In 2000, researchers took a coring from the bed of a small, shallow lake in the Eastern Sierra Nevada mountains.
They analyzed the organic matter and chemicals in the sediments to reconstruct a climate record of the past 10,000 years. They then compared it with reconstructions of ancient ocean temperatures.
The results echoed previous studies that have found a link between past periods of climate warming, cool sea-surface temperatures in the eastern Pacific Ocean and centuries-long droughts in California and the West.
Does that mean that global warming is pushing California to the threshold of endless drought?
Authors of the study, published Thursday in the online journal Scientific Reports, cautioned that it remains uncertain whether climate change driven by greenhouse gas emissions will affect ocean dynamics in the same way as past climatic shifts.
The mechanics of global warming, caused by a rise in heat-trapping gases, differ from past warming related to changes in the Earth's orbit, solar radiation or decreased volcanic activity.
“We don’t know how the Pacific Ocean is going to respond,” said Glen MacDonald, the paper’s lead author and a
Some global warming studies predict that Northern California could grow wetter. And a 2014 study co-authored by UCLA climate scientist Alex Hall concluded that overall rainfall amounts in Southern California won't change much in coming decades.
Nonetheless, MacDonald said his paper is a warning that "we simply cannot take off the table the possibility that the eastern Pacific will cool relative" to western ocean waters "and we'll have La Niña-like conditions exacerbating aridity in California."
MacDonald and his co-authors from several universities used variations in the coring from Kirman Lake to track climatic shifts.
When it was dry, the Mono County lake contracted, and parts of it turned to marsh, altering the composition of sediments. Fossil pollen and charcoal analysis also indicated climate-related changes in the region's vegetation and wildfire frequency.
The Kirman record, researchers concluded, points to several centuries of drought in the Middle Ages, as well as a mid-Holocene Epoch period of aridity that began about 8,000 years ago and lasted, with one notable interruption, for roughly 5,000 years.
Ocean-bed corings indicate that during the same period, sea-surface temperatures in the eastern Pacific were cooler — analogous, the authors wrote, to a multimillennial La Niña.
"It so nicely explains why California was so dry during that period. You see this great linkage to the Pacific," MacDonald said of the results.
Other studies of tree rings and ancient tree stumps submerged in Sierra lakes also have documented prolonged droughts. And previous paleoclimate research has linked western aridity with ocean conditions.
"There is little new here," Martin Hoerling, a research scientist with the National Oceanic and Atmospheric Administration, said in an email.
Hoerling, who was not involved in the study, said it was consistent with a 2007 paper he co-authored that found the West's warm and dry medieval period was strongly influenced by tropical ocean conditions, especially a cold east Pacific.
Climate models, he added, "do affirm, that when the east Pacific is cold compared to the remaining tropical [waters], the American West tends to have low precipitation."
But they also "predict a fairly uniform ocean-warming pattern … with some indication that the tropical east Pacific warms more than the rest of the tropical oceans: an El Niño-like response if you will."