Policy brief: Climate trend and global crop production since 1980

Global warming is likely already taking a toll on world wheat and corn production, according to a new study led by Stanford University researchers. But the United States, Canada and northern Mexico have largely escaped the trend.

"It appears as if farmers in North America got a pass on the first round of global warming," said David Lobell, an assistant professor of environmental Earth system science and center fellow at the Program on Food Security and the Environment at Stanford University. "That was surprising, given how fast we see weather has been changing in agricultural areas around the world as a whole."

Lobell and his colleagues examined temperature and precipitation records since 1980 for major crop-growing countries in the places and times of year when crops are grown. They then used crop models to estimate what worldwide crop yields would have been had temperature and precipitation had typical fluctuations around 1980 levels.

The researchers found that global wheat production was 5.5 percent lower than it would have been had the climate remained stable, and global corn production was lower by almost 4 percent. Global rice and soybean production were not significantly affected.

The United States, which is the world's largest producer of soybeans and corn, accounting for roughly 40 percent of global production, experienced a very slight cooling trend and no significant production impacts.

Outside of North America, most major producing countries were found to have experienced some decline in wheat and corn (or maize) yields related to the rise in global temperature. "Yields in most countries are still going up, but not as fast as we estimate they would be without climate trends," Lobell said.

Lobell is the lead author of the paper, Climate Trends and Global Crop Production Since 1980, published May 5 online in Science Express.

Russia, India and France suffered the greatest drops in wheat production relative to what might have been with no global warming. The largest comparative losses in corn production were seen in China and Brazil.

Total worldwide relative losses of the two crops equal the annual production of corn in Mexico and wheat in France. Together, the four crops in the study constitute approximately 75 percent of the calories that humans worldwide consume, directly or indirectly through livestock, according to research cited in the study.

"Given the relatively small temperature trends in the U.S. Corn Belt, it shouldn't be surprising if complacency or even skepticism about global warming has set in, but this study suggests that would be misguided," Lobell said.

Since 1950, the average global temperature has increased at a rate of roughly 0.13 degrees Celsius per decade. But over the next two to three decades average global temperature is expected to rise approximately 50 percent faster than that, according to the report of the Intergovernmental Panel on Climate Change. With that rate of temperature change, it is unlikely that the crop-growing regions of the United States will continue to escape the rising temperatures, Lobell said.

"The climate science is still unclear about why summers in the Corn Belt haven't been warming. But most explanations suggest that warming in the future is just as likely there as elsewhere in the world," Lobell said.

"In other words, farmers in the Corn Belt seem to have been lucky so far."

This is the first study to come up with a global estimate for the past 30 years of what has been happening, Lobell said.

To develop their estimates, the researchers used publicly available global data sets from the United Nations Food and Agriculture Organization and from the University of Delaware, University of Wisconsin, and McGill University.

The researchers also estimated the economic effects of the changes in crop yield using models of commodity markets.

"We found that since 1980, the effects of climate change on crop yields have caused an increase of approximately 20 percent in global market prices," said Wolfram Schlenker, an economist at Columbia University and a coauthor of the paper in Science.

He said if the beneficial effects of higher carbon dioxide levels on crop growth are factored into the calculation, the increase drops down to 5 percent.

"Five percent sounds small until you realize that at current prices world production of these four crops are together worth nearly $1 trillion per year," Schlenker said. "So a price increase of 5 percent implies roughly $50 billion per year more spent on food."

Rising commodity prices have so far benefited American farmers, Lobell and Schlenker said, because they haven't suffered the relative declines in crop yield that the rest of the world has been experiencing.

"It will be interesting to see what happens over the next decade in North America," Lobell said. "But to me the key message is not necessarily the specifics of each country. I think the real take-home message is that climate change is not just about the future, but that it is affecting agriculture now. Accordingly, efforts to adapt agriculture such as by developing more heat- and drought-tolerant crops will have big payoffs, even today. "

Justin Costa-Roberts, an undergraduate student at Stanford, is also a coauthor of the Science paper. David Lobell is a researcher in Stanford's Program on Food Security and the Environment, a joint program of Stanford's Woods Institute for the Environment and Freeman Spogli Institute for International Studies. Schlenker is an assistant professor at the School of International and Public Affairs and at the Department of Economics at Columbia.

The work was supported by a grant from the Rockefeller Foundation.

Sugarcane - a principal crop for biofuel - reduces the local air temperature compared to pasturelands or fields growing soybeans or maize, according to a new study from researchers at Stanford University and the Carnegie Institution for Science. But sugarcane's effect on temperature is a "double-edged machete," as it increases ambient temperatures compared with natural vegetation.

These small local changes should be taken into consideration in studies of global climate change, the researchers said.

The researchers looked at changes in vegetation in the Brazilian Cerrado - a vast tropical savanna lying south of the Amazon basin - large areas of which have been converted from natural vegetation to agriculture in recent decades.

Increasingly, these existing agricultural areas are now being converted to sugarcane for use in biofuel production. Brazil is now second only to the United States in ethanol production, much of which is used domestically.

What the effect on global climate would be if sugarcane farming were to expand significantly is not yet clear, said David Lobell, an assistant professor in environmental Earth system science at Stanford and center fellow at the Program on Food Security and the Environment.

"The temperature changes are happening locally, where the land-use change is happening," Lobell said. "It does not seem to spill over into other countries, for example, at least as far as we can tell right now."

But Lobell said sugarcane growing in the Cerrado is definitely expanding and given that the region encompasses approximately 1.9 million square kilometers (733,000 square miles) - an area larger than Alaska - the potential exists for a globally significant effect.

Using maps and data from hundreds of satellite images, the researchers calculated the temperature, the amount of water given off and how much light was reflected rather than absorbed for each of the different types of vegetation. They found that compared to land cultivated with other annual crops, sugarcane reduced the local air temperature by an average of 0.93 degrees Celsius (1.67 F).

But compared to the natural vegetation of the Cerrado - mainly grass and shrubs - the sugarcane fields warmed the ambient air by 1.55 C (2.79 F).

Lobell said the bulk of the temperature difference is due to evapotranspiration - the moisture released to the air through the leaves of the plants and the soil. Most of the land put into sugarcane had previously been converted from natural vegetation to pastureland, said Scott Loarie, a postdoctoral researcher at Carnegie. "If someone has a farm that once was natural vegetation, that transition to pasture and annual crops caused local warming," he said. "So now as the farm is going to sugarcane, by comparison it is cooling temperatures locally."

Their research, Direct Impacts on Local Climate of Sugarcane Expansion in Brazil, is described in the current issue of Nature Climate Change.

This local cooling does not necessarily mean that the global climate is cooling as a result. It depends in part on what happens with the agriculture that was displaced by the sugarcane, Loarie said. For example, if cattle used to graze on a tract of land and some Amazon forest is cut down to provide new pasture for them, net carbon emissions will actually increase.

"You might not make any difference as far as cooling the world globally at all; in fact, you might make the world marginally warmer," he said.

"The global implications of these local effects were not a part of this study, and any discussion of mitigating global climate should consider the potential for these land use cascades."

One of the important aspects of the study, Lobell said, is that it demonstrates how satellite data can be used in real time to understand the effects of environmental changes. Most research studying the impact of biofuel use on climate has been done with computer modeling.

"I think the coolest thing about this study is you actually can see these temperature effects happening already," Lobell said. "In terms of the more general point about bio energy, I think it is another good example of why looking only at greenhouse gases is not the full picture."

Another takeaway from the study, Loarie said, is that the temperature findings support the existing rule of thumb that biofuel crops are best located on land that is already used for agriculture. That general guideline stems from the fact that there is less carbon released to the atmosphere by converting land where the existing vegetation contains low amounts of carbon, such as pasture or crops, than by cutting down the dense, carbon-rich forests in the Amazon.

Loarie said that while the study clearly showed that planting sugarcane moves the temperature closer to what it would have been if the natural vegetation had not been removed from the land, that doesn't mean the land is any closer to its natural state in other respects.

"Converting pasture to sugarcane is definitely not ecological restoration," said Chris Field, a professor of biology and of environmental Earth system science, who was involved in the research.

"Still, the direct effect on climate is potentially important enough to play a role in future decisions about land use and land management in large parts of the tropics," he said.

The study was funded by the Stanford University Global Climate and Energy Project.

Greg Asner, a professor, by courtesy, of environmental Earth system science, is a coauthor of the paper. Lobell is also a center fellow at both the Freeman Spogli Institute for International Studies and the Woods Institute for the Environment. Field is also a senior fellow at the Precourt Institute for Energy and at the Woods Institute, and director of the Department of Global Ecology at the Carnegie Institution.