California's Holocene Ultra Drought

A paleoclimate paper published today, based upon a core sample from Kirman Lake in central-eastern California, corroborates previous evidence of an epic drought in California that lasted from 6000 BCE to 1000 BCE (with one notable interruption, the 4.2 ka event, which increased moisture in California at the same time it led to extreme aridity, at least, from Europe to South Asia).  

There were also several centuries of drought in California in the Middle Ages, the same drought that is famous for causing the collapse of the Anasazi (a.k.a. Ancient Puebloan) civilization.

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. . . .

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. . . . 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. . . .

Martin Hoerling, a research scientist with the National Oceanic and Atmospheric Administration. . .  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 . . . "affirm, that when the east Pacific is cold compared to the remaining tropical [waters], the American West tends to have low precipitation.” 

From the L.A. Times. The open access paper and its abstract are as follows:

Glen M. MacDonald, et al., "Prolonged California aridity linked to climate warming and Pacific sea surface temperature", Scientific Reports 6, Article Number 33325 (September 15, 2016)

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. 

We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. 

The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka).

In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1° to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.

The body text of the paper notes that: "In the 21st century California has experienced arid conditions and the most intense drought on record; 10 of the last 14 years have been drier than normal, and the last three have been the hottest and driest since 1895. . . . The hydroclimatology of the region and relation to Pacific Ocean forcing is representative of California in general and the Sierra Nevada where 60% of California’s managed water originates."