Postdoctoral researcher Christine Yao-Yun Chang measures photosynthesis in a soy field near Musgrave Research Farm.

Satellite data paints a portrait of global plant health

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Joe Schwartz

Ying Sun, assistant professor of geospatial sciences.

When it comes to measuring photosynthesis, green is not all that counts. A Cornell researcher is using a NASA satellite to measure photosynthesis in high resolution at the global scale, advancing how we measure plant health and its impact on food production and atmospheric carbon dioxide.

Since the 1980s, scientists have estimated plant productivity by measuring the amount of green color, known as the vegetative index, observed in satellite images. Those estimates – a proxy for potential photosynthesis – have, in turn, been used to answer questions about the Earth’s total carbon budget: how much carbon dioxide plants consume, and how much is emitted into the atmosphere as heat-trapping gas.

Now Ying Sun, assistant professor in the College of Agriculture and Life Sciences, and a team of researchers are using NASA’s Orbiting Carbon Observatory-2 (OCO-2) to more accurately measure photosynthesis – with implications for creating more reliable estimation of crop productivity and global carbon uptake in the face of increasing climate change.

Her findings are part of the Oct. 13 cover story of Science focusing on five papers about OCO-2. Sun is the author or co-author of three of those papers.

NASA launched OCO-2 in 2014 with the goal of determining global carbon dioxide levels by measuring in three different spectral bands how much sunlight reflects off CO2 molecules in the atmosphere. Sun and her colleagues recognized that OCO-2 can do even more. The satellite measures solar-induced chlorophyll fluorescence, a process that occurs only during photosynthesis.

Plants use solar energy to grow, and they release a tiny portion of that excess energy as a form of fluorescent light. By measuring that light emitted by plants, Sun is able to probe how much actual photosynthesis has taken place, rather than using estimates derived by the traditional vegetation indexes.

“The most exciting aspect of solar-induced chlorophyll fluorescence measurements from space is that they can provide a much more reliable estimate of crop productivity, and tell us what plants are actually doing and how well they are doing on the Earth’s surface,” she said.

Determining what is actually happening in plants is of grave concern. Heat and drought can slow photosynthesis even while plants remain green. In 2015 and 2016, for example, the El Niño weather patterns resulted in hot and dry conditions across much of the world, including the tropics.

Those two years coincided with the largest annual increases in atmospheric carbon dioxide concentration since measurements began in the 1950s. Atmospheric carbon dioxide increased by about 6.3 gigatons for 2015-16. This was a large spike compared with recent average increases of 4 gigatons a year despite emissions from human activities estimated to have been approximately the same as before the El Niño.

A reason, according to Sun and other researchers: Photosynthesis rates faltered in the hot, dry conditions, resulting in less carbon uptake by plants and trees.

“Our global population and demand for food production are both increasing, but we only have limited resources of land and water,” Sun said. “My goal is to provide sustainable solutions to increase food productivity while reducing negative impacts on the environment. Using this novel data set and state-of-the-art modeling tools, I feel confident about being able to help solve these systemic agricultural problems.”

Sun studies the fundamental processes that regulate interactions between the atmosphere and land-based ecosystems, with the goal of improving crop modeling and plant responses to stress, especially under climate change. Sun and her research team, including postdoctoral researcher Christine Yao-Yun Chang, are now verifying the satellite measurements by comparing the readings with ground-based measurements being taken in corn fields near the Musgrave Research Farm in Aurora, New York.

Krisy Gashler is a freelance writer for the College of Agriculture and Life Sciences.

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