Environment & Climate

How bad is the Western megadrought? Scientists look at tree rings to find context from history

Western Water Coverage | By Alex Hager

Published February 29, 2024 at 11:16 AM MST

Karen King gathers a core sample from a mountain hemlock tree at Lassen National Volcanic Park in northern California. King is the lead author on a study that puts the 21st Western megadrought into historical context.

The current Western megadrought is unlike any other dry period the region has experienced over the past 500 years.

That’s according to a new study in which scientists looked at tree rings to track changing temperatures going back to 1553. Researchers found that human-fueled climate change is driving temperatures higher, which makes soil drier and droughts more frequent, intense, and widespread.

Karen King, an assistant professor at the University of Tennessee-Knoxville and the study’s lead author, said it shows the role of temperature in shaping modern drought.

“We know that extreme heat has consequences,” King said. "We know that drought has consequences. So when they're compounded together, we can expect that those vulnerabilities are only going to be magnified, and the consequences are going to be more wide-reaching.”

The study, published in the journal "Science Advances,” analyzed cross-sections of trees from several Western states, including Utah, New Mexico, and Wyoming. The study’s authors concluded that the two-decade period from 2000 to 2020 is the warmest in half a millennium.

The consequences of dry conditions in the 21st century include significant strain on the Southwest’s most important water supply. The Colorado River, which supplies about 40 million people across seven states, has been shrinking as a result of those higher temperatures.

Policymakers around the region have struggled to rein in demand for water, even as more than two decades of dry conditions have shrunk the nation’s two largest reservoirs to record lows.

Eric Balken, executive director of Glen Canyon Institute, walks along a sandbar once submerged by Lake Powell. As the reservoir drops to record lows, it draws out of areas that were underwater for decades.

Some scientists and water managers say the Southwest’s currently dry period extends beyond the normal definition of “drought.” Instead, they say it should be categorized as “aridification,” a permanent resetting of the baseline for how much water enters the region’s streams, rivers, and reservoirs each year.

Warm temperatures are the first domino in a chain of changing conditions that impact water supply.

Since 2000, average temperatures in the upper Colorado River basin are more than 2 degrees Fahrenheit warmer than in the previous century. That region is where the Colorado River begins, mostly as snow in high-altitude portions of Colorado and Wyoming.

The new tree ring study shows how high temperatures have made the region’s soil drier. Dry soil means less water in streams and rivers.

When rain falls or snow melts, it seeps into the dirt before entering streams and rivers. When that dirt is saturated, it can’t absorb additional water, and snowmelt flows directly into nearby waterways. When the soil is dry, it acts like a sponge, soaking up water before reaching the places where humans collect it.

The data in this new tree-ring study and findings from similar research spell trouble for decision-makers trying to share a shrinking resource across a region with growing populations and a multibillion-dollar agricultural economy.

“While the future of precipitation in the region remains uncertain, projections of increasing temperatures pose a substantial risk for intensifying drought conditions and increasing water insecurity for these economically important, population-dense, and historically active megadrought regions,” the study’s authors wrote.

This story is part of ongoing coverage of the Colorado River, produced by KUNC in Colorado and supported by the Walton Family Foundation. KUNC is solely responsible for its editorial coverage.