What Is a Megadrought? Southwest U.S. at Its Driest in 1,200 Years

The southwestern U.S. is experiencing its most severe "megadrought" since at least the year 800—and it's likely to continue for years.

The severe dry conditions gripping this region of the country have been ongoing for 22 years now and have been as bad or even drier than any other U.S. megadrought within the past 1,200 years. Scientists say human-driven climate change is a significant factor.

Simply, a megadrought is an extended period of very dry conditions that last longer than a drought would normally last—though scientists have debated its exact definition.

Up until recently scientists had thought the Southwest's current megadrought may not have been as severe as another one that occurred in the 1500s. A new study, however, has concluded that it's worse.

Since the year 2000, the average soil moisture deficit has been greater than it was during even the driest parts of the megadroughts of the past 12 centuries.

The effects of the megadrought can be clearly seen. In 2021, two of the largest reservoirs in North America—Lake Mead and Lake Powell—reached their lowest levels ever recorded.

That in turn leads to increases in groundwater pumping from aquifers, leading to environmentally and ecologically harmful deficits that can take several years of wet weather to reverse.

Then there's the implications for wildfires, which we can expect more of if global warming continues to drive drought conditions, according to the Columbia Climate School.

The ongoing drought is thought to be driven in part by natural variations in the climate—the study researchers found that less severe megadroughts had occurred repeatedly in the western U.S. region from the years 800 to 1600.

Climate models suggest that the current drought would not be as severe without the added factor of human-driven climate change, which is thought to be responsible for about 42 percent of the soil moisture deficit since the year 2000, according to a University of California Los Angeles (UCLA) press release outlining the new study. It was published in the journal Nature Climate Change on Monday and involved researchers from UCLA and Columbia University.

Jason Smerdon, a paleoclimatologist at Columbia University's Lamont Doherty Earth Observatory who co-authored the work, told Smithsonian magazine back in 2020: "No matter which way you slice it, the clear indication is that the current drought ranks right up there with the worst in more than a thousand years, and there's a human influence on this of at least 30 percent and possibly as much as 50 percent in terms of its severity."

California drought
A sign highlighting drought effects is seen near dry, cracked mud at the edge of a farm in California in April, 2009. The U.S. southwest is experiencing a megadrought that is as bad or worse than any other in 1,200 years, a February 2022 study has suggested. David McNew/Getty