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

Governments must do more to diversify the types of crops grown throughout the world. If they don’t, climate change may jeopardize the global food supply, a leading agriculture researcher told a Stanford audience.

Cary Fowler, a senior advisor and former executive director of the Global Crop Diversity Trust, was a driving force behind the creation of the Svalbard Global Seed Vault in Norway. Commonly known as the “doomsday vault,” the repository of ancient and modern seeds from around the world ensures that future generations will have access to a wide enough range of crop traits to adapt global agriculture to a changing climate.

Dr. Cary Fowler in Svalbard, Norway, the home of the Svalbard Global Seed Vault.

During a May 6 talk sponsored by FSE as part of the center’s Food and Nutrition Policy Symposium, Fowler warned that increasingly high temperatures and water shortages interfere with the natural growing cycles of many crops and can even reduce the nutritional quality of some plants. Higher temperatures also give way to new pests, diseases, and soil microorganisms that threaten yields.

 “The biggest impacts from climate change will be in sub-Saharan Africa,” Fowler said, a region where many people already suffer serious poverty and hunger, and where crop yields lag behind the rest of the world. Fowler said that as climate pressure on agriculture intensifies, the world can expect to see an uptick in civil conflict, restrictive trade policies, and suffering among the world’s poorest people.

“Crops are going to be facing new combinations of conditions for which there is no historical experience,” said Fowler. “They will require new combinations of traits” that can only be developed by preserving genetic diversity and proactively breeding new varieties.

 “There are 1.3 billion people living on subsistence farms today,” said Dr. Cary Fowler to a Stanford audience on May 6. “How will they adapt to climate change without access to diversity?”

Fowler called for the U.S. and foreign governments to embrace their “inherited evolutionary responsibility” for preserving the huge diversity of crops grown by farmers throughout human history.

The United States is the ideal candidate to lead the world in using crop genetic diversity to adapt agriculture to climate challenges, he said. “The U.S. is well-positioned to research diversity, model future climate and assemble seed packages,” enlisting farmers in the U.S. and abroad in “another mass adaptation experiment” like the one American agriculture undertook in the 18^th and 19^th centuries.

 “I know that sounds like a wild and crazy idea,” Fowler said. “But I haven’t heard any alternatives to it. If we’re assuming we’re going to have development without diversity, that would really be a historically unprecedented experiment.”

 “If agriculture doesn’t adapt,” he added, “neither will we.”

A diverse history

In the late 1700s the United States food system lacked diversity and infrastructure. “Very few of the crops we grow now in the U.S. are native,” said Fowler. Early on, “it wasn’t always evident what crops from abroad would grow well in the U.S.”

The government soon set out to expand and diversify American agriculture. U.S. Navy ships collected seeds on overseas voyages, and U.S. diplomats brought back new crops from postings abroad. Government-sponsored expeditions sought out foreign plants with specific disease-resistant traits. The U.S. signed two dozen seed-exchange agreements with other countries, and lowered taxes on imported seeds to boost global crop exchange.

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“The United States amassed a much more diverse array of seeds and crops as a result,” said Fowler. One program introduced 600 new apple varieties, 700 new types of pears, and 353 new varieties of mangoes to American farmers.

But the United States did not simply collect new crops. It also invested in research to develop new varieties, including through plant breeding.

Genetic erosion

Research into plant breeding quickly yielded many of the modern varieties of crops we grow today in the United States.

“With plant breeding came the rise of modern varieties that had useful traits like disease resistance,” said Fowler. A small handful of new varieties quickly gained popularity with American farmers, who now had a choice about whether or not to save seeds and grow many varieties of a crop at once. Most farmers chose not to, instead relying on the same few mainstream varieties their neighbors were growing.

This shift has led to what Fowler described as the “genetic erosion” of agriculture, a trend that can only be reversed by reviving the tradition of seed saving and plant breeding on a global scale.

Seed banks

“I have probably been to more seed banks than any other person,” said Fowler. Seeds from most crops can survive hundreds or even thousands of years in storage, but most storage facilities lack the physical security to provide lasting safe haven. Many seed banks are poorly built, too warm or humid for long-term storage, and vulnerable to natural disasters. Other facilities suffer damaged during civil wars and uprisings.

Even if banks are physically secure, said Fowler, most simply do not operate on a large enough scale to protect global crop diversity. “Most crops in the world have between one and 10 total seed samples in storage, and most have no plant breeders working on them at all,” said Fowler.

The doomsday vault

In 2005 Fowler was chosen to lead an international coalition to build the Svalbard Global Seed Vault. The Norwegian government owns the facility, and it is also managed by the Global Crop Diversity Trust and the Nordic Genetic Resource Center. 

The vault is built into the side of a mountain in the far north of Norway, said Fowler, because the ideal temperature for storing seeds is minus 18 degrees Celsius.

Inside the frozen walls of the vault are shelves full of boxes holding duplicate seeds from smaller seed banks around the world. Foreign governments that contribute samples pay nothing for storage, and the seed packages are never opened by vault staff, said Fowler.

 “The vault now houses seeds from over 864,000 varieties of plants,” said Fowler, adding that not a single sample has ever been lost.

Seed storage boxes at the Svalbard Global Seed Vault.

The facility’s nickname, “the doomsday vault,” comes not only from its rugged physical location but from its capacity to withstand disasters – something its planners took great care to design. “We calculated how high the water would go if all ice in the world melted and we had the world’s largest ever tsunami,” said Fowler. “The vault is five stories above that.”

“Not a solution”

Fowler emphasized that no doomsday vault, no matter how secure its walls or how ample its seed collection, can solve the problem of crop genetic erosion. Building a vault “doesn’t mean that we as a society are getting serious about adapting agriculture to climate change,” Fowler said. Plant breeding and crop research programs focused on developing new climate-resilient varieties are just as crucial as saving seeds.

Although a few major staple crops like rice, wheat and corn are continually bred and improved in research labs around the world, most crops are largely ignored by researchers. For example, there are only six breeders of yams worldwide.

“Why conserve it if you’re not going to use it?” Fowler asked. “We are acting like crops are going to adapt by themselves, and we are assuming all but a handful of crops are unimportant.”

Quoting Charles Darwin, Fowler added that “it is not the strongest of the species that survives, nor the most intelligent, but the one most responsive to change.”

Full video and audio recordings of Dr. Fowler's May 6 lecture, and his interview with FSE director Roz Naylor, are available here.