Usually, increasing agricultural productivity depends on adding something, such as fertilizer or water. A new Stanford University-led study reveals that removing one thing in particular – a common air pollutant – could lead to dramatic gains in crop yields. The analysis, published June 1 in Science Advances, uses satellite images to reveal for the first time how nitrogen oxides – gases found in car exhaust and industrial emissions – affect crop productivity. Its findings have important implications for increasing agricultural output and analyzing climate change mitigation costs and benefits around the world.

“Nitrogen oxides are invisible to humans, but new satellites have been able to map them with incredibly high precision. Since we can also measure crop production from space, this opened up the chance to rapidly improve our knowledge of how these gases affect agriculture in different regions,” said study lead author David Lobell, the Gloria and Richard Kushel Director of Stanford’s Center on Food Security and the Environment.

A NOx-ious problem

Nitrogen oxides, or NOx, are among the most widely emitted pollutants in the world. These gases can directly damage crop cells and indirectly affect them through their role as precursors to formation of ozone, an airborne toxin known to reduce crop yields, and particulate matter aerosols that can absorb and scatter sunlight away from crops.

While scientists have long had a general understanding of nitrogen oxides’ potential for damage, little is known about their actual impacts on agricultural productivity. Past research has been limited by a lack of overlap between air monitoring stations and agricultural areas, and confounding effects of different pollutants, among other challenges to ground-based analysis.

To avoid these limitations, Lobell and his colleagues combined satellite measures of crop greenness and nitrogen dioxide levels for 2018-2020. Nitrogen dioxide is the primary form of NOx and a good measure of total NOx. Although NOx is invisible to humans, nitrogen dioxide has a distinct interaction with ultraviolet light that has enabled satellite measurements of the gas at a much higher spatial and temporal resolution than for any other air pollutant.

“In addition to being more easily measured than other pollutants, nitrogen dioxide has the nice feature of being a primary pollutant, meaning it is directly emitted rather than formed in the atmosphere,” said study co-author Jennifer Burney, an associate professor of environmental science at the University of California, San Diego. “That means relating emissions to impacts is much more straightforward than for other pollutants.”

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Calculating crop impacts

Based on their observations, the researchers estimated that reducing NOx emissions by about half in each region would improve yields by about 25% for winter crops and 15% for summer crops in China, nearly 10% for both winter and summer crops in Western Europe, and roughly 8% for summer crops and 6% for winter crops in India. North and South America generally had the lowest NOx exposures. Overall, the effects seemed most negative in seasons and locations where NOx likely drives ozone formation.

“The actions you would take to reduce NOx, such as vehicle electrification, overlap closely with the types of energy transformations needed to slow climate change and improve local air quality for human health,” said Burney. “The main take-home from this study is that the agricultural benefits of these actions could be really substantial, enough to help ease the challenge of feeding a growing population.”

Previous research by Lobell and Burney estimated reductions in ozone, particulate matter, nitrogen dioxide, and sulfur dioxide between 1999 and 2019 contributed to about 20% of the increase in U.S. corn and soybean yield gains during that period – an amount worth about $5 billion per year.

Future analysis could incorporate other satellite observations, including photosynthetic activity measured through solar-induced fluorescence, to better understand nitrogen dioxide’s effects on crops’ varying degrees of sensitivity to the gas throughout the growing season, according to the researchers. Similarly, more detailed examination of other pollutants, such as sulfur dioxide and ammonia, as well as meteorological variables, such as drought and heat, could help to explain why nitrogen dioxide affects crops differently across different regions, years, and seasons.

“It’s really exciting how many different things can be measured from satellites now, much of it coming from new European satellites,” said study coauthor Stefania Di Tommaso, a research data analyst at Stanford’s Center on Food Security and the Environment. “As the data keep improving, it really drives us to be more ambitious and creative as scientists in the types of questions we ask.”
 

Lobell is also a professor of Earth system science in Stanford’s School of Earth, Energy & Environmental Sciences, the William Wrigley Senior Fellow at the Stanford Woods Institute for the Environment, and a senior fellow at the Freeman Spogli Institute for International Studies and the Stanford Institute for Economic Policy Research. Burney also holds the Marshall Saunders Chancellor’s Endowed Chair in Global Climate Policy and Research at UC San Diego and is a research affiliate at UC San Diego’s Policy Design and Evaluation Laboratory, a fellow at the Stanford Center on Food Security and the Environment, and head of the Science Policy Fellows Program at UC San Diego.

Like a lot of people, Colin Kahl long thought of Washington, D.C. as the place to be when it comes to matters of international security. Today, Kahl, who served as national security adviser to former Vice President Joseph Biden, has a different opinion.

"A lot of the most cutting-edge policy questions and international security challenges of this century are, in a strange way, west coast issues," said Kahl, who took over as co-director of social sciences for Stanford's Center for International Security and Cooperation (CISAC) in early September. He points to the role of technology in reshaping the global balance of power, the increasing importance of the Asia-Pacific region to the U.S. economy and security, and the country's changing demographics.

Kahl is one of three new directors at research centers run by The Freeman Spogli Institute for International Studies (FSI). Also in September, Anna Grzymala-Busse took over as director of The Europe Center (TEC) and David Lobell became the Gloria and Richard Kushel Director of the Center on Food Security and the Environment (FSE).

In separate interviews, the incoming directors outlined goals that differed in substance, but had similar objectives: to focus on issues that have historically been important to their centers while advancing work on new and emerging challenges. All three also talked about further leveraging Stanford's interdisciplinary approach to education and research.

"The centers within FSI all address research and policy challenges that are constantly changing," said Lobell, a professor of earth system science who joined FSE in 2008, three years after it was formed. "As part of FSI, we have unique opportunities to better understand the interplay of our specific area within the broader context of international security."

Michael McFaul, FSI's director, said the new leaders take over at an exciting time for their respective centers — and for FSI.

"Coming into a new academic year, I am excited about the tremendous momentum within FSI and its six research centers," said McFaul, who is also the Ken Olivier and Angela Nomellini Professor of International Studies. "Our ability to generate interdisciplinary, policy-oriented research, to teach and train tomorrow's leaders, and to engage policymakers has never been stronger."

Big Data & Food

As FSE's director and a researcher himself, Lobell says he's excited about the potential for technology to solve longstanding questions surrounding food security and world hunger. Satellite imagery of small-scale farming around the globe, for instance, is rapidly advancing efforts to improve crop productivity. "Historically it's been really hard to get good data," said Lobell, whose recent projects include using machine learning to identify poverty zones in rural Africa and map yields of smallholder farms in Kenya. 

"The measurement possibilities from new and different data technologies are going to be really important going forward," said Lobell, who is also looking to add expertise in water management and micronutrients, either by funding new graduate fellowships or hiring new faculty.

Europe and Beyond

For her part, Grzymala-Busse's primary goals at The Europe Center are to develop its international intellectual networks and strengthen its long-term institutional footing. "I am excited to build on our existing strengths and bring together even more historians, anthropologists, economists, and sociologists," said Grzymala-Busse, who joined Stanford faculty in 2016 and teaches political science and international studies. "Europe is ground zero for a lot of what's happening in the world, whether the rise of populism or the economic crises, and you can’t understand these developments without understanding the history, cultures, and economics of the region."

A Third Nuclear Revolution

For CISAC, international security is no longer just about nuclear security, says Kahl, who is one of two co-directors at the center; Rodney Ewing serves as the center's co-director of science and engineering, while Kahl oversees the social sciences.

Kahl says that nuclear weapons will remain a key focus for the center as North Korea, Iran, Russia, and China move to build or modernize arsenals. But, the center will also look at emerging technologies that are becoming serious threats. He cites as examples the rapid rise of cyberattacks, pandemics and biological weapons, and artificial intelligence and machine learning.

"My plan is to ensure that Stanford continues to play a profound leadership role in the most critical security issues facing the world today," said Kahl, who came to Stanford last year as the inaugural Steven C. Házy Senior Fellow, an endowed faculty chair at FSI.

Said McFaul, "We welcome three remarkable individuals with the skills and vision to guide their respective centers into the future."

The rising level of carbon dioxide in the atmosphere means that crops are becoming less nutritious, and that change could lead to higher rates of malnutrition that predispose people to various diseases.

That conclusion comes from an analysis published Tuesday in the journal PLOS Medicine, which also examined how the risk could be alleviated. In the end, cutting emissions, and not public health initiatives, may be the best response, according to the paper's authors.

Research has already shown that crops like wheat and rice produce lower levels of essential nutrients when exposed to higher levels of carbon dioxide, thanks to experiments that artificially increased CO2 concentrations in agricultural fields. While plants grew bigger, they also had lower concentrations of minerals like iron and zinc.

Read the entire story at NPR.