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NY Times, Sept. 15 2015
Study Finds Snowpack in California’s Sierra Nevada to Be Lowest in 500 Years
By NICHOLAS ST. FLEUR
The snow that blanketed the Sierra Nevada in California last winter, and
that was supposed to serve as an essential source of fresh water for the
drought-stricken state, was at its lowest levels in the last 500 years,
according to a new study.
The paper, published on Monday in the journal Nature Climate Change,
used tree-ring data from centuries-old blue oaks to provide historical
context for the mountain range’s diminished snowfall. As of April 1, the
snowpack levels were just 5 percent of their 50-year historical average.
The paper is the first to create a model that describes temperature and
precipitation levels on the Sierra Nevada that extend centuries before
researchers started measuring snow levels each year.
“The 2015 snowpack in the Sierra Nevada is unprecedented,” said Valerie
Trouet, one of the authors of the study and a paleoclimatologist at the
University of Arizona. “We expected it to be bad, but we certainly
didn’t expect it to be the worst in the past 500 years.”
Snowmelt from the Sierra Nevada fills reservoirs that provide a third of
all of the drinking water for the state of California, as well as water
to fight wildfires and to generate electricity.
“The scope of this is profound,” said Thomas Painter, a snow hydrologist
with NASA’s Airborne Snow Observatory, adding that models like the one
developed in the study suggested a dry future for California in years
beyond the current drought. “This has been a very bad drought, and being
able to understand the context of it is extraordinarily important.”
To determine snowpack levels from 500 years ago, the research team
combined two data sets of blue oak tree rings. The first set provided
historical precipitation levels from more than 1,500 blue oaks from 33
sites in California’s Central Valley. The team compared part of that
data from the years 1930 to 1980 with actual snowpack measurements and
found that both findings matched.
Using this correlation, the team combined the precipitation data with a
second data set of tree rings that looked at winter temperatures from
1500 to 1980.
After analyzing the data, the team determined with its model that
snowpack levels as low as this year’s were a once-in-1,000-years event.
But because of rising temperatures caused by human activities, the
researchers said they thought that snow droughts would become much more
frequent.
California has a Mediterraneanlike climate, which means that it receives
most of its precipitation in the winter and is dry during the summer.
The blue oaks that encircle the Central Valley and cover the rolling
foothills of the Sierra Nevada serve as a good indicator of snowfall on
the mountains because they are very sensitive to winter precipitation,
according to David W. Stahle, a geoscientist from the University of
Arkansas and an author of the paper.
Many of the winter storms that pile snow on the Sierra Nevada also fall
as rain on the blue oaks. The trees use the moisture stored in the soil
to grow during the spring and summer, and the width of their tree rings
reflects the amount of precipitation from the preceding winter. Wide
rings indicate wet winters, while narrow rings denote dry ones.
“Having an ultrasensitive record of wet-season precipitation in ancient
blue oak trees is a gift of nature to the modern water-dependent world,”
Dr. Stahle said in an email.
Some researchers said the results were valuable to understanding the
current drought. Others found the results to be less surprising.
“I don’t think anything they say is alarmingly shocking,” said David
Rizzardo, the chief snow surveyor at the California Department of Water
Resources. “From a department perspective, you can go back 500 years or
10,000 years, it doesn’t really change the context of the here and now.
We’re stuck in this situation.”
Noah Diffenbaugh, a climate scientist at Stanford, said the study
provided valuable information about the historical context of the
drought, which would help in understanding its causes. He said that,
when combined with previous studies, the new findings helped “provide
strong evidence that global warming has substantially increased the
probability of getting these extremely low snow conditions.”
A. Park Williams, a bioclimatologist at Columbia University, said the
study added to evidence that rising temperatures had exacerbated the
lack of snow in California.
“We are now migrating into this new world where temperatures are
higher,” Dr. Williams said. “So even though the chances of an event like
this were extremely unlikely in the past, in the future it will be more
likely to occur.”
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