The European Union led the world in wheat production and exports in 2014-15. Yet Europe is also the region where productivity has slowed the most. Yields of major crops have not increased as much as would be expected over the past 20 years, based on past productivity increases and innovations in agriculture.

Finding the causes of that stagnation is key to understanding the trajectory of the global food supply.

Logically, it would seem that climate change would affect crops. But in the overall picture of agriculture, it's hard to figure out how much. European farming is a complex venture, and other possible stagnating factors include changes in government policy. For example, farm subsidies are no longer based on productivity and the use of fertilizer is now controlled to reduce runoff into water supplies. Ongoing positive factors include improvements in farm management practices and advances in crop genetics.

Historically, scientists relied on models to estimate the effects of climate change. Now Stanford's Frances C. Moore has for the first time statistically quantified the relative importance of climate in the stagnation of European crops. She found that warming and precipitation trends are affecting European grain harvests. Moore is a PhD candidate in the Emmett Interdisciplinary Program in Environment and Resources.

"This study is sobering in that it shows climate drags on some of the crops in this region," said David Lobell, co-author of the paper. "Yet this new approach to looking at the problem will help us understand more quickly what impacts require more attention, and that can only be positive in the long term." Lobell is an associate professor of environmental Earth system science and the deputy director of the Center on Food Security and the Environment at Stanford. He is also a senior fellow in the Freeman Spogli Institute for International Studies and the Stanford Woods Institute for the Environment. He studies ways to improve crop yields in major agricultural regions, with emphasis on adaptation to climate change.

"This is a major step in using quantitative analysis to disentangle the effect of climate change in a complicated system," said Dáithí Stone, a pioneer in comparing actual seasonal weather forecasts with what those forecasts would have been if human activities had not emitted greenhouse gases. "It demonstrates that the signal has become large enough that we may see the effect of climate change in a complicated system like agriculture." Stone is a research scientist in the Computational Chemistry, Materials and Climate Group of Berkeley Lab.

How wheat and corn and barley grow

Moore considered two factors in the study: actual crop yields and expected crop yields given historic temperature and precipitation trends. She applied statistical analyses to look for patterns in regional maps of actual European yields of wheat, maize (known in the United States as corn), barley and sugar beets, from 1989 to 2009.

The study found that climate trends can explain 10 percent of the slowdown in wheat and barley yields, with changes in government policy and agriculture likely responsible for the remainder of the stagnation. Moore found evidence that long-term temperature and precipitation trends since 1989 reduced overall European yields of wheat by 2.5 percent and barley by 3.8 percent, while slightly increasing maize and sugar beet yields.

Moore also wanted to find out to what extent farmers had adapted their practices to accommodate changing conditions. She applied power analysis, a statistical tool to test the effect of adaptation. But she discovered the test was not effective in the context of this study.

"We think farmers have been hurt already by warming and drying trends in Italy," Moore said. Undaunted by the limits of statistical analysis to measure farmer adaptation, she is planning another way to find out. "I have been doing this work in front of a computer – in the future I would like to go to Italy," she said. "It would be interesting to talk to the farmers."

Leslie Willoughby is an intern at Stanford News Service.

Media Contact

Frances C. Moore, Emmett Interdisciplinary Program in Environment and Resources: (617) 233-3380, fcmoore@stanford.edu

Dan Stober, Stanford News Service: (650) 721-6965, dstober@stanford.edu

New research by a Stanford team shows that climate change is expanding the amount of U.S. agricultural land that is suitable for harvesting two crops per growing season, a system known as double cropping. The practice offers higher productivity and more income for American farmers, but future yield losses from climate change may still outstrip the gains from double cropping. 

In a new study in the journal Environmental Research Letters, Stanford PhD student Christopher Seifert and professor David Lobell find that between 1988 and 2012, the area of farmland in the United States on which farmers were able to harvest two crops per year on the same plot of land grew by as much as 28 percent as a result of warmer temperatures and later fall freezes. Applying their model to two future climate change scenarios, the team projects that the amount of land suitable for double cropping in the United States – in this case, winter wheat followed by soybeans – may double or even triple by the end of the century.

Seifert and Lobell’s analysis includes 22 U.S. states east of the continental divide. They define the area suitable for double cropping as having at least 750 mm per year of rainfall and a 75 percent likelihood that both crops will survive to harvest.

The team built a first-of-its-kind model for a double cropping combination of winter wheat and soybeans, to measure the expansion of farmland that has become theoretically suitable to double cropping since 1988. Combining the model with existing U.S. government data, they find that their estimate of 28 percent growth closely mirrors the actual observed expansion of double cropping in the United States over this time period.

Seifert and Lobell then applied their model to two future climate change scenarios and found that as average temperatures rise, the area suitable for double cropping will likely grow steadily until 2060, then spike sharply between 2060 and 2080. Expansion is projected to slow between 2080-2100, as parts of the South become unsuitable due to a lack of the cold winter temperatures that winter wheat requires.

An expansion of double cropping area could be an important tool for U.S. farmers to protect against the negative effects of climate change on agriculture productivity. Yields of major staple crops like corn, soybeans and wheat are already showing increasing vulnerability to extreme heat, especially for plants that go through critical growth stages such as pollination during the hot summer months. Double cropping can help protect against these risks, and provide other benefits such as year-round ground cover that reduces soil erosion.

The new study does not incorporate data about yields, potential yields, or the changing moisture requirements of each crop as temperatures rise. Adding these factors to future analysis will improve scientists’ understanding of the value of double cropping, said lead author Seifert, a PhD student in environmental earth system science at Stanford.

The study also suggests that the negative impacts that climate change is expected to have on crops like corn and soybeans will likely be larger than the boost that double cropping can offer.

“Double cropping can be an important tool, but it’s important not to overstate its potential to ‘save’ American agriculture from climate change,” said co-author David Lobell, a professor of environmental earth system science and the deputy director of the Center on Food Security and the Environment at Stanford (FSE). FSE is a joint effort of the Freeman Spogli Institute for International Studies and the Stanford Woods Institute for the Environment.

“In the United States, double cropping can potentially make agriculture more resilient to climate change by improving overall productivity and by increasing farmers’ annual incomes,” said Seifert. “But the gains from double cropping will probably not be able to make up for the overall drop in crop yields that we expect to see with future climate change.”

CONTACT:

Christopher Seifert, Ph.D. student, Environmental Earth System Science, Stanford: cseifert@stanford.edu

David Lobell, Professor, Environmental Earth System Science, Stanford: dlobell@stanford.edu

Laura Seaman, Communications Manager, Center on Food Security and the Environment: lseaman@stanford.edu, 650-723-4920

To predict how agriculture will be affected by future climate change, scientists often rely on a single crop model – a computer simulation of how a specific crop’s yield responds to temperature changes. By combining 30 such models into a single study, and comparing each model against data from existing experimental wheat fields around the world, a team of researchers including Stanford professor David Lobell have developed a more powerful and accurate way to predict future wheat yields.

In a new analysis published in Nature Climate Change, the team’s results support previous work suggesting that wheat yields around the world are sensitive to rising temperatures. Using the new method of analysis, the team estimates an average six percent future yield loss for every one degree Celsius rise in global mean temperature.

“Combining 30 models gives us a much greater ability to predict future impacts and understand past impacts,” said Lobell. “This is a clear step forward.”

Lobell is professor of environmental earth system science in the School of Earth Science at Stanford and the deputy director of the Center on Food Security and the Environment. He is a senior fellow at the Stanford Woods Institute for the Environment and at the Freeman Spogli Institute for International Studies.

The estimated six percent yield loss for every degree increase is equivalent to about a quarter of the current volume of wheat traded globally in 2013. Yields at some sites, notably those in Mexico, Brazil, India and Sudan, show simulated wheat yield losses of more than 20 percent - in Sudan’s case, more than 50 percent - under a scenario in which global mean temperature rises by two degrees Celsius.

With higher temperatures also comes an increase in the variability of wheat yields, both by location and between years. More fluctuation in wheat yields could mean greater global price volatility for the staple crop.

Approximately 70 percent of the wheat produced today is grown either on irrigated plots or in rainy regions. The research team accounted for this factor by focusing its simulations on multiple regional-specific varieties of wheat that are commonly grown under these conditions.

The new paper includes several suggestions for avoiding some of the predicted yield losses. For example, some varieties of wheat are more heat tolerant than others, and farmers in the places hardest hit by rising temperatures could switch varieties to capitalize on this heat resistance. The effects of rising temperatures could also be managed, in part, by adjusting sowing and harvesting dates, or changing the way fertilizers are applied to crops.

Contact: David Lobell, dlobell@stanford.edu