Simply put, a megadrought is a period of extreme dryness that lasts for decades. Within that period there may be occasional better, wet, years, but the respite is brief. The dryness soon returns and drought maintains its long-term grip.

For the Southwest — including Arizona, Nevada, Utah and parts of California, Colorado and New Mexico — the drought has lasted two decades.

There have been wet years here and there, including the winter of 2016-17, when huge storms hit California (and led to a different set of problems, including a nightmarish wildfire season) and 2019, when a wet spring lifted much of Arizona out of drought, briefly, for the first time in years.

But most of the region has been in chronic drought since 2000. Not coincidentally, that was the last time Lake Mead, the giant reservoir on the Colorado River that is now at a historic low level, was anywhere near full.

The Southwest is an arid region, and much of it is classified as desert. “Normal” means high heat, low humidity and relatively little precipitation. But normal also meant the region usually got enough precipitation, from late summer through winter, to avoid the worst.

Scientists have identified long periods over the past 2,000 years in the Southwest when that normal pattern was disrupted, most likely by natural variability in Pacific Ocean temperatures. Cooler water created atmospheric conditions that blocked most storms from reaching the region.

Researchers found the evidence for these megadroughts in the annual growth rings in the trunks of ancient trees. Rings that are close together are a sign of stunted growth. And in the Southwest, what stunts growth is a lack of moisture in the soil.

These megadroughts have lasted for decades. One in the 2nd century lasted for 50 years. Others, in the 9th, 12th, 13th and 16th centuries, lasted between 30 and 40.

The current Southwestern drought is the driest 20 year period since the last megadrought in the late 1500s, and the second-driest since the 800s. Time will tell whether it lasts as long, or longer.

Some natural climate variability is at work now, too, so conditions could swing toward the wet side for long enough to pull the region out of drought. (Although water scarcity would still be an issue — the Southwest is now home to tens of millions of people, industries and agriculture that have created huge demand for water.)

But those ancient megadroughts occurred long before smokestacks and tailpipes started spewing carbon dioxide into the air, warming the planet and changing the climate. Global warming is affecting droughts now, and accounts for about half of the severity of the current Southwestern drought.

With its warmer temperatures and shifts in precipitation patterns, human-caused climate change reduces the odds of a given year being a wet one and makes it less likely that the region will have a few good years in a row. In short, climate change makes it more likely that this megadrought will continue.