NASA Earth has released a series of satellite photographs showing the transformation of one of the most famous mountain ranges on Earth.
The comparison photographs taken over the last 15 years show the gradual “drying” of the magnificent Sierra Nevada mountains in America’s west.
While the region is known for its extreme “boom and bust” cycles, NASA says the mountains have ultimately seen less snow accumulation in recent years.
Alongside the comparison images, NASA also posted a timelapse video of springtime in the snow-covered region, spanning a time frame from 2006 to 2021. The difference is stark, showing while there are still wet cycles, the dry cycles of the region are becoming harsher and more frequent.
“The Sierra Nevada has seen less snow accumulation in recent years, a decline that plays a role in water management and response to drought,” NASA said in its tweet.
The photos were all snapped at similar times each year, around April 1, which falls around halfway through the water year (October 1 to September 30), this is the time when the snowpack has usually peaked and is beginning to melt.
These snow totals at the time are important for forecasts of spring and summer water supplies. Wet winters, like those in 2006 and 2017, tend to build a deep snowpack while the region reached historically low levels in 2015 after four years of hot and dry conditions.
“Snow in the Sierra Nevada tends to be boom or bust, and much of it tends to come from atmospheric river events,” McKenzie Skiles, a snow scientist at the University of Utah, told NASA.
“Over the past 10 years, there have been fewer boom years, while the dry years have been getting drier.”
Benjamin Hatchett, a snow scientist at the US Desert Research Institute said these drier phases are expected to continue.
“In the past few years, we have had prolonged, multi-week winter dry spells, and we expect more of them going forward in a warming world,” he said.
“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.”
Both scientists are particularly concerned in changes in albedo, which is the brightness of the Earth’s surface and how much light it is capable of reflecting, and the feedback this creates.
Less snow results in fewer dark-coloured surfaces to absorb energy and can cause more snow to melt.
A study published by the American Association for the Advancement of Science last year showed that 2000-2018 was the driest period in the US Southwest since the late 1500s. The past three years have continued the trend with 2021 shaping up to becoming the driest year in modern history for the region, and California’s reservoir levels are dropping.
“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,” Hatchett added.