‘Megadrought’ vexes the West
Supercharged by climate change, worst weather event in centuries points to a much drier future
Since 2000, the West has been stricken by a dry spell so severe that it ranks among the biggest “megadroughts” of the past 1,200 years. But scientists have found that unlike the decades-long droughts of centuries ago, this one has been supercharged by humanity’s heating of the planet.
Researchers analyzed the dry and wet cycles that have swept across western North America over centuries by examining the ancient records inscribed in the growth rings of trees.
Cores extracted from thousands of trees enabled the researchers to reconstruct soil moisture and examine the West’s hydrological history, including long droughts that appear as sets of narrow growth rings running through the wood.
Using data from trees at 1,586 sites across the region, from Montana to northern Mexico, the scientists identified four other intense droughts, in the late 800s, the mid-1100s, the 1200s and the 1500s.
The driest 19-year period struck in the late 1500s, and the second-worst drought of that duration has afflicted the region since 2000.
The research team, led by scientists at Columbia University’s Lamont-Doherty Earth Observatory, also used 31 climate models to estimate the influence of higher temperatures unleashed by climate change from 2000 through 2018.
They found the region’s average temperature during those years was 1.2 degrees Celsius (2.2 degrees Fahrenheit) hotter than it would have been without human-caused warming, and they estimated climate change was responsible for 47% of the drought’s severity.
The scientists concluded that in fueling the
drought, global warming has turned what would otherwise have been a relatively moderate event into one of the most severe megadroughts since 800 A.D.
The findings, published April 17 in the journal Science, add to the growing body of research revealing major challenges for the American West as the planet heats up with the burning of fossil fuels.
As temperatures rise, the region is being ravaged by more dangerous heat waves and more intense wildfires. And the hotter, drier conditions of the past two decades offer a preview of how climate change will continue to complicate efforts to manage water in the arid West, where rivers and groundwater basins were already being over-pumped before rising temperatures piled on added pressure.
Speaking about the study in interviews, several academic researchers and managers of water districts said the findings indicate the region must prepare for having less water in the long run, through both dry and wet periods, as hotter temperatures crank up evaporation losses and leave less water flowing in streams and rivers.
Alongside other scientific work, they said, the latest research shows the warming climate is responsible for a large portion of the drought over the past two decades and will continue to have an influence in shrinking the available supplies, even in years with average amounts of rain and snowfall.
“We all should expect to have longer, drier, hotter droughts. That should be what we anticipate in the future and probably not in the too-distant future,” said Connie Woodhouse, a paleoclimatologist at the University of Arizona who wasn’t involved in the study. She said this research, along with work by other scientists, offers a “heads-up” that records of past hydrology can no longer be considered alone without factoring in climate change.
This takeaway applies to water sources all over the West, and it’s increasingly been at the center of discussions about rethinking management of the Colorado River. Long overallocated and taxed by years of hot drought, the river’s largest reservoirs now sit less than half full.
“We need to understand that these warmer temperatures that we’re experiencing, and will continue to experience at some level, will make things worse,” Woodhouse said.
Other researchers have said infrastructure and water management systems that were developed during wetter times will have to undergo substantial shifts. They’ve suggested regions across the West will need a host of adaptation efforts tailored to local circumstances, including building new infrastructure, deploying new technologies, and rewriting water policies and rules.
There is still plenty of room to make more progress conserving water on farms, in industries, and in cities, said Margaret Garcia, an Arizona State University professor who focuses on water infrastructure and management.
In most Western states, irrigation for agriculture accounts for 75% or more of total water use. Garcia said farming areas can improve irrigation efficiency or shift to less water-intensive crops, yielding big water savings.
Urban areas have made dramatic conservation gains with more efficient fixtures, Garcia said, and still can conserve more. Preparing for the effects of higher temperatures will require different strategies depending on the area, Garcia said, such as efforts to reduce water demand, repair leaky pipes, or build more facilities to recycle wastewater for reuse.
“I think it’s something that we all should be conscious of, not panicking about. It’s a slow-moving kind of problem,” Garcia said. “We’ll want to look at our water infrastructure and where there’s opportunities to make it more efficient, to perhaps look at where our vulnerabilities are.”
What’s crucial, she said, is to start thinking about options now, and to have community conversations about how to prepare.
Fueled by climate change
Scientists have discovered that past megadroughts in the West persisted much longer than the current two-decade drought. The shortest event lasted nearly 30 years, from 1575-1603.
The megadrought in the 1200s was the longest, lasting nearly a century. During this epic drought, the Ancestral Puebloan people, who had farmed and built villages in the Four Corners region, are thought to have abandoned their cliffside homes at Mesa Verde and Chaco Canyon.
The long-lived trees that recorded the ancient droughts included ponderosa pines, Douglas firs, piñon pines and blue oaks, among others.
Park Williams, a climatologist at Columbia
and the study’s lead author said he estimates his team used data from more than 30,000 trees. The datasets were compiled over decades by hundreds of other scientists, who extracted wood cores by boring into tree trunks using tools called increment borers.
Analyzing these giant sets of data, Williams and his colleagues were able to reconstruct soil moisture and compare the different droughts. They found that without the effects of human-caused warming, the post-2000 drought still would have been a drought, but it wouldn’t be on par with the megadroughts of centuries ago.
The data also revealed how conditions can flip dramatically between dry and wet. Just before the latest dry spell, the scientists found, the years from 1980 through 1998 saw the wettest 19-year period in at least 1200 years.
In the study, the researchers wrote that natural variability could very well end the drought in the coming years, “and this transition may be underway after a wet 2019.” On the other hand, Williams noted that past megadroughts have been punctuated by individual wet years in parts of the region, only to continue.
Much of Arizona had a wet winter, which filled the reservoirs on the Salt and Verde rivers that supply Phoenix.
But looking more broadly across 11 Western states, drought has flared up again over the past year.
As of this week, the U.S. Drought Monitor website shows that 50% of the West, from Washington to New Mexico, is now abnormally dry or experiencing drought conditions. That’s a dramatic increase from the same time last year when less than 17% of the region was classified as being dry or in drought.
And when the next big wet period comes, Williams said, it will be important to remember that the region over the long term is still getting drier, as it has been over the past half-century.
“The warming process is really in its infancy,” he said. And as temperatures continue to rise, “our sense of what average is, is going to have to change.”
How much additional warming occurs, and how much aridification it fuels, will depend to a large degree on how much more planet-warming pollution humanity pumps into the atmosphere in the coming years, Williams said.
“Even though the West is going to dry, the amount of drying that we see in the West in the next 50 years to 100 years is going to be substantially impacted by global carbon emissions today,” Williams said. “If we were to make major changes in energy on a global scale today that result in less emissions, then drought would probably not be as bad as it would be otherwise in the second half of the century.”
‘A hotter and drier future’
A growing number of scientific studies have documented how warmer temperatures caused by accumulating greenhouse gases have begun to affect the western United States.
Scientists have estimated that about half the decline in the Colorado River’s flow since 2000 has been due to higher temperatures. Researchers with the U.S. Geological Survey found the river is sensitive to warming and could lose about one-fourth of its flow by 2050 as temperatures continue to climb.
In other research, scientists have found that farmers in parts of the western U.S. who depend on snowmelt runoff to help irrigate crops will likely be hit hard by decreases in snow with climate change and that this could affect food production.
Some researchers have focused on how the shrinking snowpack in the mountains will gradually erode the ability of water managers to use that snowpack in calculating the available water supply each year.
They’ve found that by mid-century, more than two-thirds of areas in the West that rely on snowmelt runoff will see their ability to predict seasonal drought significantly hampered as the mountains are increasingly denuded of snow.
Alongside these long-term decreases in average snowpack, other signs of drying have emerged. Scientists have determined that the climate dividing line along the 100th meridian — which explorer John Wesley Powell identified in the 1800s as the boundary between the arid plains of the West and the wetter eastern U.S. — has been gradually shifting eastward.
With the science pointing to more drying in the decades to come, some managers of water agencies said the latest research underlines why it’s critical to plan for worst-case scenarios.
“The implications of the study are alarming,” said Kathryn Sorensen, director of Phoenix’s Water Services Department. “Looking forward, this study and others tell us that we can expect surface water availability to diminish over time, perhaps dramatically.”
Sorenson’s department serves about 1.6 million people, and most of the water comes from rivers.