NASA scientists have found that satellite-tracked plant fluorescence can predict sudden droughts months in advance, helping to mitigate and understand the impact of the carbon cycle during drought. Credit: NASA Science Visualization Studio
An unusual spike in plant productivity may signal impending severe soil water loss. NASA satellites track this subtle glow, offering early warnings of potential sudden droughts in various landscapes.
Occurring quickly and with little warning, the drought that gripped much of the United States in the summer of 2012 was one of the most extensive the country has seen since the long-running Dust Bowl of the 1930s. The “sudden drought” brought on by extreme heat that burned moisture from soil and plants resulted in mass crop failure and economic losses costing more than $30 billion.
While archetypal droughts can develop over seasons, sudden droughts are characterized by rapid drying. They can persist for weeks and are difficult to predict. In a recent study, a team led by scientists at NASA’s Jet Propulsion Laboratory in Southern California was able to detect signs of sudden drought up to three months before it began. In the future, such notice may assist mitigation efforts.
How did they do it? Following the glow.
In a field in western Kentucky, a machine sprays cover crops to prepare for planting season. NASA scientists are looking to space-based tools to help predict the rapid, undetectable droughts responsible for severe agricultural losses in recent years. Photo: USDA/Justin Peay
A signal observed from space
During photosynthesis, when a plant absorbs sunlight to convert carbon dioxide and water into food, its chlorophyll will “leak” some unused photons. This faint glow is called solar-induced fluorescence, or SIF. The stronger the fluorescence, the more carbon dioxide a plant takes from the atmosphere to fuel its growth.
Although the aurora is invisible to the naked eye, it can be detected by instruments aboard satellites such as NASA’s Orbiting Carbon Observatory-2 (OCO-2). Launched in 2014, OCO-2 has observed aurora over the US Midwest during the growing season.
Growing plants emit a form of light that is detected by NASA satellites orbiting hundreds of miles above Earth. Parts of North America seem to flash in this visualization of an average year. Gray indicates areas with little or no fluorescence; red, pink and white indicate high fluorescence. Credit: NASA Science Visualization Studio
The researchers compared years of fluorescence data to an inventory of flash droughts that hit the U.S. between May and July from 2015 to 2020. They found a domino effect: In the weeks and months leading up to a flash drought, vegetation initially thrived when conditions changed warm and dry. The flowering plants emitted an unusually strong fluorescent signal for the time of year.
But by gradually depleting soil water reserves, plants pose a risk. When extreme temperatures hit, already low moisture levels plummeted and a sudden drought developed within days.
The team correlated the fluorescence measurements with moisture data from NASA’s SMAP satellite. Short for Soil Moisture Active Passive, SMAP tracks changes in soil water by measuring the intensity of natural microwave emissions from the Earth’s surface.
The scientists found that the unusual fluorescence pattern correlated extremely well with soil moisture losses in the six to 12 weeks before a sudden drought. A consistent pattern emerged across landscapes, from the temperate forests of the eastern US to the Great Plains and western scrublands.
For this reason, plant fluorescence “shows promise as a reliable early warning indicator of sudden drought with enough time to take action,” said Nicholas Parazu, an Earth scientist at JPL and lead author of the recent study.
Jordan Gert, a scientist with the National Weather Service’s Office of Observatories who was not involved in the study, said he was pleased to see the work on flash droughts given the changing climate. He noted that agriculture benefits from predictability whenever possible.
While early warning cannot eliminate the impact of sudden droughts, Gert said, “farmers and ranchers with advanced operations can better use irrigation water to reduce impacts on crops, avoid planting crops that are likely to fail, or to plant a different type of crop to achieve the most ideal yield if they have a lead time of weeks to months.”
Carbon tracking
In addition to trying to predict sudden droughts, scientists wanted to understand how they affect carbon emissions.
By converting carbon dioxide into food during photosynthesis, plants and trees are carbon “sinks,” absorbing more CO2 from the atmosphere than they release. Many types of ecosystems, including farmland, play a role in the carbon cycle—the constant exchange of carbon atoms between the land, atmosphere, and ocean.
The scientists used measurements of carbon dioxide from the OCO-2 satellite, along with advanced computer models, to track carbon uptake by vegetation before and after sudden droughts. Heat-exposed plants absorb less CO2 from the atmosphere, so the researchers expected to find more free carbon. Instead, they found a balancing act.
High temperatures before the onset of the sudden drought tempted plants to increase carbon uptake compared to normal conditions. This anomalous uptake was, on average, sufficient to fully offset the reductions in carbon uptake due to the hot conditions that followed. The surprising discovery could help improve carbon cycle model predictions.
Celebrating its 10th year in orbit this summer, the OCO-2 satellite maps natural and man-made carbon dioxide concentrations and vegetation fluorescence using three camera-like spectrometers tuned to detect CO2’s unique light signature. They measure the gas indirectly by tracking how much reflected sunlight it absorbs in a given column of air.
