In the summer of 2012, a severe drought swept across much of the United States, causing widespread crop failure and economic losses exceeding $30 billion. Unlike typical droughts that develop gradually over seasons, this “flash drought” took hold within weeks, making it difficult to predict and mitigate. However, a recent study published in the journal Geophysical Research Letters and led by scientists from NASA’s Jet Propulsion Laboratory in Southern California has discovered a way to detect signs of flash droughts up to three months before they strike.
The key to this early detection lies in a faint glow emitted by plants during photosynthesis, known as solar-induced fluorescence (SIF). When a plant absorbs sunlight to convert carbon dioxide and water into food, some unused photons “leak” from its chlorophyll. The stronger the fluorescence, the more carbon dioxide the plant is taking from the atmosphere to fuel its growth. While invisible to the naked eye, this glow can be detected by instruments aboard satellites like NASA’s Orbiting Carbon Observatory-2 (OCO-2).
By comparing years of fluorescence data to an inventory of flash droughts that hit the U.S. between May and July from 2015 to 2020, researchers uncovered a domino effect. In the weeks and months leading up to a flash drought, vegetation initially thrived as conditions turned warm and dry, emitting an unusually strong fluorescence signal for the time of year. But, by gradually drawing down the soil’s water supply, the plants created a risk. When extreme temperatures hit, the already low moisture levels plummeted, and flash drought developed within days.
Plant fluorescence “shows promise as a reliable early warning indicator of flash drought with enough lead time to take action,” says lead study author Nicholas Parazoo, an Earth scientist at JPL.
To validate their findings, the team correlated the fluorescence measurements with moisture data from NASA’s SMAP (Soil Moisture Active Passive) satellite, which tracks changes in soil water by measuring the intensity of natural microwave emissions from Earth’s surface. They found that the unusual fluorescence pattern correlated extremely well with soil moisture losses in the six to 12 weeks before a flash drought, across diverse landscapes from the temperate forests of the Eastern U.S. to the Great Plains and Western shrublands.
While early warning can’t eliminate the impacts of flash droughts, it can help farmers and ranchers better prepare.
“Farmers and ranchers with advanced operations can better use water for irrigation to reduce crop impacts, avoid planting crops that are likely to fail, or plant a different type of crop to achieve the most ideal yield if they have weeks to months of lead time,” explains Jordan Gerth, a scientist with the National Weather Service Office of Observations who was not involved in the study.
In addition to predicting flash droughts, scientists also wanted to understand how these events impact carbon emissions. Plants and trees are carbon “sinks,” absorbing more CO2 from the atmosphere than they release during photosynthesis. Researchers expected to find more free carbon during flash droughts, as heat-stressed plants absorb less CO2. However, they discovered a surprising balancing act.
Warm temperatures prior to the onset of flash drought tempted plants to increase their carbon uptake compared to normal conditions. On average, this anomalous uptake was sufficient to fully offset decreases in carbon uptake due to the hot conditions that followed. This unexpected finding could help improve carbon cycle model predictions.
The OCO-2 satellite, celebrating its 10th year in orbit this summer, plays a crucial role in mapping natural and human-made carbon dioxide concentrations and vegetation fluorescence. Using three camera-like spectrometers tuned to detect the unique light signature of CO2, the satellite measures the gas indirectly by tracking how much reflected sunlight it absorbs in a given column of air.
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