Snowfall is declining in the Sierra Nevada mountains Today’s Image of the Day from NASA Earth Observatory shows the extent of snowfall across the Sierra Nevada mountains, which has been declining in recent years. 

According to the California Department of Water Resources, snowpack accounts for about 30 percent of California’s water supply in a typical year. 

In the spring and summer, meltwater from Sierra Nevada snowpack helps replenish rivers and reservoirs.

The extent of the seasonal snowpack in the Sierra Nevada depends on whether a winter is wet or dry.

“Snow in the Sierra Nevada tends to be boom or bust, and much of it tends to come from atmospheric river events,” said McKenzie Skiles, a snow scientist at the University of Utah. “Over the past ten years, there have been fewer boom years, while the dry years have been getting drier.”

Benjamin Hatchett, a snow scientist at the Desert Research Institute, said there have been prolonged, multi-week winter dry spells in the past few years. We can expect more of them going forward in a warming world, he added. 

“The dry spells allow snow to start ripening earlier and melting to begin, especially if temperatures are above normal. During spring, warm and dry weather can help maintain rapid melting, especially if the snow surface becomes dirty and the albedo declines,” explained Hatchett.

The albedo is the whiteness of Earth’s surface, which determines how much light it reflects or absorbs.

Less snowfall leads to more dark-colored surfaces that absorb energy and warm up the local environment. In a reinforcing feedback loop, this causes more snow to melt and creates more dark surfaces for further warming.

Hatchett has also been studying changing snow levels in the Sierra Nevada. His team found that from 2008 to 2017, the snow level moved 72 meters higher in elevation per year – resulting in less snow cover and less water stored in the snowpack.

“There are multiple drivers of the snow trends, and their roles vary and interact,” said Hatchett. “For instance, less frequent storms (drought years) mean there is less precipitation that can build a snowpack, and the corresponding increase in dry days means there are more opportunities to melt existing snow.” 

“Also, increases in temperatures on wet days reduce the fraction of precipitation falling as snow; increases in dry day temperatures help contribute to warming the snowpack to the melting point. And humidity plays an important role in midwinter and spring melt events.”

“We are in a transitional period from the climate we expected – that is admittedly wetter than many other times in the past – into somewhat uncharted territory of a warmer world.”

The images, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, show just how much the snow levels in the Sierra Nevada fluctuate from year to year. 

Image Credit: NASA Earth Observatory 

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By Chrissy Sexton, Earth.com Staff Writer