Pacific Ocean’s response to greenhouse gases could extend California drought for centuries

Warming forces have caused millennia of dryness in California’s prehistory, and greenhouses gases could do the same. Clues from prehistoric arid periods in California show that today’s increasing greenhouse gas levels could lock the state into drought for centuries, according to a study by an international research collaboration led by Glen MacDonald from UCLA and including Katrina Moser from Western University and her former graduate student, Amy Bloom.

The study, published in the journal Nature Scientific Reports, looked at how natural climatic forces contributed to centuries-long and even millennia-long periods of aridity in California during the past 10,000 years. These phenomena — sun spots, a slightly different earth orbit, a decrease in volcanic activity — intermittently warmed the region through increased radiative forcing, and recently have been joined by a new force: greenhouse gases.

As long as warming forces like greenhouse gases are present, the resulting radiative forcing can extend drought-like conditions more or less indefinitely, says MacDonald, a distinguished professor of geography and of ecology and evolutionary biology at UCLA.

“Radiative forcing in the past appears to have had catastrophic effects in extending droughts,” says MacDonald, an international authority on drought and climate change. “When you have arid periods that persist for 60 years, as we did in the 12th century, or for millennia, as we did from 6,000 to 1,000 BCE, that’s not really a ‘drought.’ That aridity is the new normal.”

Moser adds, “Owing to the importance of California’s economic and agricultural activities in North America, and the dependence of these activities on water, prolonged aridity in California in the future will have far-reaching effects across the United States, into Canada and beyond.”

Researchers tracked California’s historic and prehistoric climate and water conditions by taking a sediment core in the Sierra Nevada mountains. They pulled a 5 cm wide, 3m deep cylinder of sediment from the bottom of Kirman Lake and analyzed it in 1 cm sections, creating the most detailed and continuous paleo-environmental record of California ever.

As part of the study, Bloom, now an assistant professor at Illinois State University, developed a mathematical model to reconstruct drought over thousands of years using diatoms. Moser is an expert in diatoms, which are microscopic algae that have cell wall made of opaline silica.

“Opaline silica is basically like glass,” explains Moser, a geography professor at Western’s Faculty of Social Science. “When the diatoms die, the organic parts decay and you are left with the resistant cell wall that is beautifully ornamented and can be identified to the species level.”

These microfossils accumulate in the lake sediments and due to the fact that different species require different environmental conditions, lots can be learned about the environment at the time the lake sediments were deposited from the diatom fossil assemblage.

Bloom’s mathematical model was used to infer lake salinity and depth, which could then be used to determine periods of aridity, which was instrumental in determining California drought records for the past 10,000 years.

The team also correlated their findings with other studies of California climate history, and for the first time, cross-referenced these with histories of the Pacific Ocean’s temperature taken from marine sediment cores and other sources.

The researchers discovered that periods of increased radiative forcing could produce drought-like conditions that extended indefinitely and that these conditions were closely tied to prolonged changes in Pacific Ocean surface temperatures.

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Katrina Moser and members of her research team prepare for core sampling.
Aerial view of Kirman Lake.
Diatoms sample from Kirman Lake. Diatoms are microscopic algae that have cell wall made of opaline silica.