The snow season, which started this month, is off to a good start.
A series of December storms covered the Sierra Nevada with heavy snow, leaving the snowpack at 106% of average, according to the state’s snow survey.
But a new study suggests that Californians won’t always be able to rely on melting snow to trickle down the mountains each spring, filling state reservoirs for use over the long, dry summers.
According to a new report, the size of the Sierra snowpack could shrink by as much as 79% by the century’s end if humans don’t limit greenhouse emissions.
“We are currently relying on snow to store the precipitation we get in the winter for use during the summer,” said Andrew Jones, a climate scientist at the Lawrence Berkeley National Laboratory in Berkeley, who worked on the study. “In the future, we’ll have to radically restructure the way we manage water in California.”
Experts estimate that the Sierra Nevada snowpack provides about one-third of the state’s fresh water each year.
In the new work, published last month in the journal Geophysical Research Letters, scientists looked specifically at how future climate conditions could affect the snowpack directly upstream of 10 of the state’s largest reservoirs.
Using five climate models that assume continued high global greenhouse gas emissions, the researchers found that on average, the snowpack could face a 54% reduction in volume in the next 20 to 40 years and a 79% reduction in the next 60 to 80 years.
However, not all regions of the state would be affected equally by this loss of winter water storage.
Reservoirs in the northern part of the state would be more affected than those in the central and southern regions, the authors said. That’s because the Sierra Nevada is not quite as high in Northern California as it is in other parts of the state.
“As the world is warming, the snowline is moving up the mountains,” Jones said. “That means that lower elevations will be affected more quickly by this reduction than those in higher elevations.”
The snowpack above the state’s northern reservoirs such as Shasta, Oroville and Folsom is expected to shrink by 59.5% in 20 to 40 years and by 83.8% by the end of the century.
Reservoirs in the central part of the state could see reductions of 48.4% by midcentury and 73.4% by the end of the century, according to the report. In the southern regions, the snowpack could shrink by 48.8% by midcentury and 75.6% by the end of the century, the authors said.
That doesn’t mean less precipitation will fall up north in the future, but that it will come as rain rather than snow.
That’s a problem for water managers trying to balance both water storage and flood control.
“Under our current system, we would have to let much of that rain flow to the ocean because we wouldn’t be able to store it in the reservoirs,” Jones said.
The study also revealed that the time of year when the snowpack starts to melt could move up by as much as four weeks by 2099.
Scientists now expect the snowpack to reach its peak at the beginning of April. But years from now, the peak would probably be at the beginning of March, the authors found.
“That means that not only will there be less storage of snow, but also the time period over which we need to store water is longer,” Jones said. “The summer period is essentially growing.”
The new work is not the first to warn that the size of the Sierra Nevada snowpack will probably be drastically reduced in the future, or that the melting season could start earlier in the year.
“The snowpack literature is converging pretty much to the same general conclusions,” Alan Rhoades, a Berkeley Lab postdoctoral fellow and the study’s lead author, said in a statement.
However, the study is among the first to break down how global climate change will affect water flow in distinct regions of California.
“Scientists created climate models to understand global climate systems, but now people are asking what is going to happen in my local watershed,” Jones said.
It should also be noted that the authors don’t expect the march toward a smaller snowpack to be a straight line, but rather a winding road.
The winter of 2016-17 created one of the largest snowpacks in California history, loading reservoirs and rivers for months, even to the point of flooding some communities. Overall, the snowpack was 163% of average, which, when it melts, equals 46 inches of rain.
That record relieved some water deficits the state had been suffering during a years-long drought. Then, after last season, the snowpack went back to below-average levels.