By Katherine Dempsey
Nov 13, 2014
Ancient shorelines in the Great Basin of the western United States reveal clues to severe ancient droughts and that could help us better predict climate change for the future.
Scientists in the U.S. and elsewhere across the globe are trying to use lake clues signifying wet and dry periods over thousands of years in order to tie these periods to past climate changes. A warming planet is expected to cause increased dryness and drought in many areas, including the western U.S.
Nevada State Climatologist Douglas P. Boyle studies past shorelines at terminal lakes (lakes with evaporation as the only outlet) such as Nevada’s Walker Lake and other closed lakes because they so sensitive to climate fluctuations. His research group creates lake-level simulations using hydrologic and atmospheric models and historic data in order to gain insight about previous climate.
Walker Lake levels are dropping as the drought in California and Nevada persists, said Boyle, an associate professor in the geography department at the University of Nevada, Reno. Without sufficient water upstream for crops, farmers are now pumping groundwater, he said, but it’s uncertain how long the drought will persist, and groundwater is a finite source.
At the Comer Abrupt Climate Change Conference held in southwestern Wisconsin this fall, he gathered with other top climate change scientists and showed them how rising and falling lake levels depict climate change. He presented preliminary results from research conducted with his doctoral advisee Benjamin Hatchett and others at the University of Nevada.
“We can try to figure out how those climates existed and why they existed and how the earth responded to those climates to give us just a better understanding of climate change, why it happens and what are the impacts,” Boyle said.
Low shorelines signify dry climate and high shorelines signify wet climate. Boyle’s team created corresponding “wet” and “dry” simulations of Walker Lake depths using the average monthly precipitations and average monthly temperatures from the 20 wettest and 20 driest water years from 1920 to 2011. Their model of the Walker River Basin produced “runoff” based on the synthetic dry and wet climates, resulting in both dry and wet simulated lake levels.
Boyle’s group has mapped their wet and dry simulations against the most recent “highstand” of Walker Lake in the late 1800s and its shoreline during the Medieval Climatic Anomaly, which struck the Great Basin of the U.S. with two “megadroughts.” One of the megadroughts spanned 240 years, 850 to 1,090 A.D., and the other spanned at least 180 years – 1140 to 1320 A.D. Throughout these times, annual precipitation was at least 40 percent less than precipitation in our present-day climate, according to early results from the Nevada State Climate Office at the University of Nevada, Reno.
Other researchers have used datable objects like shells, wood and trees to find the historical locations of the past shorelines. Boyle said closed-basin lakes are “extremely sensitive” to varying climate and provide a barometer of what the climate was like.
Walker Lake’s elevation above sea level is 3,914 feet this year compared to more than 4,100 for the most recent highstand. For consistency, Boyle’s team documents shoreline elevations above sea level rather than water depths that can vary from place to place in a lake. The current level is only about three feet higher than the level of a “dry simulation” run after the Comer conference. The “wet simulation” was 344 feet higher than the 2014 level. The 2014 level is 52 feet lower than the MCA lowstand during severe and prolonged drought periods.
Boyle noted, however, that Walker Lake’s 2014 elevation is due to agricultural water consumption upstream rather than climate. If farming weren’t a factor, the lake level would likely be nearly 200 feet higher at present, making it 147 feet higher than the MCA low, according to new simulations run after the Comer Conference.
Boyle’s group also tried to figure out what made the climate change, examining the atmosphere of a subset of the 20 wet and dry water years. They noticed high pressure over the West Coast during drought periods but low pressure during wet periods. High pressure blocked the storm track, meaning it couldn’t carry storms to Nevada and California. Conversely, low pressure let the storms pass over the western Great Basin.
High pressure pushing the storm track to the Pacific Northwest during the last three winters has created the drought, Boyle said.
Now, Boyle’s team needs to figure out what created the atmospheric behaviors – the high pressure and low pressure – that accompanied the dry and wet climates from the subset, he said. Plus, they need to learn more about the link between those behaviors and “paleo periods” of wetness and dryness corresponding to 13,000-14,000 years ago and the MCA, respectively.
“We’re trying to understand from the historic period where we do know what the atmosphere was like for wet years and dry years and try and better understand how the atmosphere might have been behaving in the paleo wet and paleo dry periods,” Boyle said.
Learning why climate changed during those paleo periods could give us more information about how the planet might act in response to global warming driven by human fossil fuel use, Boyle said.
“We have an incredible amount of CO2 and other greenhouse gases that have been put into the atmosphere,” Boyle said. “We’re still learning how the earth is responding to that.”
Boyle is also working with climate scientist Sean Birkel to create a climate model of China’s Tarim Basin. They are trying to pinpoint the climate conditions that would allow lake Lop Nor to display a highstand of about 2,625 feet, which was the level of the shoreline at around 1850. Lop Nor has been dry since around 1950.
At the Comer Conference, Birkel spoke about the difficulty of trying to simulate Lop Nor’s level. These problems stem from the Tarim Basin’s large size – about 1,200 miles long. Two key atmospheric circulation systems control precipitation and influence climate in the region: monsoon-driven precipitation in the south and westerly wind-driven precipitation in the north. Factoring in both of these systems complicates modeling.
Birkel is helping to develop Climate Adaptation and Sustainility (CLAS) software for the University of Maine’s Climate Change Institute.
Yonaton Goldsmith, a Ph.D. student in geochemistry at Lamont-Doherty Earth Observatory at Columbia University, studied China’s Dali Lake, comparing lake records with Chinese cave records to find out how the intensity of the East Asian summer monsoon varied throughout the last 16,000 years. The lake is located on the edge of the monsoon’s reach.
Three types of evidence help determine lake levels – lake sediments containing shells, beach ridges, and alluvial deposits. “If we find lake sediments that have shells in them, we can say that the lake was higher,” he told scientists at the Comer Conference. Beach ridges signify the lake elevation, and alluvial deposits show lower lake level.