Tenaya Lake, elevation 8,150 feet, at Yosemite National Park. One of numerous Sierran lakes with Medieval-era tree stumps rooted at its floor.
by Chris Gray
Dec 04, 2008
If the past 2,000 years are any guide, California could dry out significantly, with or without global warming. But global warming isn’t helping.
The Patagonian drylands, looking west toward the Argentine Andes.
The 20th century stands out as a very wet period while climate change and urban sprawl in the state are all but certain to make the current century a dry one and threaten the state’s supply of water.
With global warming displacing the jet stream of high-altitude winds and the water it carries to the north, dry conditions that were the norm prior to the 20th century are all but certain to return and threaten the way of life in sunny California.
All across the warming Earth, the climate of already arid lands will march toward desert, posing water shortage in the American Southwest. But, in some places such as Africa, agriculture could be devastated.
“It’s bad news,” said Jay Quade, a geoscience researcher at the University of Arizona who traveled to South America to study its past climate. “It means famine in many parts of the world.” He said tensions over the arid border of India and Pakistan likely would be exacerbated by a drier climate.
In California, lakes east of the Sierra Nevada have been lowered to provide water that supports the way of life in the famously sunny state.
Artifacts found on the dry bed of Owens Lake in California. These date from times in the past when the bed was exposed and dry due to drought.
Scott Stine, an environmental scientist from California State University, East Bay, has found stumps of trees rooted in the artificially exposed bed of Mono Lake. The trees grew hundreds of years ago when the lake was naturally low due to drought.
The streams that feed this landlocked lake near the Nevada border have been diverted to the city of Los Angeles, and these diversions have drawn Mono Lake to low levels, exposing the stumps.
For thousands of years, the landlocked lakes of eastern California and western Nevada have risen and fallen in response to natural changes in water supply, but rainfall during the last century provided an abundance of runoff for the tributary streams that flow into these lakes.
“With or without global warming, there is no reason to think that this wet period will continue,” Stine said.
Quade found similar patterns in South America. A warmer world means a drier place for regions across the globe that are already arid, his research shows. Land use could change dramatically.
“Just small shifts in the rain belts could really shift what agriculture takes place,” Quade said.
All this would be bad news for California. The unusually wet conditions have provided a reliable source for drinking water for urban areas across the state, as well as agricultural irrigation in the Central Valley and Southern California, Stine said.
Mono Lake, one of several lakes showing evidence of a dry past, has dropped 45 feet since Los Angeles completed its aqueduct in the 1940s. By imposing drought, global warming could increase the drawdown.
“Nature had pulled Mono Lake down even lower than Los Angeles had taken it,” said Stine, a professor of geography and environmental sciences.
Stine said that traces of the isotope Carbon-14 in stump wood reveals that the lake stood at low levels for centuries during medieval times prior to 1350.
While glacial ice cores serve as windows into the past in the Arctic and Antarctic, pluvial lakes — landlocked lakes without an outlet to the sea — are the gauge into the prehistoric climate of the world’s deserts.
“Obviously, high stands of lakes indicate wet times, low stands of lakes indicate dry times,” Stine said.
High in the Andes of Bolivia and in the lifeless reaches of Chile’s Atacama Desert, Quade also found these landlocked long dried out lake beds, a good place to study past changes in climate.
Lake Titicaca, on the border of Bolivia and Peru, is the largest lake in South America, and a little less than half the size of Lake Ontario. But geomorphic records indicate a much larger lake, called Lake Tauca, once stretched far beyond Titicaca’s modern shores.
“You can look up the side of the hill and see a shoreline” from the past, Quade said.
Quade’s research dates Lake Tauca’s peak depths to between 14,000 and 16,000 years ago, about the time that large armadas of icebergs were released from huge but thinning ice sheets into the North Atlantic Ocean. The rush of freshwater and ice stopped normal circulation of the ocean currents and suddenly sparked an advance of another relatively short-lived ice age in the Northern Hemisphere.
Quade hypothesizes that the cold North Atlantic Ocean shifted the wet, tropical climate of the equator 20 degrees to the south, greening South America’s deserts, such as the Atacama, and raising the level of Lake Titicaca fivefold.
“The idea that Lake Titicaca was five times its present size is staggering today,” Quade said.
Both Quade and Stine recently presented their findings at the Comer Foundation’s abrupt climate change conference in Wisconsin. They joined other leading climate scientists to discuss their latest research from across the globe supported by the foundation.
One of the current scientific tasks in climatology has been to find out what would happen if a Heinrich Event, a sudden change in climate caused by the release of ice into the North Atlantic, reoccurred. The phenomenon is named for German researcher Harmut Heinrich, who first identified the cyclical nature of these upheavals.
“Catastrophic collapse of Greenland ice in the next 100 years would be great for Bolivia,” Quade said. “It would mean that it would rain like hell.”
Wallace Broecker, the Columbia University climatologist who first figured out the oceans’ circulation patterns, said at the conference that such a collapse would be unlikely, though. In a warming world, there is probably not enough ice left to cut off the ocean conveyor system that he identified. Previous collapses occurred in the midst of receding ice ages, triggering a secondary advance.
Researchers at Princeton University and the University of Miami proposed the theory that, as the world gets warmer, the earth’s deserts will get drier, because of shifts in weather patterns. Conversely, when the earth gets colder, its deserts get wetter.
Quade’s studies in the southern reaches of South America help bolster this potentially alarming trend. Laguna Cari Laufquen, in the arid Patagonia region of Argentina, went through its wettest period, not after the last Heinrich Event, but at the very coldest part of the last major ice age, 22,000 years ago.
Broecker and Stine have been collaborating with Quade’s work at Cari Laufquen in Argentina.
Since the height of the last ice age, the Argentine lake has shrunk to half its ancient levels — levels that would theoretically continue to drop as global temperatures rise.
Most of the American West has long been vulnerable to periodic drought. But with the world now warming, both the severity and the length of droughts might well increase, according to Stine.
“You would expect a warming planet to impose drier conditions on mid-latitude arid lands,” Stine said. But he worried the past may not be a reliable indicator of the West’s uncertain future. “We have turned our atmosphere into an artifact. With the atmosphere now composed of so many greenhouse gases, we don’t know what the future holds.”
