. agriculture may be nearing its maximum efficiency, she says. But Don Duvick, a retired plant breeder from Pioneer Hi-Bred International, is sanguine. "If there were a trend to higher temperature, the breeders would churn out hybrids that can take it," he says. More vulnerable to climate change are developing countries, where temperatures are already high, soils are often poor, and management has a long way to go. "Climate change will put them even further behind," Rosenzweig says.
The study is not the last word, Rosenzweig and others emphasize. Lobell and Asner looked at only one aspect of climate; its variability and the number of extreme events, such as floods, also strongly affect yields. In addition, temperature shifts might influence harvests in other ways, such as by bringing more or less land into production, says Mark Rosegrant, director of the International Food Policy Research Institute in Washington, D.C. It may also be dicey to generalize from these counties to the nation as a whole. "It's so simplified, it's hard to say that this is truly reality," Rosenzweig says.
No one, however, disputes the bottom line: Small, gradual shifts in climate can play an important role in yield trends and therefore food supply. What needs to be done next is to expand the study to other regions of the world--something that Lobell is already working on--and to other crops.