The #megadrought parching 75 percent of the Western US, explained — Vox

West Drought Monitor March 16, 2021.

From Vox (Lili Pike):

Rising temperatures and lack of rain threaten to decrease water supplies and bring more wildfires this summer and in the years to come.

The Western US is in the midst of yet another dangerous dry spell. The drought has been building over the past year, and since November, a greater stretch of the West has been in the most severe category of drought than at any time in the 20 years that the National Drought Mitigation Center has been keeping records.

Western states are already facing water shortages, and with the National Weather Service projecting that the dry stretch will continue, the problems that accompany droughts are likely to pile up heading into this summer…

That last drought also led to other fallouts: billions of dollars in economic losses as farmers were forced to let fields lie fallow and a 50 percent drop in electricity production from dams. It also contributed to the death of over 100 million trees, which fuels bigger wildfires, like the ones that ripped through the West last summer. If the current drought continues, similarly stark consequences lie ahead.

Unfortunately, these droughts in rapid succession aren’t an aberration but rather a sign of what’s to come. Climate change is driving more severe droughts and spurring longer, more troubling “megadroughts” across the Western states. Here’s what you need to know about what the future holds for these states as temperatures rise.

The latest episode in a megadrought

This time last year, the West was relatively drought-free after a wet winter in 2019. But by now, the region has swung from 27 percent in drought to 77 percent, according to the latest data from the US Drought Monitor released March 11.

What happened?

Over the past year, the drought has been building due to a lack of rain, a weak summer monsoon in the Southwest, and intense summer heat waves. “If I had to pinpoint one thing that really drove the drought to where we are right now, it was the heat of last summer,” said Brian Fuchs, a climatologist at the University of Nebraska’s National Drought Mitigation Center.

High summer temperatures sucked the moisture out of the soil and evaporated water resources.

The Four Corners, where Utah, Arizona, New Mexico, and Colorado meet, has been the epicenter of this drought, Fuchs said. The dark splotch on the map below shows that those states as well as Nevada have been experiencing the most intense drought.

US Drought Monitor March 16, 2021.

Now the West is in the winter wet season, but due, in part, to the La Niña weather pattern, too little rain and snow is falling to make up for the preceding dry months.

Some rain and snow may still fall, but the National Weather Service’s seasonal forecast projects that drought conditions will persist across the Western US through May, the end of the current forecast period. “We do have some time to maybe put a dent in some of these deficits that we’ve seen through the winter,” said Fuchs. “Now the idea that we are going to catch up completely — that’s going to be tough.”

The trajectory of this drought episode remains unclear, but scientists say that it is actually part of a bigger megadrought — a decades-long dry spell, punctuated by severe droughts. This megadrought began around 2000, and as the chart below shows, the majority of land in the West has been in some level of drought ever since.

West drought time series ending March 18, 2021 via the US Drought Monitor.

To understand why the West is in a megadrought, the role of climate change, and what it means for the region’s future, we first need to look at some historical clues.

What’s behind longer and more intense droughts? Climate change.

Based on data from tree rings and other ecological records of weather and climate patterns of the last few thousand years, we know that the West is no stranger to drought. In an April 2020 tree ring study published in Science, researchers found that several megadroughts occurred between 850 and 1600 — before humans started pumping massive amounts of greenhouse gases into the atmosphere. These droughts were likely caused by cool temperatures in the Pacific Ocean that prevented rainfall from reaching the Southwest.

While natural variability has been a factor in recent droughts, the current megadrought is also being driven by climate change, according to the study. Higher temperatures, caused by greenhouse gases, have increased evaporation and decreased precipitation in the spring across the region. The researchers were able to identify that climate change accounted for 46 percent of the drought’s severity.

Without climate change, there still would have been a drought, but “anthropogenic warming was critical for placing 2000–2018 on a trajectory consistent with the most severe past megadroughts,” they wrote. The current megadrought, which they traced from 2000 through 2018, was the second driest 19-year episode in the 1,200-year record.

This finding is not just important for how we understand the current crisis, but also for the coming decades in the Western US as temperatures continue to climb.

The latest National Climate Assessment, authored by 13 US federal agencies in 2018, laid out a grim future for the Southwestern states: Rising temperatures will increase the likelihood of megadroughts in the region and make droughts more frequent and severe, according to the scientific literature cited.

While annual precipitation in the Southwest may not necessarily decrease, the hotter annual temperatures will burn off more moisture, contributing to droughts, the researchers explained in the Science tree ring study.

Leave a Reply