How severe is the megadrought in the West?

Summer for many Americans is the time to enjoy being outside. But for much of the United States, this year’s extreme drought, wildfires, smoke and heat waves have made enjoying outdoor activities nearly impossible and continue to threaten the livelihoods and health of people and ecosystems across the country. With summer 2021 barely half over, and conditions likely to worsen in coming months, these extreme conditions provide a stark reminder that the chronic impacts of climate change will be one of our greatest 21st-century challenges.

As bad as 2021 has been, the story of drought in the West doesn’t begin this year. Since 2000, severe drought has drained western reservoirs, increased ground-water extraction, promoted giant wildfires and forest die-off, and coincided with ever-intensifying heat waves. We’ve had a bit of a bad run.

But how bad has that run really been? Pretty bad, actually. In 2020, our research team published a study demonstrating that 2000 to 2018 was among the worst 19-year drought periods in at least 1200 years, second only to a so-called “megadrought” in the late 1500s.

Now, 2021 is shaping up to be the region’s most severe drought year in modern history, pushing the 2000s drought into its 22nd year, an over two-decade-long event that will likely be the West’s driest 22-year period in at least 1,200 years. In other words, 2021 will probably be remembered as a fork in the road for western drought, when an already long and severe drought had a big growth spurt and entered legitimate megadrought territory.

The term megadrought arose in the 1990s through the study of tree-ring records. Measurements from many thousands of trees across the West give us an exceptionally accurate record of annual soil-moisture conditions that stretches back more than 1,200 years. It is in this record that the story of megadroughts — severe droughts that stretched on for multiple decades or even a century — has been revealed.

From 800 AD to 1600 AD, the West suffered four megadroughts, each pummeling the region with intense and prolonged dry conditions. A megadrought in the 1200s is especially infamous because it lasted the better part of a century and coincided with the depopulation of indigenous cliff-dwelling settlements in the Southwest. The last of the megadroughts left its mark in the late 1500s, lasting approximately 30 years and including the year 1580 CE, the worst single drought year in at least 1,200 years.

As of July, our projected estimates indicate that 2021 will very likely finish among the worst three to five drought years in the past 1,200 years. These exceptionally dry conditions will push the 2000s drought to the top, overtaking the 1500s megadrought as the event with the driest 22-year period in more than a millennium. Will the current drought soon end — or will it survive to 30 years, the age of the 1500s megadrought, or persist even longer? We don’t know, but it will take more than just one or two lucky wet years to make up for the dryness accumulated since 2000.

A difference between the current megadrought and those of the past is that it has not exclusively been a matter of chance — this drought has been strengthened by human-caused climate change. Warming from greenhouse-gas emissions enhances the atmosphere’s thirst for moisture from soils, plants and lakes. Warming also reduces mountain snowpack and may even push storm tracks north — away from the dry southwestern United States. Based on climate model simulations, our best estimate is that human-caused warming trends account for 30 to 45 percent of the severity of the 2000s drought so far. In other words, if the last two decades of fickle storms in the West had occurred without human-caused warming, the resulting drought would have been serious, but not in the same ballpark as the megadroughts of the past.

This assessment that warming worsens droughts in the West is not based solely on climate modeling. The Earth has been faithfully storing clues about its environmental history in more than just tree rings. Shorelines and mud sediments from ancient lakes, vegetation preserved in pack-rat nests, and other natural archives point to profound drying across the West as the globe warmed coming out of the last glacial period approximately 12,000 to 15,000 years ago, and again roughly 6,000 years ago when the Northern Hemisphere received its most sunlight in 100,000 years. The message from these past periods is clear: when the globe warms, the West dries.

 Multiple lines of evidence indicate that human-caused warming will continue to load the dice toward increasingly severe and longer-lasting droughts in the western US. A western water crisis may very well be underway and the ever-increasing risks require that drought resilience locally must be immediately pursued, while greenhouse-gas reductions must be an urgent priority globally.

Yes, there’s been a lot of bad news lately, but the good news is that our science has given us the ability to anticipate the future. That power has alerted us to the seriousness of the risks we may face, but it has also given us the power to influence how the future will unfold.

The choice is ours, but as we ponder our decision it may be wise to reflect on the words of the late Nobel laureate, Sherwood Roland: “What’s the use of having developed a science well enough to make predictions if, in the end, all we’re willing to do is stand around and wait for them to come true?”

Park Williams is an associate professor of hydroclimatology in the Geography Department at UCLA. Williams’ research focuses on the causes and consequences of drought and he teaches courses on global climatology and climate change. Follow him on Twitter: @peedublya

Benjamin Cook is an adjunct associate scientist at the Lamont-Doherty Earth Observatory of Columbia University. His research focuses on drought, climate change, and interactions between the earth surface and the climate system. He teaches courses on global climatology and climate change. Follow him on Twitter: @DustyBowl

Jason Smerdon is a Lamont research professor at the Lamont-Doherty Earth Observatory and co-director of the Undergraduate Program in Sustainable Development, both at Columbia University. He studies climate variability and change over the last two millennia and is coauthor of the book “Climate Change: The Science of Global Warming and Our Energy Future.”