Stephanie Pappas 1 day ago
Deep drought and early heat waves are setting the western half of the United States up for a dry, fiery summer.
About 91% of the West was in drought as of June 22, with 55% experiencing extreme or exceptional drought, while two back-to-back heat waves exacerbated the drying of soils and vegetation, increasing the potential for wildfires. Meanwhile, water supplies are dwindling: Lake Mead, the reservoir formed by the Hoover Dam, is the lowest it has been since the 1930s when it was first filled, and California’s reservoirs are 50% lower than usual for this time of year, according to the Associated Press.
The current conditions are part of a 22-year-long drought in the western U.S., the likes of which have not been seen for more than 400 years prior to 2000. But this summer is likely to eclipse the drought of the last two decades, said Benjamin Cook, a climate scientist at the NASA Goddard Institute for Space Studies.
“Even in the context of those last 20 years of drought this year really stands out,” Cook told Live Science. “For the Southwest, it’s very likely to be the worst single year of drought since 2002.”
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In the immediate term, drought in the West is promoted by La Niña conditions, which were in place through winter 2020-2021, according to Scott Handel, a meteorologist at the National Oceanic and Atmospheric Administration (NOAA). During La Niña, surface temperatures in the eastern Pacific Ocean are cooler than usual, which tends to promote drier weather in the West. This natural variation in Pacific temperatures is now in a neutral state (neither La Niña or El Niño), Handel told reporters on June 17, and a reversal to El Niño conditions — which bring more moisture to the Southwest — is not likely through the winter.
Extended drought is part of the history of the U.S. West. Modern meteorological records in the West go back little more than a century, but tree rings hold a longer record of wet and dry years. These data, stretching back 2,000 years in some cases, reveal that long, deep droughts occur even without the influence of climate change. The period between 800 and the 1500s, in particular, was marked by several long droughts. The most comparable to today’s 22 year-long dry spell occurred in the 1500s and ultimately lasted 30 years, said Park Williams, a bioclimatologist at UCLA. A less-severe drought in the 1200s lasted a full century.
The current drought ranks with these “megadroughts” of the past, but it’s being exacerbated by climate change. Over the past seven decades, soil moisture has trended lower and lower on the hottest days in the Southwest, according to research published June 17 in the journal Nature Climate Change. The top 5% of hottest days in the Southwest are now 22% drier than they were in the 1970s, the study found, with the situation worse in California and Nevada, where they are 30% drier. This dryness is due to declines in winter and spring moisture, study author Karen McKinnon, a climate scientist at UCLA, told Live Science.
Climate change causes more winter precipitation in Western states to fall as rain instead of snow, while warmer spring temperatures lead to more evaporation and transpiration, or surface moisture lost to the atmosphere early in the season. These declines in snow and spring moisture reduce river flow in the spring and summer.
And these changes feed back on themselves, said Connie Woodhouse, a paleoclimatologist at the University of Arizona who studies past streamflow. Multiple years of drought in a row yield extra-dry soils, which take up water that would otherwise travel into rivers and reservoirs.
“A lot of the snow when it melted didn’t go into the rivers, it went into replenishing the soil moisture,” Woodhouse told Live Science.
A feedback loop connects drought, dry soils and heat waves. In moist conditions, solar energy goes into heating and evaporating water before it starts increasing the air temperature. In drier conditions, with nowhere for that energy to go, temperatures climb quickly. With the right weather conditions, such as a high-pressure ridge that parked over the western U.S. the week of June 14, this can lead to record-breaking heat.
What this all adds up to, Williams said, is a region already prone to drought being pushed to the edge by the influence of climate change.
“By our calculations, if we were to remove the global warming trend then we’d still be in a bad drought and it would still be one of the worst droughts in the last 400 years,” Williams told Live Science, “but it wouldn’t even come close to contending with the megadrought of the 1500s.”
The summer ahead
The immediate impact of the drought will be to push the potential for wildfires earlier in the season by two to four weeks, said Gina Palma, a fire meteorologist with the Department of Agriculture. Fuel for wildfires, especially sagebrush, juniper and pinyon pine, is drying out earlier than usual, Palma told reporters during the June 17 news briefing.
“What we are seeing now in June is typically what we would see in July or August,” she said.
The greatest risk will be at high elevations and in foothills, Palma said. One silver lining of drought is that it reduces grass growth at lower elevations, providing less fuel for grass fires. In June, areas of greatest risk are in the Southwest and along the Pacific Coast, Palma said: New Mexico, Arizona, southern Colorado, Utah, eastern Nevada, Northern California and parts of Oregon and Washington. July may bring relief for Arizona and New Mexico, as long-term forecasts are predicting a rainy monsoon season for these regions, but that month will push above-normal fire risk into the Pacific Northwest and Northern Rockies, with much of California still under elevated threat. By August, monsoon moisture is expected to push north, shifting the area of concern into the Northern Rockies, Dakotas, Pacific Northwest and the foothills and higher elevations of California, Palma said.
“Much of the West will have been above-normal at some point,” she said.
Actual ignition of fires still depends on a few factors, though. One is the prevalence of fire weather, which means gusty winds and dry lightning. The other is human activity. Many fires are sparked by people. Reducing those opportunities with fire bans and responsible behavior can reduce the ultimate impact of Western wildfires.
Drought also increases the demand for water both by individuals and agriculture (which uses 80% to 90% of all water used in the West, according to the USDA). Water restrictions are already in place in many Western cities as water managers warn of limited supply; but crops are a tougher nut to crack, NASA’s Cook said.
“That’s what we’ve seen in previous drought years, municipalities able to step up and conserve water and change their water use in ways that did not severely impact day-to-day lives,” he said. “But there is much less flexibility [in agriculture]. An orange grove or an almond grove in the Imperial Valley [in California] needs a certain amount of water, and that’s kind of it.”
In a warming world, Western drought is likely to become more and more normal. The baseline climate will make it easier to fall into drought, Cook said, and harder to climb out. This raises questions about how the region will adapt in the future.
“As long as life has been in western North America, it has probably been constrained by water availability,” Williams said. “And as water is becoming less available, we are going to have to figure out how to use less of it.”
Originally published on Live Science
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