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Climate 101is a Mashable series that answers provoking and Anne Marie Gonzales Archivessalient questions about Earth’s warming climate.

The water keeps going down.

Almost the entire Southwest is mired in various stages of drought as of April 21, 2021, resulting in falling water levels at the nation's two largest reservoirs, Lake Mead and Lake Powell. The consequences could be unprecedented. For the first time in Lake Mead's 85-year existence, water levels may drop below a point this summer that triggers water cuts in Arizona and Nevada. (This would largely mean cuts to farmers and agriculture.)

Geological and climate records show that sustained droughts, lasting decades, come and go in the Southwest. But the current prolonged drying trend, which started some 20 years ago, is exacerbated by a rapidly warming climate. This makes the current drought not just long, but especially intense.

"It's two decades long and probably the worst drought in at least 400 years," said Benjamin Cook, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory who studies drought.

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The big picture is clear. In the last 50 years, precipitation trends in the Southwest haven't changed much and remained mostly flat, explained Jonathan Overpeck, a climate scientist at the University of Michigan who researches Southwestern drought. Yet, the amount of water flowing in the region's major artery, the Colorado River, has dropped significantly — by 16 percent — in the last century. The land is drying out, too. "The only thing that’s changing in a big way is temperature," said Overpeck.

"[Droughts are] going to get worse and worse unless we stop global warming," he added.

Indeed, Earth's temperature will continue to rise in the coming decades, but just how much depends on the amount of heat-trapping greenhouse gases civilization emits. Currently, humanity is on course to warm Earth by over 5 degrees Fahrenheit (3 degrees Celsius) by the century's end. The last time it was that warm, sea levels were some 30 feet higher and giant camels roamed the high Arctic.

A warmer Southwest — some regions of which have warmed by well over 3.6 F (2 C) since the late 19th century — evaporates more water from rivers and the nation's sprawling reservoirs. On mountains, more snow directly evaporates into the air in a process called sublimation, meaning less water ultimately flows into rivers. And, crucially, the region's trees and plants are losing bounties of water to the warming, dry air. "They’re losing a lot of moisture," explained Overpeck. These factors add up to a prolonged drying trend out West.

This certainly isn't a normal drought. "The reason this drought is so exceptional is likely because of climate change," emphasized Cook. "It makes it easier to get into a drought, and harder to get out. It makes droughts a bit more intense than they used to be."

But should today's drought be called a megadrought? Megadrought is a fuzzy term with no standard definition. Overpeck said "megadrought" is often used to describe a drought that's at least two decades long. Cook noted some researchers call it a megadrought, while others don't, but belaboring the nomenclature doesn't change what really matters: The drought is exceptionally dry and intense, and human-caused warming is likely playing an outsized role.

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Droughts are cumulative, meaning it's unlikely one good year of rain will eliminate a long regional drought. In 2019, for example, we saw normal rainfall in parts of the Southwest and a pretty wet year in California. But 2020 dashed hopes for climbing out of a prolonged drought. On top of warmer temperatures, the region's typical summer rainfall failed, and California received just half of its normal precipitation this winter. An important rain-fed reservoir has dried up in Northern California. The drought continues.

It's always possible a surprise late-season rain or snow changes the trajectory of the current drought, like 2014's impressively wet "Miracle May." But that's unlikely. "That's a hope against hope," said Overpeck. "That's like buying a lottery ticket."

Overall, the evidence points to an increasingly drying Western world. This demands improved water conservation, especially in water-gulping agriculture. Lake Mead, the nation's largest reservoir, is now about 40 percent full. "It's pretty dire," said Overpeck.

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