Sierra Nevada snow pack – the larger picture

According to a story in the Arizona Daily Star, University of Arizona researchers claim “The combination of drought and high temperatures that shrank Sierra Nevada snow pack and brought water shortages and destructive fires to California this year may have no precedent in 500 years, according to a study of tree-ring records.”

This makes for ominous headlines that will get some press. However, had the researchers gone farther into the past, they would have found that extreme drought and low snow pack conditions were more common, and all due to natural variation of the climate.

Here is what the IPCC has to say about North American droughts in their 4th Assessment Report:

Climate Change 2007: Working Group I: The Physical Science Basis The Record of Hydrologic Variability and Change in the Americas

Multiple proxies, including tree rings, sediments, historical documents and lake sediment records make it clear that the past 2 kyr [2,000 years] included periods with more frequent, longer and/or geographically more extensive droughts in North America than during the 20th century. Past droughts, including decadal-length ‘megadroughts’, are most likely due to extended periods of anomalous SST [sea surface temperature], but remain difficult to simulate with coupled ocean-atmosphere models. Thus, the palaeoclimatic record suggests that multi-year, decadal and even centennial-scale drier periods are likely to remain a feature of future North American climate, particularly in the area west of the Mississippi River.

There is some evidence that North American drought was more regionally extensive, severe and frequent during past intervals that were characterised by warmer than average NH summer temperatures (e.g., during medieval times and the mid-Holocene). There is evidence that changes in the North American hydrologic regime can occur abruptly relative to the rate of change in climate forcing and duration of the subsequent climate regime. Abrupt shifts in drought frequency and duration have been found in palaeohydrologic records from western North America. Similarly, the upper Mississippi River Basin and elsewhere have seen abrupt shifts in the frequency and size of the largest flood events. Recent investigations of past large-hurricane activity in the southeast USA suggest that changes in the regional frequency of large hurricanes can shift abruptly in response to more gradual forcing. Although the paleoclimatic record indicates that hydrologic shifts in drought, floods and tropical storms have occurred abruptly (i.e., within years), this past abrupt change has not been simulated with coupled atmosphere ocean models. Decadal variability of Central Chilean precipitation was greater before the 20th century, with more intense and prolonged dry episodes in the past. Tree-ring based precipitation reconstructions for the past eight centuries reveal multi-year drought episodes in the 14th and 16th to 18th centuries that exceed the estimates of decadal drought during the 20th century.

I understand from the Star story that the researchers used a pre-existing 500-year database of tree rings, so they cannot be accused of “cherry-picking”on purpose, although it so appears. I do not have access to the full paper, so I don’t know if the researchers tempered their claims with the longer history. In my opinion, however, based on what was reported in the Star, the researchers appear to be “crying wolf.”

Related articles:

Drought in the West
Droughts in the Southwest put in perspective

University of Arizona Scientists Find Evidence of Roman Period Megadrought