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
The project is supported under the NSF Science, Engineering and Education for Sustainability Fellows (SEES Fellows) program, with the goal of helping to enable discoveries needed to inform actions that lead to environmental, energy and societal sustainability while creating the necessary workforce to address these challenges. Sustainability science is an emerging field that addresses the challenges of meeting human needs without harm to the environment, and without sacrificing the ability of future generations to meet their needs.
West Africa is highly vulnerable to climate hazards and better quantification and understanding of the impact of climate change on crop yields are urgently needed. Here we provide an assessment of near-term climate change impacts on sorghum yields in West Africa and account for uncertainties both in future climate scenarios and in crop models. Towards this goal, we use simulations of nine bias-corrected CMIP5 climate models and two crop models (SARRA-H and APSIM) to evaluate the robustness of projected crop yield impacts in this area. In broad agreement with the full CMIP5 ensemble, our subset of bias-corrected climate models projects a mean warming of +2.8 °C in the decades of 2031–2060 compared to a baseline of 1961–1990 and a robust change in rainfall in West Africa with less rain in the Western part of the Sahel (Senegal, South-West Mali) and more rain in Central Sahel (Burkina Faso, South-West Niger). Projected rainfall deficits are concentrated in early monsoon season in the Western part of the Sahel while positive rainfall changes are found in late monsoon season all over the Sahel, suggesting a shift in the seasonality of the monsoon. In response to such climate change, but without accounting for direct crop responses to CO2, mean crop yield decreases by about 16–20% and year-to-year variability increases in the Western part of the Sahel, while the eastern domain sees much milder impacts. Such differences in climate and impacts projections between the Western and Eastern parts of the Sahel are highly consistent across the climate and crop models used in this study. We investigate the robustness of impacts for different choices of cultivars, nutrient treatments, and crop responses to CO2. Adverse impacts on mean yield and yield variability are lowest for modern cultivars, as their short and nearly fixed growth cycle appears to be more resilient to the seasonality shift of the monsoon, thus suggesting shorter season varieties could be considered a potential adaptation to ongoing climate changes. Easing nitrogen stress via increasing fertilizer inputs would increase absolute yields, but also make the crops more responsive to climate stresses, thus enhancing the negative impacts of climate change in a relative sense. Finally, CO2 fertilization would significantly offset the negative climate impacts on sorghum yields by about 10%, with drier regions experiencing the largest benefits, though the net impacts of climate change remain negative even after accounting for CO2.
In a recent speech, Stanford professor Rosamond Naylor examined the wide range of challenges contributing to global food insecurity, which Naylor defined as a lack of plentiful, nutritious and affordable food. Naylor's lecture, titled "Feeding the World in the 21st Century," was part of the quarterly Earth Matters series sponsored by Stanford Continuing Studies and the Stanford School of Earth Sciences. Naylor, a professor of Environmental Earth System Science and director of the Center on Food Security and the Environment at Stanford, is also a professor (by courtesy) of Economics, and the William Wrigley Senior Fellow at the Freeman Spogli Institute for International Studies and the Stanford Woods Institute for the Environment.
"One billion people go to bed day in and day out with chronic hunger," said Naylor. The problem of food insecurity, she explained, goes far beyond food supply. "We produce enough calories, just with cereal crops alone, to feed everyone on the planet," she said. Rather, food insecurity arises from a complex and interactive set of factors including poverty, malnutrition, disease, conflict, poor governance and volatile prices. Food supply depends on limited natural resources including water and energy, and food accessibility depends on government policies about land rights, biofuels, and food subsidies. Often, said Naylor, food policies in one country can impact food security in other parts of the world. Solutions to global hunger must account for this complexity, and for the "evolving" nature of food security.
As an example of this evolution, Naylor pointed to the success of China and India in reducing hunger rates from 70 percent to 15 percent within a single generation. Economic growth was key, as was the "Green Revolution," a series of advances in plant breeding, irrigation and agricultural technology that led to a doubling of global cereal crop production between 1970 and 2010. But Naylor warned that the success of the Green Revolution can lead to complacency about present-day food security challenges. China, for example, sharply reduced hunger as it underwent rapid economic growth, but now faces what Naylor described as a "second food security challenge" of micronutrient deficiency. Anemia, which is caused by a lack of dietary iron and which Naylor said is common in many rural areas of China, can permanently damage children's cognitive development and school performance, and eventually impede a country’s economic growth.
Although hunger is more prevalent in the developing world, food insecurity knows no geographic boundaries, said Naylor. Every country, including wealthy economies like the United States, struggles with problems of food availability, access, and nutrition. "Rather than think of this as 'their problem' that we don't need to deal with, really it's our problem too," Naylor said.
She pointed out that one in five children in the United States is chronically hungry, and 50 million Americans receive government food assistance. Many more millions go to soup kitchens every night, she added. "We are in a precarious position with our own food security, with big implications for public health and educational attainment," Naylor said. A major paradox of the United States' food security challenge is that hunger increasingly coexists with obesity. For the poorest Americans, cheap food offers abundant calories but low nutritional value. To improve the health and food security of millions of Americans, "linking policy in a way that can enhance the incomes of the poorest is really important, and it's the hard part,” she said.” It's not easy to fix the inequality issue."
When asked whether there were any "easy" decisions that the global community can agree to, Naylor responded, "What we need to do for a lot of these issues is pretty clear, but how we get after it is not always agreed upon." She added, "But I think we've seen quite a few success stories," including the growing research on climate resilient crops, new scientific tools such as plant genetics, improved modeling techniques for water and irrigation systems, and better knowledge about how to use fertilizer more efficiently. She also said that the growing body of agriculture-focused climate research was encouraging, and that Stanford is a leader on this front.
Naylor is the editor and co-author of The Evolving Sphere of Food Security, a new book from Oxford University Press. The book features a team of 19 faculty authors from 5 Stanford schools including Earth science, economics, law, engineering, medicine, political science, international relations, and biology. The all-Stanford lineup was intentional, Naylor said, because the university is committed to interdisciplinary research that addresses complex global issues like food security, and because "agriculture is incredibly dominated by policy, and Stanford has a long history of dealing with some of these policy elements. This is the glue that enables us to answer really challenging questions."
Abstract: A rapidly growing body of research examines whether human conflict can be affected by climatic changes. Drawing from archaeology, criminology, economics, geography, history, political science, and psychology, we assemble and analyze the 60 most rigorous quantitative studies and document, for the first time, a striking convergence of results. We find strong causal evidence linking climatic events to human conflict across a range of spatial and temporal scales and across all major regions of the world. The magnitude of climate’s influence is substantial: for each one standard deviation (1σ) change in climate toward warmer temperatures or more extreme rainfall, median estimates indicate that the frequency of interpersonal violence rises 4% and the frequency of intergroup conflict rises 14%. Because locations throughout the inhabited world are expected to warm 2σ to 4σ by 2050, amplified rates of human conflict could represent a large and critical impact of anthropogenic climate change.
Abstract: Are violent conflict and socio-political stability associated with changes in climatological variables? We examine 50 rigorous quantitative studies on this question and find consistent support for a causal association between climatological changes and various conflict outcomes, at spatial scales ranging from individual buildings to the entire globe and at temporal scales ranging from an anomalous hour to an anomalous millennium. Multiple mechanisms that could explain this association have been proposed and are sometimes supported by findings, but the literature is currently unable to decisively exclude any proposed pathway. Several mechanisms likely contribute to the outcomes that we observe.
Forward-thinking companies, government organizations, and NGOs are beginning to link their efforts to build markets, promote environmental conservation, and reduce poverty in developing economies.
Join GDP for a discussion that explores potential synergies and challenges associated with linking these efforts. The panelists will share their own experiences and other promising models currently employed by companies, NGOs and government organizations around the world.
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Rosamond Naylor is the William Wrigley Professor in Earth System Science, a Senior Fellow at Stanford Woods Institute and the Freeman Spogli Institute for International Studies, the founding Director at the Center on Food Security and the Environment, and Professor of Economics (by courtesy) at Stanford University. She received her B.A. in Economics and Environmental Studies from the University of Colorado, her M.Sc. in Economics from the London School of Economics, and her Ph.D. in applied economics from Stanford University. Her research focuses on policies and practices to improve global food security and protect the environment on land and at sea. She works with her students in many locations around the world. She has been involved in many field-level research projects around the world and has published widely on issues related to intensive crop production, aquaculture and livestock systems, biofuels, climate change, food price volatility, and food policy analysis. In addition to her many peer-reviewed papers, Naylor has published two books on her work: The Evolving Sphere of Food Security (Naylor, ed., 2014), and The Tropical Oil Crops Revolution: Food, Farmers, Fuels, and Forests (Byerlee, Falcon, and Naylor, 2017).
She is a Fellow of the Ecological Society of America, a Pew Marine Fellow, a Leopold Leadership Fellow, a Fellow of the Beijer Institute for Ecological Economics, a member of Sigma Xi, and the co-Chair of the Blue Food Assessment. Naylor serves as the President of the Board of Directors for Aspen Global Change Institute, is a member of the Scientific Advisory Committee for Oceana and is a member of the Forest Advisory Panel for Cargill. At Stanford, Naylor teaches courses on the World Food Economy, Human-Environment Interactions, and Food and Security.
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Mark C. Thurber is Associate Director of the Program on Energy and Sustainable Development (PESD) at Stanford University, where he studies and teaches about energy and environmental markets and policy. Dr. Thurber has written and edited books and articles on topics including global fossil fuel markets, climate policy, integration of renewable energy into electricity markets, and provision of energy services to low-income populations.
Dr. Thurber co-edited and contributed to Oil and Governance: State-owned Enterprises and the World Energy Supply (Cambridge University Press, 2012) and The Global Coal Market: Supplying the Major Fuel for Emerging Economies (Cambridge University Press, 2015). He is the author of Coal (Polity Press, 2019) about why coal has thus far remained the preeminent fuel for electricity generation around the world despite its negative impacts on local air quality and the global climate.
Dr. Thurber teaches a course on energy markets and policy at Stanford, in which he runs a game-based simulation of electricity, carbon, and renewable energy markets. With Dr. Frank Wolak, he also conducts game-based workshops for policymakers and regulators. These workshops explore timely policy topics including how to ensure resource adequacy in a world with very high shares of renewable energy generation.
Dr. Thurber has previous experience working in high-tech industry. From 2003-2005, he was an engineering manager at a plant in Guadalajara, México that manufactured hard disk drive heads. He holds a Ph.D. from Stanford University and a B.S.E. from Princeton University.