A warm winter left very little snow in California’s Sierra Nevada, and now the extreme heat is accelerating the rapid melting of the mountains.
The Sierra snowpack averages 48% this time of year, according to state data, down from 73% of the average in late February.
When hydrologist Newsha Ajami skied near Lake Tahoe in early March, she saw the snow from the last round of storms quickly disappearing from the slopes, and many of the ski lifts were closed.
“There was a lot of empty land, an empty mountain with no snow,” he said. “It was almost over. It was scary.”
California depends on the Sierra snowpack for about 30% of its water, on average. But the unusual warmth across the West this winter, which broke records in many places, it brought more precipitation as rain instead of snow.
Scientific research has shown that climate change is caused by humans pushing the central snow lines in the mountains and changing the flow time.
Warming driven by the use of fossil fuels and rising levels of greenhouse gases also brings longer and longer heat waves.
California’s snowpack typically peaks around April 1. But this year, state estimates from across the Sierra Nevada show that snowpack has been shrinking since Feb. 25, and rapid snow loss will continue this week as the West fires a wave that is predicted to break records in many areas.
National Weather Service said “unusual summer-like heat” this week will bring high temperatures 15 to 30 degrees above normal across the Southwest. Areas expected to record heat include Los Angeles, Palm Springs, Fresno and Phoenix.
The National Weather Service has warned that the heat wave, in addition to bringing risks of heat stress, will also create dangerous conditions in rivers as the rapid melting of snow causes rising waters and rapid currents.
There is more snow melting in some parts of the mountains than in others. In the southern Sierra, snowpack stands at 71% of average, while the northern Sierra is only 28%.
Despite the lack of snow, rainfall this winter has been slightly above average across the country. And California’s largest reservoirs, boosted by heavy flows from the past three years, are at 122% of average.
“The dams are full of water. It should be good this year. But does this mean we are good in the long run? I don’t think so,” said Ajami, who leads a new program focused on risk, resilience and recovery from extreme weather events at Stanford University’s Doerr School of Sustainability.
The water infrastructure system that California built over the last century, he said, relies heavily on snow that naturally stores water and then gradually releases snowmelt into reservoirs for use by cities and farms.
“The challenge we are facing right now is, that cycle has really been changed, so we don’t really have a system that can be properly managed under the current circumstances we are facing,” said Ajami. “It’s a huge problem, and we really need to go back and look and see how we can rethink and re-use these systems.”
He said that means many efforts, such as changing the way dams work and directing floodwaters to replenish depleted groundwater. Efforts to improve the health of forests and mountain meadows, he said, are also important so that the country can absorb and store water naturally.
The Colorado River, another a great source of water of Southern California, declined in the latter part of the century between severe droughts worsened with rising temperatures. This year, snowpack in the upper Colorado River watershed stands at 59% of average, and that will mean less snowmelt feeding the river’s lakes, which are shrinking to record low levels.
Ajami pointed out that extreme heat not only causes ice to melt faster but also creates sublimationwhere ice is directly converted to water vapor. And when hot conditions leave mountain soils dry, melting snow can soak into the ground before water flows into streams and rivers.
“The whole system is under pressure,” Ajami said. “Because of climate change, it’s affecting the way the water cycle behaves.”
Record warmth this winter, with some states seeing seasonal temperatures more than 3 degrees above average, has brought “a classic signature of warm weather to the mountain snowpack,” said Daniel Swain, a UC Agriculture and Natural Resources climate scientist.
That signature of winter warmth, he said, was under the snow “because it fell as rain rather than snow, because it’s on the wrong side of the freezing line, or because it fell as wet snow at first and melted quickly.”
