Study shows warming temperatures shifting snow to rain across Sierra Nevada
Special to The Union
TREES MOVING UPHILL: The great tree migration
Sierra Nevada trees have noticed it’s getting warmer, and they are voting with their feet.
Seedlings of three species of cold-loving conifers are no longer sprouting and growing in middle elevations where they used to be common, state biologists with the California Department of Fish and Wildlife found.
They counted young trees at different elevations in the Sierra Nevada and compared their findings to detailed surveys taken 80 years earlier. Of 12 tree species studied, three of them – red fir, western white pine and mountain hemlock – already live in the highest ranges of the Sierra Nevada.
But their lower-elevation boundaries have moved uphill.
The mountain hemlock, a towering conifer that likes the coolest climes, made the biggest shift. Seedlings now sprout 700 feet higher – the length of more than two football fields – than they did 80 years ago, the study found. (Nevada County’s higher elevations offer an important refuge to this tree, researchers added.)
Higher-elevation boundaries, however, have not risen. That means high-altitude trees have less surface area to populate, leading to greater species competition, researchers wrote.
– Trina Kleist
For more stories:https://www.theunion.com/news/local-news/the-science-of-warmer-winters-sierra-temps-rising-faster-greater-impact-on-snowpack/
ENGLISH MOUNTAIN, Calif. – Both the history and the future of local water appear on this mountain 28 miles northeast of Nevada City.
The day Jessica Erickson measured snowpack for Nevada Irrigation District, a below-freezing wind smacked her cheeks, pine trees threw long shadows and the afternoon sun cast an orange glow onto the peak above her. Snow still lay thick on English Mountain on that late day in April 2017. Last winter was the third-snowiest in NID’s watershed on record, said Erickson, NID’s senior hydrographer.
English Mountain rises roughly in the center of NID’s watershed and forms its highest peak. For 90 winters, ample snowpack in this watershed has worked like a frozen reservoir. Most years, it holds water all winter, then sends spring snowmelt flowing to western Nevada County and beyond. Its water nourishes more than 95,000 people and agriculture worth $98 million in three counties, Operations Manager Chip Close said.
But rising temperatures are shrinking that frozen reservoir. Wintertime temperatures already have risen 2 degrees Fahrenheit across the Sierra Nevada. As a result, snowfall already is shifting to rain, and the range’s average snowline is rising, research shows. Snowpack is forecast to shrink by nearly half within 30 years; researchers predict it will shrink by as much as 75 percent within 60 years.
The biggest impact comes at elevations below 8,000 feet, more research reveals. Nearly all of NID’s snow-producing watershed lies below that level; so does about 60 percent of all historic Sierra Nevada snowpack.
That means this mountainside where Erickson measures NID water — straddling 7,100 feet elevation — will get rain more often than snow on its granite face. The future’s “new normal” remains unclear, but new research predicts the English Mountain area would look less often like its historical average, in 2016.
More often, it will look like it did in 2015, a year of both record-high temperatures and record-low precipitation, when precious little snow fell anywhere in the Sierra.
These three trends are linked: Rising wintertime temperatures in the Sierra Nevada cause snow to shift to rain at middle elevations and snowpack to shrink. They imply fundamental changes for water supply systems in Nevada County and across California, scientists warn.
“Going forward, it is likely to become more difficult to store and manage municipal, agricultural and ecological water needs within a warmer climate,” wrote researchers Neil Berg and Alex Hall, of the Department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles, in “Geophysical Research Letters” in March 2017.
HISTORIC PATTERN CHANGING
The historic pattern that has supplied water to western Nevada County works like this:
In October, NID’s reservoirs wait at their lowest levels of the year, pulled down by the demand of summer and fall. Water left in the reservoirs is called “carry-over.”
Then, cold winter storms dump snow on the Sierra Nevada. Below-freezing temperatures keep most of the snow there until late spring. Customer demand perks up in April, when the irrigation season starts, and NID uses reservoir water to supply that demand.
As reservoir water flows out over late spring and summer, snowpack slowly melts and refills the reservoirs. Snowmelt peaks in June. Newly filled reservoirs supply customers into October, when demand slows down again and the reservoir levels drop, according to NID data.
This cycle across the Sierra Nevada historically has supplied roughly 60 percent of California’s developed water, state officials said.
NID: 149 ACRES ABOVE 8K FEET
On her snowpack measuring expeditions, Erickson catches the scents of sagebrush, cedar, red fir and sugar pines. She spots bear and rabbit tracks, and she hears the calls of mountain chickadees and the rush of wind and, in the spring, the trickle of busy creeks.
Erickson and her team measure snow on English Mountain and four more areas in NID’s upper-elevation watershed. These measurements gauge the amount of water stored in the snowpack which, for NID, ranges from above 5,500 feet to 8,373 feet — the peak of English Mountain. In a typical winter, this frozen reservoir stores about 125,000 acre-feet of water — enough to supply NID’s local customers for a typical year.
That snowy reservoir sits on more than 44,000 “run-off acres” — that is, mountain slopes and valleys within the NID watershed where snowpack historically forms in winter, holds water for several months, and slowly releases it as snowmelt in spring, Erickson said. (Imagine an area nearly the size of 500 Nevada County Fairgrounds.)
Higher elevations usually have deeper snow, but less area to collect it, according to NID records. About 149 run-off acres sit above 8,000 feet.
All the rest of NID’s snowpack formation area — 99.7 percent — lies below 8,000 feet.
RISING TEMPS, LESS SNOW
The average low temperature during the winter in the Sierra Nevada already has risen 2 degrees over the last 123 years, according to records kept by the Western Regional Climate Center in Reno.
As temperatures rise, the snowline — the point on the mountain where falling precipitation turns to snow — also rises. In the Sierra, that has been observed for most winters since the mid-1990s.
“In the past 20 years, there is a lot of variability” in the snowline, wrote regional climatologist Nina Oakley of the Climate Center. “But the general pattern is that average freezing (elevation) levels are not as low” as they were previously.
Temperatures will continue to warm, scientists project: Wintertime averages in the Sierra could rise another 7 degrees Fahrenheit by the end of the century, Berg and Hall predicted.
(That forecast is based on more nuanced climate models developed recently by UCLA scientists to reflect real conditions in the Sierra Nevada. It assumes current levels of greenhouse gas emissions continue. If emissions are reduced, temperatures still would rise, but less, the models predict. If nations including the United States meet the lower emissions standards set in the 2015 Paris Agreement on climate, Sierra Nevada temperatures are likely to rise about 3 degrees Fahrenheit by 2100, Berg and Hall forecast.)
Rising temperatures will reduce Sierra snowpack: Acreage of snowpack is likely to shrink by 48 percent by 2080, according to UCLA scientist Daniel Walton.
The water content of snow is likely to drop by 45 percent within 30 years, and by 75 percent by 2100, according to Daniel Cayan of the Scripps Institution of Oceanography at UC San Diego.
Within NID’s snowpack formation area, that translates to losing the equivalent of most of Rollins Reservoir — which holds 66,000 acre-feet of water — 32 years from now.
BELOW 8K FEET, LOSE MORE SNOW
Sierra Nevada snow is especially sensitive to rising temperatures because of the range’s elevation: Most of the range’s snowpack-formation area — including the NID watershed — lies in the elevations most impacted by warmer winters.
That’s because the average snowline, historically, has been just below 7,000 feet, according to weather data.
“In the Sierra, most snowpack forms at 6,000 to 9,000 feet, so the warming climate is going to impact the lower elevations much more. That’s where we mostly see this transition” from snow to rain, said regional climatologist Daniel McEvoy at the Western Regional Climate Center.
When warming temperatures and drought come together, the impacts also are greater at elevations below 8,000 feet.
Soumaya Belmecheri’s team at the University of Arizona compared the width of tree rings to recorded Sierra winter temperatures, precipitation and snowpack thickness to model backwards in time. They found the effect of high temperatures during the 2012-15 drought was stronger at elevations below 7,000 feet than above 8,700 feet, she wrote in 2015 in Nature magazine.
Berg and Hall’s research predicts greater impacts below 8,200 feet.
While this recent drought was extreme, drought in general is part of California’s normal pattern of extreme variability. From year to year, both temperature and precipitation vary wildly; it’s the most variable climate in the nation, according to the National Oceanic and Atmospheric Administration.
“Normal” includes minor droughts, coming every 7 to 10 years, and severe droughts every 15 to 40 years, according to weather records. Scientists still are researching the effect of warming temperatures on snowpack during California’s years of “average” temperature and precipitation.
“It is not a simple task to quantify these things, and they are highly variable in time and space,” Oakley cautioned.
(For more about research on rising temperatures in the Sierra Nevada, see “The science of warmer winters” at http://www.TheUnion.com )
SNOWPACK SHRINKING ACROSS THE WEST
Beyond the Sierra Nevada, snowpack across the western United States already has shrunk 10 percent to 20 percent since the 1980s, as warming temperatures shift snow to rain.
Regional snowpack declines already have led to less water in the Columbia and Colorado rivers, wrote researcher John Fyfe of the Canadian Centre for Climate Modelling and Analysis in Victoria, British Columbia.
Western snowpack is predicted to shrink another 30 percent in the next two decades, Fyfe added.
(Southern California gets roughly 60 percent of its water from the Colorado River system.)
CARRY-OVER: THE KEY TO WATER SECURITY
For local water supplies, rising temperatures mean NID’s “run-off acres” — the areas where mountain snowpack historically forms a frozen reservoir for summertime use — more often will receive sparse snow.
As snow shifts to rain, NID will adapt its operations, but it will not be able to capture as much water, said NID Water Resources Superintendent Sue Sindt.
Snow melts slowly, but wintertime rain runs off quickly. Reservoirs will fill before customer demand kicks in — as happened in spring 2017. Water that can’t be held in the reservoirs will spill over the dams — as happened this year — and flow on downstream.
Customer demand over spring and summer will draw down the reservoirs, but in a warmer future, little snowmelt will flow in to backfill those lakes — a critical change, Sindt explained.
Carry-over storage — the water left in reservoirs at the end of the irrigation season in October — will be less.
“Absent knowing what the following winter is going to be like, carry-over storage is the only known water supply we have” going into the next year, Sindt added.
And because the next year could come up dry, maintaining as much carry-over as possible is important to water security for the whole community, she said.
PREPARE FOR CHANGE
The role of rising temperature in the shrinking of snowpack “foreshadows major future impacts on the state’s water supplies,” University of Arizona’s Belmecheri concluded.
Reliance on that one supply format — melting snow — has left water managers across California vulnerable, agreed NID General Manager Remleh Scherzinger.
“It’s extraordinarily important to the district to be flexible in its operations, both now and into the future, given the uncertainty of climate change,” Scherzinger said.
Trina Kleist is a Grass Valley freelance writer whose clients include Nevada Irrigation District. She may be contacted at email@example.com or 530-575-6132.
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