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WATER POLITICS

Via National Geographic, a report on the western US drought:

There have been brief moments of reprieve in the drought that has stretched on since 2000 in the western United States—a water-rich 2011, a snow-laden 2019—but those breaks have only highlighted the more dramatic feature of the last few decades: unrelenting dryness.

Without human-driven climate change forcing Earth’s temperatures up, the ongoing drought would still be painful and parched. But it would be unexceptional in the grand scheme of the past 1,200 years. A new study in Nature Climate Change shows that Earth’s warming climate has made the western drought about 40 percent more severe, making it the region’s driest stretch since A.D. 800. And there’s a very strong chance the drought will continue through 2030.

“Not only is this drought continuing to chug along, it’s proceeding at as full-steam pace as it ever has been,” says Park Williams, a climate scientist at UCLA and an author of the new research.

Soil moisture is at historic lows

For millennia, the most certain climate truth of the U.S. West has been that conditions change. A pulse of wet months or years will turn mountainsides and valleys lush green, then just as certainly a dry stretch will parch the green to brown.

But understanding what controls that variability, and its relationship to climate change, is critically important to everyone living in the region’s boom-bust water cycle.

Williams and his colleagues wanted to understand how intense the current drought has been compared to past events throughout the Southwest, from northern Mexico up to Idaho. In 2020, they published a study that examined 1,200 years of regional drought as recorded by the growth patterns of trees.

Trees track weather and climate in their rings, laying down thicker layers of new wood in years where moisture is plentiful. A stressful year of drought or extreme heat often results in a thin, undersized ring—a sign the tree put its energy into surviving rather than growing. Scientists use rings to tease out historical changes in temperature, precipitation, and other climate-relevant information. By reading the rings of more than 1,500 trees across the western U.S., some of which were growing as early as 800, and stitching together the story they told, the team previously created a map of soil moisture—a way of measuring the intensity of drought—across the region.

Soil moisture is a particularly useful way of assessing drought, says Brad Udall, a climate scientist at Colorado State University. Most people don’t think much about soil moisture, but it is “the reservoir upon which all life depends,” he says, because it sustains plants, the basis of all other life.

But hot, dry air sucks moisture out of the soil. And if rain or snow don’t replenish that invisible reservoir, the deficit grows, swelling the empty bucket that must be refilled before water can flow to rivers, streams, and aquifers. Recent research has shown that in years with low soil moisture, rivers and streams flow lower, since “any available water just gets sucked up by the soil,” says Udall. That’s a huge problem for the Colorado River and other watercourses across the West, which have seen flows dwindle to record lows in recent years.

In their 2020 paper, Williams and colleagues reported that between the years 800 and 2018, there were 40 droughts, four of which were exceptionally severe “megadroughts”: the first in the late 800s, another in the mid-1100s, one in the 1200s, and a fourth megadrought—the most intense on record—from 1575 to 1593.

When they published their analysis, the current drought looked like it would rank as one of the worst droughts on record, but not quite: The fourth megadrought in the 1500s had been drier, and all the previous megadroughts had lasted longer. But 2019, when they were analyzing the data, was a wet year. “We really thought it might be ending,” says Williams.

But it didn’t end: 2020 and 2021 were even drier. The summer of 2021 saw record-breaking heat waves that sucked any trace of moisture from the soil and drew down reservoirs so low that many farmers received little or no water to irrigate their crops. Meager water levels at major reservoirs along the Colorado River triggered mandatory cuts in the amount of water states like Arizona and Nevada could take.

When Williams and his colleagues added these most recent years to their analysis, it became abundantly clear that the current drought is now the driest on record (though not yet the longest, as several of the previous megadroughts lasted longer). In fact, two exceptionally dry years, 2002 and 2021, were most likely the most parched in the last 300, and two of the thirstiest in the historical record.

“This would have been a garden-variety drought,” says Connie Woodhouse, a climate scientist at the University of Arizona, who was not involved with the study. “But with warming…wow. And this is because of our own doing.”

Climate change supercharge to extreme drought

It didn’t have to be this way.

Human-caused climate change, the team found, turned the drought from bad to terrible. Without the extra shove from climate change, the last 20 years would have been run-of-the-mill dry, somewhere in the middle of the pack of the 10 driest periods in the past 1,200 years.

But climate change has made the atmosphere in the region hotter and drier and supercharged soil moisture loss. It was responsible for about 42 percent of the soil dryness over the 2000-2021 period—making 2021 alone about 20 percent drier than it otherwise would have been.

That’s roughly the same as the effect other studies have found from regional climate change, says Udall. Climate change is estimated to have driven about half the losses in the Colorado River’s flow in recent years, for instance—a huge effect with enormous real-world consequences. The amount of water available to states dependent on the Colorado is now being renegotiated because of drought-induced shortages. In Arizona, farmers are already readjusting their expectations and practices. Fire risks have skyrocketed. Entire communities are re-assessing their water plans.

Global temperatures have risen about 1.1 degrees Celsius (2 degrees Fahrenheit) since the late 1800s—so far. They are forecast to rise much more in coming decades unless governments make swift, decisive cuts to greenhouse gas emissions.

When the team published their first analysis a few years ago, it seemed like the end of the drought might be nearing. But even in that short time the intensifying effects of the warming climate have changed the game. “(The study authors) just had to wait two more years to see these records are being broken now,” says Danielle Touma, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado. “It’s amazing how quickly we’re seeing these events become more and more extreme.”

The kinds of impacts climate scientists expected to see in the next few decades are playing out right now, she says.

When will the drought end?

 No one can say for sure when this drought will end, but if history is any indication, it eventually will.

The problem is, unlike in the past, the underlying climate is so much hotter, and the drought-hole so much deeper, that erasing the soil moisture deficit wrought by 20-plus years of aridity will be harder than ever.

“The inertia of a drought of this magnitude is unlikely to be broken with a single year of good precipitation,” says Williams. “It’ll take a number of wet years to dig ourselves out.”

That’s a scary thought to many farmers and others who already have been waiting years for reprieve. To assess the likelihood that the region would switch out of this super-arid phase in the next few years, the study team examined the wet-dry patterns in any given 40-year stretch in their 1,200-year-long record. They then layered those historical patterns on top of the current status to estimate the odds of the drought lasting another year, or two, or even more. They repeated the exercise over and over to get a better sense of the odds.

Even absent climate change, there’s a very high chance the drought would last through 2023; in 94 percent of their simulations, it goes on through next year, and in 33 percent of their simulations it lasts all the way to 2030. But layer climate change’s added pressure on top and it’s suddenly much more likely the drought will continue at least through 2023: 94 percent of the time they saw it stretch that far. In 75 percent of the simulations, it went on until 2030.

“That would be absolutely devastating, honestly,” says Don Cameron, a farmer in California’s San Joaquin Valley. “If it went on that long, you’d see California agriculture just devastated. There’s no way around it.”

Climate change has so thoroughly reshaped the West that it’s not enough to think about just what it will take to rebound after this drought, says Touma

“Maybe we can recover for a couple of years. But there’s always going to be the next one, and it’s going to be worse because of warming temperatures,” says Touma. “The question is…how will we adapt to a world where this is even more commonplace and normal?