As the planet warms, dire predictions of coastal flooding, inland droughts, ruined farmland, and global food shortages fill the news and research journals. But for all the talk of the future, scientists have little data on how climate change has already affected agriculture. A new study hopes to shed some light on this area.
"It's a frustration having to always answer questions about the future and having everyone think of climate change as something in the future," says David Lobell, an agricultural scientist at Stanford University in Palo Alto, California. "It's not something we have to anticipate. It's something we have to learn from and deal with right now."
Lobell and colleagues analyzed agricultural records of corn, rice, wheat, and soybeans from 1980 to 2008. Those four crops make up 75% of the calories consumed by the world's population. Using historical weather data on temperature and precipitation, the researchers constructed a trend line of weather patterns, controlling for a certain amount of seasonal variation, and linked it to crop data year by year. They also constructed a second trend line that assumed no warming during the period, and then compared the two.
Worldwide, the authors report online today in Science, yields of corn and wheat declined by 3.8% and 5.5%, respectively, compared with what they would have been without global warming. Rice and soybean production remained the same. But the trends vary considerably from region to region. Unlike most other regions, the United States and Canada saw no climate-linked decline in food production during this period. Lobell says that this is consistent with data from the Intergovernmental Panel on Climate Change (IPCC) showing that the eastern part of the United States has not warmed as much as other parts of the world for unknown reasons.
The most surprising finding, Lobell says, was how widely temperature trends varied by region and how dramatically they affected crop production. Rainfall, by contrast, seemed to contribute little to agricultural production trends. Although new technology and better farming practices have led to an overall increase in productivity, Lobell says, this increase isn't keeping pace with warming. As the group reports in the paper, a rise in temperature of 1°C tends to lower yields by 10% in countries that aren't at high latitudes.
While the paper is "an interesting contribution to the discussion," says John Reilly, an agricultural and energy economist at the Massachusetts Institute of Technology in Cambridge, Massachusetts, its conclusions are "not compelling." Many caveats and uncontrollable factors—including the price of crops, the latitude at which they are planted, and specific advances in technology—could contribute to the changes in trend that the authors see, weakening their link between yields and warming. "It's a careful set of work, but it's just a hard area to work in," Reilly says.
Reilly points out that IPCC predicts an increase, not a decrease, in global crop production, as more CO2 in the atmosphere is beneficial for plant growth. But Lobell says his analysis suggests that any advantage this CO2 boost confers is already being pushed to its limit, because warming itself harms crops.
The next step, Lobell says, is to look at areas where temperature is changing most rapidly to see how farmers are adapting their agricultural methods in response-and which types of agricultural improvements work best.