When California Gov. Jerry Brown stood in a snowless Sierra Nevada meadow on April 1 and ordered unprecedented water restrictions because of the drought, it was the first spring in 75 years of observation that the area lacked snow.
Now, six months later, researchers say this year’s record-low snowpack may be far more historic -- and ominous -- than previously realized.
“We were expecting that 2015 would be extreme, but not like this,” said senior study author Valerie Trouet, a paleoclimatologist at the University of Arizona.
The paper is the latest in a series of studies that have sought to characterize the depth of California’s four-year drought and place it in a broader context.
While those studies have used tree ring records and other proxies to estimate past temperatures, precipitation and dryness, this is the first to examine Sierra Nevada snowfall specifically.
Snowpack is a key factor in California’s water supply: Melting Sierra Nevada snow helps replenish and sustain state reservoirs and provides the state with roughly a third of its water. Because of that, researchers and state officials began monitoring snowpack in the 1930s and have established 108 measuring stations throughout the Sierra Nevada.
This spring, researchers found that the April 1 snow water equivalent was only 5 percent of average since monitoring began. In the case of the meadow in Phillips -- where Brown ordered water use reductions -- spring snowpack usually reached a height of 5 1/2 feet during that time of year.
To reconstruct past snow conditions, Trouet and her colleagues compared data from the reporting stations as well as two previous tree ring studies. The first involved data from 1,500 blue oak trees extending back to the year 1400 and was used to estimate rainfall. The second study used tree ring data from different trees to model past temperatures.
“What we know about snow and how it varies from year to year is that there are two important climatic factors that play a role,” Trouet said. “One of them is the amount of precipitation that falls and the other is the temperature at the time that precipitation falls. With higher temperatures, your precipitation is going to fall as rain.”
When researchers put all the data into a chronology, they saw how exceptional the 2015 snowfall was: The chance that such a “snow drought” would affect the entire Sierra Nevada more than once every 500 years was less than 5 percent, researchers concluded.
For lower mountain elevations, where temperatures are higher, the return period was estimated to be 1,000 years. At higher elevations, where temperatures are much more likely to reach freezing and cause any precipitation to fall as snow, the return period was just 95 years.
The researchers noted that while California’s total precipitation in 2015 fell within the bounds of natural variability, winter temperatures were among the highest ever recorded. Global warming threatened to shorten the period between repeat events, according to the authors.
“With anthropogenic warming, those high temperatures are going to be rising,” Trouet said. “We can assume that the return interval is going to get shorter.”
Scientists said the research helped put California’s current drought in perspective.
The work offers “another piece of the puzzle in an increasingly converging picture of a really exceptional California drought,” said Benjamin Cook, a climate scientist at NASA Goddard Institute for Space Studies, who was not involved in the research.
And although forecasters said it was increasingly likely that a powerful El Nino would result in a wetter than usual fall and winter, it may not contribute to large snowpack.
“Temperatures this winter may be warmer than usual, despite an increased likelihood of wet conditions,” wrote Daniel Swain, a Stanford University doctoral student in environmental and earth system science and the author of California Weather Blog.
“In that sense, the present paper is very relevant,” wrote Swain, who was not involved in the research. “Even with increased precipitation, snow at lower elevations may actually be below average if temperatures are warm.”