Folsom Lake's water levels usually rise in May when the Sierra Nevada snowpack melts away and flows down to the reservoir near Sacramento.

However, the drought that has gripped most of the United States West this year is now so severe that the lake is about half as full as it usually is. Purple lupine fields line large swaths of the dry lakebed in place of water.

Megadrought in the US

Folsom Lake's plight is emblematic of the western United States' worsening drought. As of May 6, 67 percent of the country experienced "severe" drought or worse, with a whopping 21% still experiencing "exceptional" drought, the worst level in the US Drought Monitor's system. Water levels are moving toward a point that would cause the first formal scarcity declaration for the Colorado River basin at Lake Mead, one of the two large reservoirs on which some 40 million Americans depend.

According to experts, the situation is unlikely to change in the foreseeable future, as 2021 looks set to prolong the "megadrought" that has gripped the country largely unabated since 2000.

"It's just climate change that propelled this case to be one of the worst in 500 years," says Ben Cook, a climate scientist at Columbia University's Lamont Doherty Earth Observatory.

Drought

Drought can manifest itself in a variety of ways. There's  "meteorological drought," which measures the amount of rain or snow accumulated against a long-term average. "Hydrological drought" refers to the amount of water that runs into streams and waterways and is deposited in mountain snowpack and underground aquifers. There's the agricultural or ecological drought, which affects the soil, plants, and livestock.

Dry conditions are nothing new in the American West, which has seen centuries of water booms and busts.

Since the establishment of the Drought Monitor in 2000, the country has been in a state of drought nearly every year. According to a team of experts, the 20-year drought is comparable to every other in the past 1,200 years.

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Megadrought

They combined hundreds of tree-ring records from around northern Mexico and the western United States to create a database dating back to about 800 AD. Trees keep track of rainy and dry years by rising faster and leaving tighter rings as their roots feel wetter soil. The researchers used tree rings and climate simulations to create a soil moisture pattern, which shows how severe droughts have been.

Previous megadroughts lasted decades-"20, 30, even 40 years, far above what we've had to deal with in the last 100 years," Cook says. Droughts like the Dust Bowl of the 1920s lasted just five to ten years, which was catastrophic for those who lived through it but much less destructive than a multi-decade drought.

Worsened by Climate Change

By contrast, the latest dry spell is already long and heavy. It is only second on their list to the famine of the 1500s, which happened in a world unaffected by human-caused climate change. That should give us pause, according to Cook, because it demonstrates that the West will naturally swing into such severe drought states without the added nudge of climate change. An extra push from humans could exacerbate the results.

And, according to their report, that is precisely what has happened: climate change has driven this "megadrought" into dangerous territory. It would have been poor regardless-their predictions say it would have been the eleventh most severe on record-but the extra heat from climate change accelerated the drying, bringing it to the second most severe drought in the last 1200 years.

Human-caused climate change, combined with human reshaping of natural hydrological systems-by damming waterways, planting large swaths of crops, and other methods-has changed the baseline conditions so drastically that there is no way to return to what was once considered normal. The physics is clearly incompatible.

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