Centuries-Long California Droughts
California experienced centuries-long droughts in the past 20,000 years that coincided
with the thawing of ice caps in the Arctic, according to a new study by UC Davis doctoral
student Jessica Oster and geology professor Isabel Montañez.
The finding, which comes from analyzing stalagmites from Moaning Cavern in the central Sierra Nevada,
was published online November 5, 2009 in the journal Earth and Planetary Science Letters.
Stalagmites As Climate Recorders
The sometimes spectacular mineral formations in caves such as Moaning Cavern and Black Chasm
build up over centuries as water drips from the cave roof. Those drops of water pick up trace
chemicals in their path through air, soil and rocks, and deposit the chemicals in the stalagmite.
"They're like tree rings made out of rock," Montañez said. "These are the only climate
records of this type for California for this period when past global warming was occurring."
Matching the Drought Record to Arctic Thawing
At the end of the last ice age about 15,000 years ago, climate records from Greenland show a
warm period called the Bolling-Allerod period. Oster and Montanez's results show that at the
same time, California became much drier. Episodes of relative cooling in the Arctic records,
including the Younger Dryas period 13,000 years ago, were accompanied by wetter periods in California.
A Shifting Jet Stream
The researchers don't know exactly what connects Arctic temperatures to precipitation over
California. However, climate models developed by others suggest that when Arctic sea ice
disappears, the jet stream -- high-altitude winds with a profound influence on climate -- shifts
north, moving precipitation away from California.
Current Arctic Thawing and California Drought?
"If there is a connection to Arctic sea ice then there are big implications for us in California,"
Montañez said. Arctic sea ice has declined by about 3 percent a year over the past three decades,
and some forecasts predict an ice-free Arctic ocean as soon as 2020.
Linking the Stalagmite Record to Climate
Oster's analysis of the past is rooted in a thorough understanding of the cave in the present. Working
with the cave owners, she has measured drip rates, collected air, water, soil and vegetation samples,
and studied what happens to the cave through wet and dry seasons to determine how stalagmites are
affected by changing conditions.
Isotope Chemistry
Oster collected stalagmites and cut tiny samples from them for analysis. The ratio of uranium to its
breakdown product, thorium, allowed her to date the layers within the stalagmite. Isotopes of oxygen,
carbon and strontium and levels of metals in the cave minerals all vary as the climate gets wetter or drier.
"Most respond to precipitation in some way," Oster said. For example, carbon isotopes reflect the
amount of vegetation on the ground over the cave. Other minerals tend to decrease when rainfall is
high and water moves through the aquifer more rapidly.
Oxygen-18 isotopes vary with both temperature and rainfall. Measuring the other mineral
compositions provides more certainty that the changes primarily track relative rainfall.
One-Centry Resolution
The stalagmite records allowed Oster and Montañez to follow relative changes in precipitation in
the western Sierra Nevada with a resolution of less than a century.
"We can't quantify precipitation, but we can see a relative shift from wetter to drier conditions
with each episode of warming in the northern polar region," Montañez said.
Other authors on the paper are Warren Sharp, a geochronologist at the Berkeley Geochronology Center,
and Kari Cooper, associate professor of geology at UC Davis. The research was funded by the
National Science Foundation.
About UC Davis
For 100 years, UC Davis has engaged in teaching, research and public service that matter to California
and transform the world. Located close to the state capital, UC Davis has 31,000 students, an annual
research budget that exceeds $500 million, a comprehensive health system and 13 specialized research
centers. The university offers interdisciplinary graduate study and more than 100 undergraduate majors
in four colleges — Agricultural and Environmental Sciences, Biological Sciences, Engineering, and
Letters and Science — and advanced degrees from six professional schools — Education, Law, Management,
Medicine, Veterinary Medicine and the Betty Irene Moore School of Nursing.
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| Photo of a stalagmite that has been cut to reveal its internal growth structure and to allow compositional sampling. Image by Isabel Montañez, Department of Geology, University of California at Davis. Enlarge. |
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