NASA's key to predicting flash drought

An unusual increase in plant productivity may indicate impending severe soil water loss. NASA Satellites track this subtle glow, providing early warnings of potential droughts across the diverse landscape.

The drought that raged quickly and without warning, which swept across much of the United States in the summer of 2012, was one of the most widespread the country had seen since the years-long Dust Bowl of the 1930s. The “flash drought,” fueled by extreme heat that burned moisture from soil and plants, led to widespread crop failure and economic losses costing more than $30 billion.

While typical droughts may develop over the course of seasons, flash droughts are characterized by rapid drying. It can take hold within weeks and is difficult to predict. in Recent studyA team led by scientists from NASA's Jet Propulsion Laboratory in Southern California was able to detect signs of a sudden drought three months before it began. In the future, this advance notice could assist in mitigation efforts.

How did they do it? By following the glow.

In a field in western Kentucky, a sprayer covers crops in preparation for the planting season. NASA scientists are looking to space-based instruments to help predict rapid, subtle droughts responsible for massive agricultural losses in recent years. Source: USDA/Justin Beus

A signal seen from space

during PhotosynthesisWhen a plant absorbs sunlight to convert carbon dioxide and water into food, chlorophyll will “leak” some unused photons. This faint glow is called solar induced fluorescence, or SIF. The stronger the fluorescence, the more carbon dioxide the plant takes in from the atmosphere to support its growth.

While the glow is invisible to the naked eye, it can be detected by instruments aboard satellites such as NASA's Orbiting Carbon Observatory 2 (OCO-2). OCO-2 was launched in 2014 to monitor the American Midwest Glowing during the growing season.

Growing plants emit a form of light that can be detected by NASA satellites orbiting hundreds of miles above Earth. Parts of North America appear to shine in this visualization, which depicts an average year. Gray color indicates areas with little or no fluorescence; Red, pink and white indicate high fluorescence. Source: NASA Scientific Visualization Studio

The researchers compared years of luminescence data with a list of flash droughts that struck the United States between May and July from 2015 to 2020. They found a domino effect: In the weeks and months leading up to the flash drought, plants initially thrived as conditions changed. Warm and dry. The blooming plants gave off an unusually strong fluorescent signal for this time of year.

But by gradually reducing the soil's water supply, the plants have created a danger. When extreme temperatures arrived, already low humidity levels dropped, and sudden drought set in within days.

The team linked the fluorescence measurements to humidity 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 correlates very well with soil moisture loss during the six to 12-week period before a sudden drought. A consistent pattern emerged across diverse landscapes, from the temperate forests of the eastern United States to the Great Plains and scrublands of the West.

For this reason, plant fluorescence “shows promise as a reliable early warning indicator of sudden drought with sufficient lead time to take action,” said Nicholas Barrazo, an earth scientist at the University of California. Jet Propulsion Laboratory He is the lead author of the latest study.

Jordan Gerth, a scientist with the National Weather Service's monitoring office who was not involved in the study, said he's happy to see action on flash droughts, given our changing climate. He noted that agriculture benefits from predictability whenever possible.

While early warning cannot eliminate the effects of sudden drought, Gerth said, “farmers and ranchers with advanced operations can make better use of water for irrigation to reduce crop impacts, avoid planting crops that are likely to fail, or plant a different type of crop.” . To achieve optimal return if they have weeks to months of lead time.

Track carbon emissions

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 act as carbon “sinks,” absorbing more carbon dioxide from the atmosphere than they release. Many types of ecosystems, including agricultural lands, play a role in… Carbon cycle – The continuous exchange of carbon atoms between the Earth, the atmosphere and the oceans.

Scientists used carbon dioxide measurements from the OCO-2 satellite, along with advanced computer models, to track carbon uptake by plants before and after sudden droughts. Plants exposed to heat stress absorb less carbon dioxide from the atmosphere, so the researchers expected to find more free carbon. What they found instead was a balancing act.

Warm temperatures before the onset of sudden drought tempt plants to increase their carbon uptake compared to normal conditions. This anomalous uptake was, on average, sufficient to compensate for the complete reduction in carbon uptake due to the ensuing warm conditions. This surprising discovery could help improve carbon cycle model predictions.

The OCO-2 satellite celebrates its 10th year in orbit this summer, mapping natural and man-made objects. Carbon dioxide concentrations And plants Optical Using three camera-like spectrophotometers tuned to detect the unique optical signature of carbon dioxide. They measure the gas indirectly by tracking how much reflected sunlight it absorbs in a given column of air.