Tuesday, December 29, 2020

If ancient drought choked the Southwest for centuries, what does the future hold?

Posted By on Tue, Dec 29, 2020 at 7:10 AM

The ancient people of western Utah’s Danger Cave lived well. They ate freshwater fish, ducks and other small game, according to detritus they left behind. They had a lush lakeside view with cattails, bulrushes and water-loving willows adorning the marshlands.

But over time, the good life became history. As heat and drought set in, the freshwater dried up, and the ancients were forced to survive by plucking tiny seeds from desert shrubs called pickleweed. Archaeologists know this from a thick layer of dusty chaff buried in the cave’s floor.

It might be ancient history, but science tells us that the past could also become the future. In fact, thanks to global warming, regional climate patterns linked to extended periods of heat and drought that upended prehistoric life across the Southwest thousands of years ago are setting up again now.

“The benefit of any kind of paleoclimate data is that it tells us what nature is capable of,” said Matthew Lachniet, a paleoclimatologist at the University of Nevada, Las Vegas.

The climate risk across the Southwest is actually growing, based on Lachniet’s recent study of a different cave about 200 miles across the Great Basin, which covers most of Nevada and the western half of Utah.

His geochemical data from Leviathan Cave in Nevada shows that drought can last 4,000 years – findings that Lachniet’s team cross-checked against paleoclimate data from the Arctic and tropical Pacific. In short, the story in the cave data suggests a worst-case scenario that could – and probably should – guide planning throughout the Southwest, home to 56 million people.



“In this case, we know that nature is capable of extended dry conditions that are even longer than they are today,” Lachniet said. “And the concern is, if we go into the future of a warm Arctic and a warm western tropical Pacific, that it will have the same effect on the climate of the Southwest” that it did during those arid times back at Danger Cave.

Lachniet’s scientific paper, released last summer in the journal Paleoceanography and Paleoclimatology, sprang from his work analyzing part of a rock pillar from Leviathan Cave, which is in Nevada’s Basin and Range National Monument.

The paleohistory that his team analyzed in the stalagmites that formed in Leviathan cover nearly the same time frame that humans first started using Danger Cave, which is now considered one of the Great Basin’s most important human history sites.

The stalagmites are created by the slow buildup of calcium carbonate layers as water droplets fall from the cave ceiling. They offer a “readable” record, much like the tree ring records that are a staple of climate science.

“We’re able to date those layers using chemistry rather than actually counting them like they do in a tree ring,” Lachniet said. “We measure the concentration of uranium naturally occurring in the calcite, and that tells us how old the stalagmite is.”

Using the top 6 inches of an 18-inch stalagmite that researchers removed from Leviathan Cave and brought back to the lab for analysis, researchers measured the length and depth of dry periods on the land surface going back about 13,400 years.

And although the study methods could not determine how much temperature changed, they did validate a period of aridification from 9,800 to 5,400 years ago as the western Pacific warmed and Arctic sea ice declined because Earth’s orbit shifted.

Lachniet explained that these same regional conditions that caused aridification in Nevada and the Great Basin before – the warming Pacific and shrinking sea ice – also could be expected to prompt a return to those ancient conditions in the Southwest, this time because of warming from greenhouse gases. Lachniet said his findings, along with other recent research, further imply that not only is Nevada expected to become warmer because of these trends but so are the Colorado Rockies and parts of the headwaters of the Colorado River.

Climate scientist Cody Routson of Northern Arizona University has looked at some of the same trends in his own research and found them “pretty eye-opening.”

The reason? Water managers and climate scientists already are alarmed about the increasingly severe “hot droughts” in the Colorado River Basin, with high temperatures coinciding with less rainfall.

And water policy is generally based on a worst-case scenario, derived from tree ring analysis going back about 1,500 years to the medieval megadrought on the Colorado River. That drought, Routson said, was “way worse than anything we have experienced.”

But this new paleoclimate data suggests that a clearer picture of what the future holds for the Southwest might be found in the climate of the early and middle Holocene, roughly the same time ancient people were adapting to the changes around Danger Cave and the drought that gripped the Southwest from 9,800 to 5,400 years ago in Lachniet’s study.

So as the new studies confirm hot, dry periods lasting thousands of years, rethinking water and development planning time frames becomes urgent for 56 million people, whose lives and economies rely on water from the Colorado River and the Rio Grande basins, Lachniet said in his stalagmite research paper.

“What we’re seeing is that temperatures are going to increase, and that ends up producing much more evaporation, more loss of water from the reservoirs, a greater demand for water in agricultural systems where people are pulling in that water for use on their crops,” Lachniet said. “So even in the absence of any change in the amount of precipitation, we would expect to have more water scarcity in the future under those warmer conditions.”

Findings like these are prompting climate scientists, like researchers in other fields, to reconsider their reluctance to share their findings outside academia.

Using the words “worst-case scenario” in a scientific paper, as Lachniet does, is pretty bold, said Andrea Brunelle, a paleoclimatologist at the University of Utah. But she added that scientists – especially climate scientists – are increasingly sharing their findings with decision-makers, not just other scientists.

“Then the next really important step,” she said, “is getting this into the hands of water managers or land managers or whoever it is that makes the plans and lays out the policies for the future.”

This idea also seems to underlie new work from a team of international climate scientists that recently made the case that climate modeling centers should include simulations of past climates in their predictions of future climate change. Besides predicting scenarios for future climate, this approach will help scientists assess the impacts of human-caused greenhouse gas emissions and propose strategies for mitigation.

“Looking to the past to inform the future could help narrow uncertainties surrounding projections of changes in temperature, ice sheets and the water cycle,” said Jessica Tierney, an associate professor in the geosciences department of the University of Arizona who is lead author of a new research review paper in the journal Science.

The team behind that paper goes a step further, urging climate model developers to consider paleohistory while predicting the future.

“If your model can simulate past climates accurately,” Tierney said, “it likely will do a much better job at getting future scenarios right.”

To the east of Danger Cave lie the glowing white Bonneville Salt Flats. They are what’s leftover from the retreat of prehistoric Lake Bonneville 16,000 years ago.

The salt flats, which for decades have hosted land speed record attempts, provided a twist to a Danger Cave tour that took place before the pandemic began in March. As cars zipped across the salt flats during international Speed Week, Ron Rood of Metcalf Archaeology told a small gathering of tourists about the changes that had reshaped the land and human life over thousands of years all across the Great Basin.

The visitors gathered around as he showed a snippet of willow basketry and passed around small plastic tubs filled with such artifacts as animal bones, evidence of dinner rubbish and hand tools. He explained how hard it was to strip the seeds from the desert plants that took over the landscape when freshwater food sources disappeared.

“Maybe,” Rood said, “we can learn some lessons from that.”

This story is part of a project covering the Colorado River Basin and water in the West. It was produced by InsideClimate News in collaboration with public radio station KUNC in Greeley, Colorado.

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