By Morgan Levey, Dec. 11, 2017
For Californians reeling from the deadly fires in Northern California and the Oroville Dam crisis that displaced thousands, the record-breaking precipitation levels last year may seem like a silver lining after more than five years of drought.
Yet it’s precisely these massive swings, from prolonged dry periods to extreme rain and mud slides that have increased the state’s vulnerability to environmental catastrophes, stressing water supplies and amplifying flooding hazards.
Experts believe climate change and global warming are the culprits for this extreme weather and warn that warming is impacting the mountains most severely. One-third of California’s water supply comes from snowpack in the Sierra Nevada mountains and snowpack reservoir that builds up during winter is in rapid decline. The Sierra Nevada is a moderate-elevation mountain range, highly susceptible to rising temperatures that change the snow level, or the point at which snow turns to rain in the atmosphere during storms.
Climate scientists are finding that it’s not just warming temperatures that are causing the transition from snow to rain but also the altering intensity and variability of winter storms in the Sierra Nevada.
“It appears that the storms themselves are getting a lot warmer,” said Benjamin Hatchett, a hydrometeorologist at the Desert Research Institute in Reno, devoted to studying natural and human-induced environmental changes. According to a new paper published in the journal Water by Hatchett and a team of climate scientists, the combined effects of rising sea surface temperatures and warming background air temperatures make conditions more favorable for warmer and wetter storms landing in the Sierra Nevada. Prolonged periods of dry weather between storms only exacerbates the water resource problems caused by rainier winter storms. The rainier winter downpours cause greater risk of flooding and leave little snowpack to stream into the water supply during summer.
“The rule of thumb is a handful of good storms make up the bulk of your water year,” said Michael Anderson, California’s state climatologist in the Department of Water Resources. But the hope is that most of these storms bring snow that stays frozen through the winter. While 2017 did bring record-breaking levels of precipitation, its snowpack was nowhere near the record-setting snowpack levels in 1983, according to Anderson.
California’s state water resource infrastructure is woefully underprepared for this type of climate shift. And it’s not just California. Nevada also relies heavily on water supplies from melting snow. The snowpack essentially acts as a secondary reservoir for the states’ water management systems. “If you don’t have a snowpack then you don’t have that ability for that spring refill, or limited opportunity for it. It puts more pressure on the summer water management objectives,” Anderson said.
Hatchett analyzed recorded snow-level data over the past decade and discovered that the snow for winter storms has risen approximately 1,200 feet in altitutde in the atmosphere between the beginning and end of the last ten years. Meanwhile, since 1951, the snow fraction or percent of precipitation that falls in a given time as snow, has steadily declined by about three percent each year.
“Over 10 years that’s 30 percent less precipitation falling as snow, so that can make a significant impact on our water resources,” Hatchett said. The last 10 years has been the steepest decadal decline in snow fraction over the past 67 years.
Since the Sierra Nevada is a maritime mountain range, it’s highly influenced by its proximity to the Pacific Ocean. A major component of climate change is warming sea-surface temperatures, making conditions preferable for atmospheric rivers, or long, narrow filaments of water vapor in the atmosphere that “favor wetter, warmer, higher snow-level types of storms,” Hatchett says. With increased evaporation potential from the warming, atmospheric rivers pick up energy and moisture across the ocean and make landfall with more intensity and variability.
“In California, we actually see precipitation totals associated with atmospheric rivers that rival that of hurricanes in the southeast United States,” Hatchett added. What Hatchett and his team are discovering is that the rise in sea-surface temperatures correspond with changes to the structure of winter storms, forcing the snow level to higher altitudes in the atmosphere and making less of this precipitation fall as snow.
Amid background warming and drying, stronger and more frequent atmospheric rivers are projected for the coming years. “I think the last couple years might look like a window into the future where we can have these persistent, very dry and warm years and then we might have one just awesome super wet year,” Hatchett said.
Long periods of dry weather increase opportunities for evapotranspiration, creating stress for water management systems and reducing water availability as temperatures rise. What does this mean for Californians? Basically the warmer the weather gets, the more often the Sierra Nevada will experience prolonged periods of drought punctuated by moments of heavy precipitation.
Hatchett’s research is key in assisting state officials regulate reservoirs and flood control. Anderson said the Department of Water Resources is dependent on this data to forecast winter storms. “The better we forecast them, the better we can start managing water that we expect to come out of them. Either as snow or as rain.”
This is not just a West Coast issue. Globally, changes in the climate system will force infrastructure to adapt. “We’ve built an infrastructure for a world that is changing,” said Richard Alley, a professor of geosciences and associate of the Earth and Environmental Systems Institute at Pennsylvania State University. “We have counted on snow pack to be our reservoir. And now we’re not going to have it because it’s reducing. And this is true in the Himalayas and this is true in California.”
Photo at top: A snowless landscape is becoming the norm in Tahoe, as seen in this photo taken early in the 2017 season at Squaw Valley. (David Metres/Courtesy of Ben Hatchett)
Note: Morgan Levey is a Comer Scholar, a Medill scholarship program supported by the Comer Family Foundation to promote graduate studies in environmental journalism.