Can the US Drought Monitor track climate change?

Known for its bright bands of yellow, orange and red, the U.S. Drought Monitor has warned farmers, residents and officials across the country of impending water shortages every week since 1999.

Drawing on data on soil moisture, temperature, snow cover, meltwater runoff, reservoir levels and more, the map has become an essential tool for determining the outlook for water supplies. water, declare drought emergencies, and decide where and when government aid should be distributed, among other things.

But this essential diagnostic tool is also struggling to keep pace with climate change, as longer and more persistent droughts hit the American West and increasingly strain groundwater supplies and the river. Colorado, according to a recent study published in the journal AGU Advances.

One problem, researchers say, is that the monitor was launched just at the start of one of the driest periods in the Southwest’s history, and it never adapted to the increasing aridity of the region.

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“The product is essential, but it is also undoubtedly, in my opinion, influenced by climate change,” said Justin Mankin, one of the study’s authors and an associate professor of geography at Dartmouth. “And we in the drought community need to have a conversation about what it means to think about drought monitoring in the context of an aridifying climate. »

The monitor provides an accurate and reliable snapshot of what’s happening in the climate system at any given time, including a mix of global warming and La Niña conditions that contribute to drought conditions in the American Southwest, according to the study.

But its onset coincided with the start of a decades-long drought in the West, including the region’s 22 driest years in at least the last 1,200 years, sometimes referred to as a megadrought.

During that time, some areas of California experienced exceptional drought — the worst of five possible categories — nine times more often than they should have, according to the Drought Monitor’s probability. Areas were in this category 18% of the time – or over a period of almost four years – compared to the normal baseline rate of 2%, according to the study.

The findings raise questions about how familiar assessment can best address long-term trends and whether a product designed for periodic anomalies can accurately capture a much larger, slower-moving crisis .

“These trends … emphasize the theoretical guidelines of the product itself, and I would argue that undermines its usefulness as a policymaker,” said Mankin, who was also the former co-lead of the National Oceanic and Atmospheric Administration Drought Task Force. .

Indeed, the monitor is much more than just a dryness measurement. It is used to inform social and economic policies, including decisions about state and local drought emergency declarations, federal funds for farmers and agribusinesses, and other drought-related aid. disasters.

The Farm Services Agency, for example, uses weekly Drought Monitor updates to distribute certain assistance programs such as emergency haying and the Livestock Forage Disaster Program. The Internal Revenue Service uses it for certain livestock-related tax deferrals, while the U.S. Department of Agriculture uses it to determine eligibility for low-interest loans.

But resources triggered by exceptional conditions can quickly become depleted if those conditions last for months or even years, said Jason Smerdon, another study author and a climate scientist at the Columbia Climate School.

“This is an emergency, but a different type of emergency,” Smerdon said. “If it’s going to be dark red all the time, then short-term emergency aid to address the challenge isn’t really the right way to look at it. This is an emergency of a much longer and lingering nature that, in my opinion, requires different planning and relief. »

Experts who work on the Drought Monitor said they were open to feedback and also acknowledged that the tool has its limitations.

“The Drought Monitor was never intended as an indicator of climate change, but rather as a real-time assessment of drought conditions,” said Mark Svoboda, director of the National Drought Mitigation Center at the University of Nebraska-Lincoln , which produces the monitor in partnership with NOAA and USDA.

The monitor uses an “evidence convergence” approach that extracts data each week from dozens of indicators, including measurements of precipitation, soil moisture, snowpack, snow water equivalents, flow riverine, evapotranspiration and groundwater and reservoir levels, he said. This means they are also somewhat beholden to the limitations of these tools.

The Los Angeles Aqueduct crosses the Owens Valley in June 2023, in Lone Pine, California.

(Brian van der Brug/Los Angeles Times)

Before its release each Thursday, more than 400 experts across the country review a draft of the update and provide feedback on its findings. It’s rare that such a product has so much input and also adds a bit of art to science, according to Svoboda, who co-founded the Drought Monitor in 1999 and was its author for 17 years.

But the problem today is that it is not yet clear whether the mega-drought in the West represents a permanent climate change that could justify a recalibration of the tools, or whether there could still be wetter days to come, a he declared. In already arid regions, who decides when small changes in humidity cross a tipping point into a new era?

“We saw these multi-decadal droughts, during which we then saw a return to a wetter regime,” Svoboda said. “So the real challenge right now, after 20 years, because of climate change, is: Are we ever going to see a step backwards? Therefore, should we classify and switch to a more arid climate in a given region? That’s the real hundred billion dollar question, and our current indicators – none of them are going to answer that, because we’re not a forecasting tool.

The good news is that the monitoring agency is generally adept at standardizing its classifications based on regional variations, for example to the extent that an “exceptional” drought might have very different implications in California than in a place like Vermont , Smerdon said.

But the weekly overview also isn’t “enough to think about where we’re going and what we’re doing in light of the increasing water pressure we’re going to experience in the West,” he said. .

For example, researchers found that Southern California was never classified as an exceptional drought area during the 23-year study period, despite the fact that millions of people in and around Los Angeles had been subject to the strictest water restrictions ever seen in the middle of the state’s three driest regions. years recorded.

Additionally, the Drought Monitor currently shows that most of California and the Southwest are out of the worst phases of drought after two recent wet winters. But groundwater supplies are still depleted and the Colorado River has not fully recovered from more than two decades of drought, with Lake Mead still measuring only about 35 percent of its capacity.

The Colorado River flows along the California-Arizona border on April 3, 2023.

(Brian van der Brug/Los Angeles Times)

“They are the central banks of the water economy in the West, but you look at the map and it’s not there in any substantial way,” Mankin said.

Svoboda said current readings in the Colorado River region represent a “double-edged sword.” Lake Mead is unlikely to reach full capacity again, but continuing to reflect this on the Drought Monitor would leave the map indefinitely red.

“The challenge is always that you don’t want to cry wolf too soon, but you certainly don’t want to wait for the wolf to eat you either,” he said of declaring a drought. “And when you come out of a drought, it’s the same thing. Drought impacts may persist.

Other tools, such as reservoir monitors from the U.S. Bureau of Reclamation and the California Department of Water Resources, are better equipped to measure water scarcity and supply in the region than the Drought Monitor, he said. he adds.

But there are potential ways to help the tool evolve with climate change, including expanding its baseline to include wetter periods before the onset of the current mega-drought, which would help even out the curve in bell and reduce the frequency of extremes, the researchers said.

Tackling a new “super-exceptional” category could also help with its calibration. This is a measure that has already been proposed for other aggravating climatic events, such as a new “category 6” for hurricanes.

However, this solution has limitations when it comes to drought, which is strongly linked to limited amounts of water, unlike hurricanes, which have no upper limit on strength, the researchers said.

Svoboda also said the Drought Monitor is limited by limited recordings in many of the products it relies on. Additionally, adjusting “normal” would also mean that all surrounding policies – like Farm Services Agency relief – would have to change as well.

Such problems do not necessarily indicate faults in the monitor, but rather highlight the need for a variety of tools, he and the researchers said. The US Drought Monitor should not be the be-all and end-all for decision-making, and should continue to be combined with reservoir observations, snow surveys, and other measurements to form a complete picture.

Mankin said he hopes the study can shed light on the need for long-term adaptation — and the increased possibility that California and other states will face much more frequent droughts than before.

However, he and Smerdon stressed that the study should not undermine the value of the Drought Monitor – or the work of those who keep it updated week after week.

“Monitoring drought and keeping a sense of how the hydroclimate fluctuates across the United States is really important, and it’s been a real game changer to have it as a resource and for planning,” Smerdon said. “I just think it needs to evolve. We need to think about what a tool like the Drought Monitor means in a changing climate.