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‘Megadrought’ persists in western U.S., as another extremely dry year develops
The long-running dry stretch rivals anything in the last 1200 years, a sign of climate-change induced “aridification.”BYALEJANDRA BORUNDAPUBLISHED MAY 7, 2021• 12 MIN READ
Water levels usually peak in May at Folsom Lake in California, rising as Sierra Nevada snowpack melts away and courses down to the reservoir, near Sacramento.
But this year, the drought that has gripped much of the U.S. West is already so strong that the lake is only half as full as normal. Instead of water, fields of purple lupines line vast swaths of dry lakebed.
Folsom Lake’s situation is emblematic of the deepening drought across the western United States. As of May 6, 67 percent of the region was in a state of “severe” drought or worse; a stunning 21 percent is already in “exceptional” drought, the worst category in the U.S. Drought Monitor’s framework. At Lake Mead, one of the two major reservoirs of the Colorado River, which some 40 million Americans depend on, water levels are creeping toward a threshold that would trigger the first official shortage declaration for the basin.
The situation is unlikely to improve in the near future, scientists say, as 2021 shapes up to extend the “megadrought” that researchers have found to be gripping the region mostly unabated since 2000.
The region would have been in a state of drought regardless, “but it’s really climate change that pushed this event to be one of the worst in 500 years,” says Ben Cook, a climate scientist at Columbia’s Lamont Doherty Earth Observatory.
Dry, dry, dry
Drought can take many forms. There’s “meteorological drought,” which compares how much rain or snow has fallen compared to a long-term average. There’s “hydrological drought,” which considers how much water flows through streams and rivers and is stored in mountain snowpack and underground aquifers. There’s the drought the soil, plants, and animals feel, called agricultural or ecological drought.
This year, by just about every measure, the West is extremely low on water. Most of California and the Four Corners states are somewhere between 25 and 50 percent of their long-term averages on most of the drought indicators. Snowpack in California on April 1, its usual peak time, hit only 59 percent of its long-term average and has already largely melted away, leaving it at a paltry 15 percent of its average at this date. Soils are parched. The Colorado and Rio Grande rivers are trickling compared to their long-term histories.
The U.S. Drought Monitor ties all of these many pieces together into a weekly, U.S.-wide snapshot, binning the on-the-ground drought situation into six categories from “none” to “exceptional.” On April 22 they reported that 21 percent of the West was in “extreme” or “exceptional” drought, the two worst levels—nearly three times the area ever listed in that category this early in the year in the Monitor’s 20-year history.
“It’s incredible, how much of the West is in extreme or exceptional drought right now,” says Sandra Postel, the director of the Global Water Policy Project, “including much of the Colorado and Rio Grande basins, the two lifelines of the Southwest.”
The conditions are influenced by many factors, including a La Niña event that began last fall, which scientists know can contribute to dry conditions in the Southwest. In a La Niña event, ocean surface waters in the Eastern tropical Pacific are relatively cool (in an El Niño, this part of the ocean is usually extra warm). That cooling shifts the position of towering high-energy clouds, which tend to form over warm water, further to the west, which in turn affects the shape of planet-spanning weather systems. The effect “is like dropping a pebble into a pond,” says Samantha Stevenson, a climate scientist at the University of California, Santa Barbara; where the pebble is dropped affects where the waves of weather move.
The shape and pattern of the big weather ripples moving away from the Eastern Pacific toward the western U.S. make it more likely that precipitation-rich storm systems curve northward toward the Pacific Northwest and Canada rather than toward the Southwest.
A healthy dose of random chance also feeds into the weather patterns that keep the West dry. But underneath the weather vagaries, human-driven climate change is making those conditions more likely.
Dry conditions are nothing new in the U.S. West, which has cycled through water booms and busts for millennia.
But the region has been in a state of drought nearly every year since 2000, when the Drought Monitor was established. That 20-year-long stretch rivals any drought in the last 1,200 years, a team of scientists reported last year.
They knit together hundreds of tree-ring records from across northern Mexico and the U.S. West, creating a record that stretches back to about 800 AD. Trees record damp and dry years, growing more vigorously when their roots feel wetter soil and leaving behind slightly thicker rings. The team combined the tree rings with climate models to build a history of soil moisture, which indicates the intensity of drought.
In that 1,200-year-long record, the region cycled through 35 major droughts, including four “megadroughts” of particularly notable strength and duration: one in the 800s; another in the 1100s; a third in the 1200s that has been linked to the collapse of Ancestral Pueblo culture in the Southwest; and one deep, intense stretch in the late 1500s that may have added insurmountable strain on Native communities in Mexico already stressed by colonization and disease.
The previous megadroughts lasted decades—“20, 30, even 40 years, really eclipsing anything we’ve had to manage for in the last 100 years,” says Cook. In the past century, droughts like the 1920s Dust Bowl generally lasted only five to 10 years—devastating for those living through it, but significantly less disruptive than a multi-decade-long drought.
This current dry stretch is already long and intense by comparison. It comes in second in their record only to the 1500s drought, which occurred in a world unchanged by human-forced climate change. That should give us pause, says Cook, because it shows the West can swing into such drastic drought states naturally, without the extra nudge of climate change. An extra push from humans could make the effects far worse.
And, according to their analysis, that’s exactly what has occurred: This “megadrought” has been pushed into extreme territory by climate change. It would have been bad no matter what—their estimates suggest it would have been roughly the eleventh most intense in their record—but the added heat from climate change supercharged the drying, pushing it up to the second most intense drought in the last 1200 years.
Human-caused climate change, in tandem with human reshaping of the natural hydrological systems—by damming rivers, growing vast fields of crops, and more—have shifted the baseline conditions so thoroughly that there is no way to return to what used to be considered normal. The physics are simply too different.
Hotter air is thirstier than dry, capable of holding 7 percent more moisture for each degree Celsius (1.8 degrees Fahrenheit) warmer it gets. Climate change has bumped average air temperatures up 1.6 degrees Fahrenheit in the region since the early 1900s. The increase means the atmosphere more readily pulls water from streams and rivers, lakes and reservoirs, and plants and soils.
The effects can feed back on themselves, exacerbating drought under some conditions. Evaporation takes a lot of energy, which is used to transform water from liquid to gas, using up energy that would otherwise be absorbed into soil as heat. As soils dry out, there’s less water to evaporate—so solar radiation just heats the ground further.
“When we sweat, water evaporates from our skin, and that evaporation acts as a cooling mechanism for our body,” says Amir AghaKouchak, a climate scientist at the University of California, Irvine. “Earth’s surface works the same way.”
Crucially, hotter air also means the precipitation that arrives is more likely to fall as rain than snow. Snow in the high mountains acts like a water tower, storing winter precipitation until it melts in spring and summer, smoothing the boom-bust seasonal precipitation cycle. With hotter air, whatever snow does fall often melts earlier in the year. Both contribute to a “snow drought” effect. Agha Kouchak and a colleague Laurie Huning recently found that in the western U.S., snow droughts lasted 28 percent longer after 2000, compared with the previous 20 years.
And the effects cascade. Less snow can lead to drier soils, which can increase the chance of heat waves, which dry soils further.
In the face of continued climate change, some scientists and others have suggested that using the word “drought” for what’s happening now might no longer be appropriate, because it implies that the water shortages may end. Instead, we might be seeing a fundamental, long-term shift in water availability all over the West.
“Climate change is leading to a steady aridification of the western U.S.,” says Cook. “We’ve shifted the baseline drier and drier, making it easier for natural variability to send us into a drought and harder for natural variability to drag us out of an event.”
Impacts on the ground
The effects of the ongoing drought are already playing out across many western states.
Along the California-Oregon border in the Klamath River basin, water reserves are so low that farmers in the region will receive only 8 percent of the water they usually get. The Yurok and Karuk tribes, which steward salmon and other fish populations along the river, are concerned that it won’t have enough water to keep the fish healthy.
The Colorado River, the source of water for almost 40 million people, is struggling as well. Even hefty March snows that pushed the overall snowpack in the headwater region up to about 85 percent of its long-term average peak couldn’t make up for the water deficit that had built up, and the snow has disappeared much more quickly than normal, says Brad Udall, a climate scientist at Colorado State University.
Lake Mead, the giant reservoir atop the Hoover Dam fed by the Colorado, is only 39 percent full, down over 130 feet since 2000. If the water level drops to below 1,075 feet (it was at 1079 at the end of April), a major water conservation plan will be triggered for the first time ever. The Bureau of Reclamation expects this to happen this June. If that threshold is passed, water allotments to the states that rely on the Colorado’s water will adjust in 2022; states on the lower reaches of the river, like Arizona and Nevada, will be hit with major cuts.
Tribes in the river basin, who have long lacked consistent access to the clean and plentiful water to which they have legal rights, are also feeling the effects of the drought.
“Navajo Nation has been the epicenter of drought for years,” says Bidtah Becker, a Navajo attorney and co-author of a new report outlining the water access challenges, highlighted by COVID-19, for tribes across the region. “This is a climate change matter, and it’s really important for the federal government to invest appropriately now so that we’re building water systems that are resilient for the future.”
The fear of another year of intense wildfire is growing across the region, as well; recent years have seen some of the largest and most disruptive fire seasons in recorded history, driven in part by extremely dry conditions. In April in California, measurements of the “fuel moisture content” in chamise, a common and flammable shrub, were at dryness levels not usually seen before June.
A good run of rain or late-season snow could offset some of the worst impacts, says Udall, but the likelihood of that showing up wanes as the season marches on. “We could maybe get lucky,” he says, but luck now would simply assuage the meteorological, short-term drought—not the bigger problem.
“As warm and hot and record-setting as it has been the last few years,” he says, “what you need to keep in mind is, these are some of the coolest temperatures you’re going to experience in the next 100 years. Because it’s just going to get hotter. You ain’t seen nothing yet.”