Let it snow, let it snow…and not melt too soon

Studying seasonal snowpack.

A nip in the air. Holidays around the corner. Best of all, those big, fat, soft white flakes floating down on the ponderosa pines up at Sugar Bowl, Heavenly Valley, or Mount Shasta. That lovely, neck-deep snowfall in the Sierras is part of the annual accumulation of snow in Northern Hemisphere mountains around the world, and it comes around as reliably as the New Year. But the elevation at which snow falls, and when snow melts, is changing.

Environmental Studies Professor Iris Stewart-Frey has been exploring seasonal snowpack over decades and trying to measure its variability. Drawing on mountains of data for an article that recently appeared in the journal Hydrological Processes, she describes the many factors that raise or reduce snowpack and snowmelt. Climate change appears to be key.

In midlatitudes in the Northern Hemisphere, warming temperatures have increased precipitation. That in turn boosts snowpack in high elevations, where temperatures remain cold all winter. But at lower elevations, warmer temperatures mean more rain and less snow, and what snow there is melts sooner.

Let It Snow Winter 2019

More rain in mountain regions can be a short-term gain, as it immediately boosts streamflow, but that comes at the expense of streamflow in July. In the past, mountain snowpack has long served as an arid region’s essential cold storage unit—a natural reservoir that slowly releases much-needed water during warmer months to the dry, hot valleys below. And it provides “the primary source of water for large populations,” Stewart-Frey notes. Those populations will now have to cope with drier late summers and autumns.

In California, measurements in some areas show a trend of earlier and earlier spring runoff—a “spring pulse onset”—as much as 20 days earlier than 1948 levels. But that’s only the beginning. Stewart-Frey predicts that, if trends of early snowmelt continue, “streamflow timing might shift by 30 to 40 days by the end of the century.”

What scientists are discovering about coral reefs and arctic tundra— that some of our planet’s natural features are more fragile than others—appears to pertain to snow-capped mountains too. First, mountains get more precipitation than surrounding areas, as moist air rises when it reaches them. Also, mountain snowpack is very responsive to atmospheric circulation patterns. Then there is the important combination of temperature and precipitation: That wonderful ski-ready powder piling up at Lake Tahoe often remains relatively close to 32 degrees Fahrenheit, so it’s very sensitive to small temperature increases. Finally, just as in the arctic, large areas of white snow cover reflect sunlight; darker earth or water absorbs light. So reduced snowpack equals increasing heat at ground level, resulting in what scientists call snow-albedo feedback. The warmer it gets, the warmer it gets.

California’s water woes are, of course, all too familiar. But Stewart- Frey notes that the political decisions affecting water will get even tougher. We will have to stretch less water to go further and will have to make some hard choices,” she writes.

And not only in California. Stewart- Frey researched measurements taken in the European Alps, Siberian uplands, and the Tibetan plateau. Many (though not all) studies point to reductions around the globe in levels of winter water storage. In Finland, greater precipitation has meant “greater maximum snow depth, but a shorter duration of the snow cover.” So even in places where accumulated snow still gets very deep, an earlier melt-out could still mean dried-up streams by early fall.

The worst danger areas? Central Asia and Chile. Some regions are “in the very precarious situation of experiencing large population growth, as well as a temporary increase in water resources due to glacial melt,” Stewart- Frey writes. “By the time water derived from glacial melt decreases, a larger population will be accustomed to a larger flow, without the resources or infrastructure that exist in California and the U.S.”

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