A critical question for agricultural production and food security is how water demand for staple crops will respond to climate and carbon dioxide (CO2) changes1, especially in light of the expected increases in extreme heat exposure2. To quantify the trade-offs between the effects of climate and CO2 on water demand, we use a ‘sink-strength’ model of demand3,4 which relies on the vapour-pressure deficit (VPD), incident radiation and the efficiencies of canopy-radiation use and canopy transpiration; the latter two are both dependent on CO2. This model is applied to a global data set of gridded monthly weather data over the cropping regions of maize, soybean, wheat and rice during the years 1948–2013. We find that this approach agrees well with Penman–Monteith potential evapotranspiration (PM) for the C3 crops of soybean, wheat and rice, where the competing CO2 effects largely cancel each other out, but that water demand in maize is significantly overstated by a demand measure that does not include CO2, such as the PM. We find the largest changes in wheat, for which water demand has increased since 1981 over 86% of the global cropping area and by 2.3–3.6 percentage points per decade in different regions.
One of the greatest challenges in monitoring food security is to provide reliable crop yield information that is temporally consistent and spatially scalable. An ideal yield dataset would not only extend globally and across multiple years, but would also have enough spatial granularity to characterize productivity at the field and subfield level. Rapid increases in satellite data acquisition and platforms such as Google Earth Engine that can efficiently access and process vast archives of new and historical data offer an opportunity to map yields globally, but require efficient and robust algorithms to combine various data streams into yield estimates. We recently introduced a Scalable satellite-based Crop Yield Mapper (SCYM) that combines crop models simulations with imagery and weather data to generate 30 m resolution yield estimates without the need for ground calibration. In this study, we tested new large-scale implementations of SCYM, focusing on three regions with varying crops, field sizes and landscape heterogeneity: maize in the U.S. corn belt (390,000 km2), maize in Southern Zambia (86,000 km2), and wheat in northern India (450,000 km2). As a benchmark, we also tested a simpler empirical approach (PEAKVI) that relates yield to the peak value of a time series of spatially aggregated vegetation indices, similar to methods used in current operational monitoring. Both SCYM and PEAKVI were applied to data from all Landsat's sensors and MODIS for more than a decade in each region, and evaluated against ground-based estimates at the finest available administrative level (e.g., counties in the U.S.). We found consistently high correlations (R2 ≥ 0.5) between the spatial pattern of ground- and satellite-based estimates in both U.S. maize and India wheat, with small differences between methods and source of satellite data. In the U.S., SCYM outperformed PEAKVI in tracking temporal yield variations, likely owing to its explicit consideration of weather. In India, both methods failed to track temporal yield changes, with various possible explanations discussed. In Zambia, the PEAKVI approach applied to MODIS tracked yield variations much better (R2 > 0.5) than any other yield estimate, likely because the frequent cloud cover in this region confounds the other approaches. Overall, this study demonstrates successful approaches to yield estimation in each region, and illustrates the importance of distinguishing between accuracy for spatial and temporal variation. The 30 m resolution of Landsat-based SCYM does not appear to offer large benefits for tracking aggregate yields, but enables finer scale analyses than possible with the other approaches.
Temperature data are commonly used to estimate the sensitivity of many societally relevant outcomes, including crop yields, mortality, and economic output, to ongoing climate changes. In many tropical regions, however, temperature measures are often very sparse and unreliable, limiting our ability to understand climate change impacts. Here we evaluate satellite measures of near-surface temperature (Ts) as an alternative to traditional air temperatures (Ta) from weather stations, and in particular their ability to replace Ta in econometric estimation of climate response functions. We show that for maize yields in Africa and the United States, and for economic output in the United States, regressions that use Ts produce very similar results to those using Ta, despite the fact that daily correlation between the two temperature measures is often low. Moreover, for regions such as Africa with poor station coverage, we find that models with Ts outperform models with Ta, as measured by both R2 values and out-of-sample prediction error. The results indicate that Ts can be used to study climate impacts in areas with limited station data, and should enable faster progress in assessing risks and adaptation needs in these regions.
The Global Development and Poverty Initiative (GDP) seminar series returns with a reprise of its most popular seminar last year. Join us for a stimulating discussion on the opportunities, obstacles, and unforeseen events encountered while conducting field research in the developing world.
The panelists will share stories of challenges and successes from their own experiences and will offer insights on conducting effective research in the field.
Encina Commons, Room 102,
615 Crothers Way,
Stanford, CA 94305-6019
(650) 723-0984
(650) 723-1919
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ebd@stanford.edu
Professor, Medicine
Professor, Health Policy
Senior Fellow, by courtesy, Freeman Spogli Institute for International Studies
Senior Fellow, Woods Institute for the Environment
eran_bendavid
MD, MS
My academic focus is on global health, health policy, infectious diseases, environmental changes, and population health. Our research primarily addresses how health policies and environmental changes affect health outcomes worldwide, with a special emphasis on population living in impoverished conditions.
Our recent publications in journals like Nature, Lancet, and JAMA Pediatrics include studies on the impact of tropical cyclones on population health and the dynamics of SARS-CoV-2 infectivity in children. These works are part of my broader effort to understand the health consequences of environmental and policy changes.
Collaborating with trainees and leading academics in global health, our group's research interests also involve analyzing the relationship between health aid policies and their effects on child health and family planning in sub-Saharan Africa. My research typically aims to inform policy decisions and deepen the understanding of complex health dynamics.
Current projects focus on the health and social effects of pollution and natural hazards, as well as the extended implications of war on health, particularly among children and women.
Specific projects we have ongoing include:
What do global warming and demographic shifts imply for the population exposure to extreme heat and extreme cold events?
What are the implications of tropical cyclones (hurricanes) on delivery of basic health services such as vaccinations in low-income contexts?
What effect do malaria control programs have on child mortality?
What is the evidence that foreign aid for health is good diplomacy?
How can we compare health inequalities across countries? Is health in the U.S. uniquely unequal?
Faculty Co-director of the Stanford Center on China's Economy and Institutions
Helen F. Farnsworth Endowed Professorship
Senior Fellow at the Freeman Spogli Institute for International Studies
Senior Fellow at the Stanford Institute for Economic Policy Research
scott_rozelle_new_headshot.jpeg
PhD
Scott Rozelle is the Helen F. Farnsworth Senior Fellow and the co-director of Stanford Center on China's Economy and Institutions in the Freeman Spogli Institute for International Studies and Stanford Institute for Economic Policy Research at Stanford University. He received his BS from the University of California, Berkeley, and his MS and PhD from Cornell University. Previously, Rozelle was a professor at the University of California, Davis and an assistant professor in Stanford’s Food Research Institute and department of economics. He currently is a member of several organizations, including the American Economics Association, the International Association for Agricultural Economists, and the Association for Asian Studies. Rozelle also serves on the editorial boards of Economic Developmentand Cultural Change, Agricultural Economics, the Australian Journal of Agricultural and Resource Economics, and the China Economic Review.
His research focuses almost exclusively on China and is concerned with: agricultural policy, including the supply, demand, and trade in agricultural projects; the emergence and evolution of markets and other economic institutions in the transition process and their implications for equity and efficiency; and the economics of poverty and inequality, with an emphasis on rural education, health and nutrition.
Rozelle's papers have been published in top academic journals, including Science, Nature, American Economic Review, and the Journal of Economic Literature. His book,Invisible China: How the Urban-Rural Divide Threatens China’s Rise, was published in 2020 by The University of Chicago Press. He is fluent in Chinese and has established a research program in which he has close working ties with several Chinese collaborators and policymakers. For the past 20 years, Rozelle has been the chair of the International Advisory Board of the Center for Chinese Agricultural Policy; a co-director of the University of California's Agricultural Issues Center; and a member of Stanford's Walter H. Shorenstein Asia-Pacific Research Center and the Center on Food Security and the Environment.
In recognition of his outstanding achievements, Rozelle has received numerous honors and awards, including the Friendship Award in 2008, the highest award given to a non-Chinese by the Premier; and the National Science and Technology Collaboration Award in 2009 for scientific achievement in collaborative research.
Stanford Center on China’s Economy and Institutions
A Stanford-led team has discovered how to estimate crop yields with more accuracy than ever before with satellites that measure a special form of light emitted by plants. This breakthrough will help scientists study how crops respond to climate change.
As Earth's population grows toward a projected 9 billion by 2050 and climate change puts growing pressure on the world's agriculture, researchers are turning to technology to help safeguard the global food supply.
A research team, led by Kaiyu Guan, a postdoctoral fellow in Earth system science at Stanford's School of Earth, Energy, & Environmental Sciences, has developed a method to estimate crop yields using satellites that can measure solar-induced fluorescence, a light emitted by growing plants. The team published its results in the journal Global Change Biology.
Scientists have used satellites to collect agricultural data since 1972, when the National Aeronautics and Space Administration (NASA) pioneered the practice of using the color – or "greenness" – of reflected sunlight to map plant cover over the entire globe.
"This was an amazing breakthrough that fundamentally changed the way we view our planet," said Joe Berry, professor of global ecology at the Carnegie Institution for Science and a co-author of the study. "However, these vegetation maps are not ideal predictors of crop productivity. What we need to know is growth rate rather than greenness.
The growth rate can tell researchers what size yield to expect from crops by the end of the growing season. The higher the growth rate of a soybean plant or stalk of corn, for instance, the greater the harvest from a mature plant.
"What we need to measure is flux – the carbon dioxide that is exchanged between plants and the atmosphere – to understand photosynthesis and plant growth," Guan said. "How do you use color to infer flux? That's a big gap."
Solar-induced fluorescence
Recently, researchers at NASA and several European institutes discovered how to measure this flux, called solar-induced fluorescence, from satellites that were originally designed for measuring ozone and other gases in the atmosphere.
A plant uses most of the energy it absorbs from the sun to grow via photosynthesis, and dissipates unused energy as heat. It also passively releases between 1 and 2 percent of the original solar energy absorbed by the plant back into the atmosphere as fluorescent light. Guan's team worked out how to distinguish the tiny flow of specific fluorescence from the abundance of reflected sunlight that also arrives at the satellite.
"I think of it like crumbs falling to the ground as people are eating. It's a very small trail," said co-author David Lobell, associate professor of Earth system science at Stanford's School of Earth, Energy, & Environmental Science. "This glow that plants have seems to be very proportional to how fast they're growing. So the more they're growing, the more photosynthesis they're doing, and the brighter they're fluorescing." Lobell is also deputy director of the Center on Food Security and the Environment.
The research team saw an opportunity to use this new data to close the knowledge gap about crop growth, beginning with a major corn- and soybean-producing region of the U.S. Midwest.
"With the fluorescence breakthrough, we can start to directly measure photosynthesis instead of color," Guan said.
The fact that fluorescence can now be detected from space allows researchers to measure plant growth across much larger areas and over long periods of time, giving a much clearer picture of how yields fluctuate under changing weather conditions.
"One of the really cool things about fluorescence is that it opens up a whole new set of questions that we can ask about vegetation, and often times it's these new measurements that drive the science forward," Lobell said.
Next steps
The research team has already identified a number of potential uses of this approach by agricultural scientists, farmers, crop insurance providers and government agencies concerned with agricultural productivity.
If there is a day when the plant is really stressed, the fluorescence will drop significantly, Lobell said. Capturing these short-term responses to environmental changes will help scientists understand what factors plants are responding to on the daily time scale.
"That helps us, for example, figure out what we need to worry about in terms of stresses that crops are responding to," Lobell said. "What should we really be focusing on in terms of the next generation of cropping systems? What should they be able to withstand that the current crops can't withstand?"
At this early stage, fluorescence measurements are relatively low-resolution (a single measurement covers about 50 square kilometers) and because it is only collected once per day, cloudy skies can interfere with the fluorescence signal. For now, researchers have to supplement the data with other information and with on-the-ground observations to refine the measurements.
"Now that we have demonstrated the concept, we hope to soon be orbiting some new satellites specifically designed to make fluorescence measurements with better spatial and temporal resolution," Berry said.
The team plans to continue its research on U.S. crop yields while expanding measurements to other parts of the world.
"In the future, we hope to directly use this technology to monitor global food production, for example in China or Brazil, or even in your backyard," Guan said.
David Lobell is also deputy director of the Center on Food Security and the Environment, and William Wrigley Senior Fellow at the Freeman Spogli Institute for International Studies and the Stanford Woods Institute for the Environment. The study was also co-authored by Youngguan Zhang of the International Institute for Earth System Sciences at Nanjing University and the German Research Center for Geosciences (GFZ); Joanna Joiner of the NASA Goddard Space Flight Center Laboratory for Atmospheric Chemistry and Dynamics; Luis Guanter of GFZ; and Grayson Badgley of Stanford's Department of Earth System Science and Department of Global Ecology at the Carnegie Institution for Science.
CONTACTS:
p> Kaiyu Guan, Stanford School of Earth, Energy, & Environmental Sciences: kaiyug@stanford.edu
Laura Seaman, Stanford's Center on Food Security and the Environment: lseaman@stanford.edu, (650) 723-4920
Agricultural crops are on the front lines of climate change. Can we expect increased food production in the context of global warming? Do our crops come pre-adapted to a climate not seen since the dawn of agriculture, or must we take bold measures to prepare agriculture for climate change? This talk will focus on the role that crop diversity must necessarily play in facilitating the adaptation of agricultural crops to new climates and environments. Genebanks, the “Doomsday Vault” near the North Pole, and possible new roles for plant breeders and farmers will be explored.
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Dr. Cary Fowler is perhaps best known as the “father” of the Svalbard Global Seed Vault, which UN Secretary General Ban Ki-Moon has described as an “inspirational symbol of peace and food security for the entire humanity.” Dr. Fowler proposed creation of this Arctic facility to Norway and headed the international committee that developed the plan for its establishment by Norway. The Seed Vault provides ultimate security for more than 850,000 unique crop varieties, the raw material for all future plant breeding and crop improvement efforts. He currently chairs the International Council that oversees its operations.
In 2005 Dr. Fowler was chosen to lead the new Global Crop Diversity Trust, an international organization cosponsored by Food and Agriculture Organization of the UN (FAO) and the Consultative Group on International Agricultural Research (CGIAR). This position carried international diplomatic status. During his tenure, he built an endowment of $130 million and raised an additional $100 million (including the first major grant given for agriculture by the Gates Foundation) for programs to conserve crop diversity and make it available for plant breeding. The Trust organized a huge global project to rescue 90,000 threatened crop varieties in developing countries – the largest such effort in history - and is now engaged in an effort Dr. Fowler initiated with the Royal Botanic Gardens (Kew) to collect, conserve and pre-breed the wild relatives of 26 major crops. He oversaw development of a global information system to aid plant breeders and researchers find appropriate genetic materials from genebanks around the world. These initiatives at the Crop Trust, positioned the organization as a major path-breaking player in the global effort to adapt crops to climate change.
Prior to leading the Global Crop Diversity Trust, Dr. Fowler was Professor at the Norwegian University of Life Sciences in Ås Norway. He headed research and the Ph.D. program at the Department of International Environment and Development Studies and was a member of the university committee that allocated research funding to the different departments.
The U.N.’s FAO recruited him in the 1990s to lead the team to produce the UN’s first global assessment of the State of the World’s Plant Genetic Resources. He was personally responsible for drafting and negotiating the first FAO Global Plan of Action on the Conservation and Sustainable Utilization of Plant Genetic Resources, formally adopted by 150 countries in 1996. Following this, Dr. Fowler served as Special Assistant to the Secretary General of the World Food Summit (twice) and represented the Consultative Group on International Agricultural Research (CGIAR/World Bank) in negotiations on the International Treaty on Plant Genetic Resources. He chaired a series of Nordic government sponsored informal meetings of 15 countries to facilitate negotiations for this treaty. And, he represented Norway on the Panel of Experts of the Convention on Biological Diversity.
Cary Fowler was born in 1949 and grew up in in Memphis, Tennessee, the son of a judge and a dietician. He studied at Simon Fraser University in Canada where he received a B.A. (honors – first class) degree. He earned his Ph.D. at Uppsala University in Sweden with a thesis on agricultural biodiversity and intellectual property rights. Dr. Fowler has lectured widely, been a visiting scholar at Stanford University and a visiting professor at the University of California – Davis. He is the author or co-author of more than 100 articles and several books including the classic Shattering: Food, Politics and the Loss of Genetic Diversity (University of Arizona Press), Unnatural Selection, Technology, Politics and Plant Evolution (Gordon & Breach Science Publishers) and The State of the World’s Plant Genetic Resources (UN-FAO).
Dr. Fowler currently serves on the boards of Rhodes College, the NY Botanical Garden Corporation, the Lillian Goldman Charitable Trust and Amy Goldman Charitable Trust. He remains associated with the Global Crop Diversity Trust as Special Advisor. He is a former member of the U.S. National Plant Genetic Resources Board (appointed by the Secretary of Agriculture) and former board and executive committee member of the International Maize and Wheat Improvement Center in Mexico. He has served as chair of the national Livestock Conservancy. He is the recipient of several awards: Right Livelihood Award, Vavilov Medal, the Heinz Award, Bette Midler’s Wind Beneath My Wings Award, the William Brown Award of the Missouri Botanical Garden and two honorary doctorates. He is one of two foreign elected members of the Russian Academy of Agricultural Sciences and is a member of the Russian Academy of Sciences.
Dr. Cary Fowler
Speaker
Senior Advisor, Global Crop Diversity Trust
