June 10, 2011

Highs and Lows in Rocky Mountain Snowpack

Snowpack levels in the Rocky Mountains, according to a new study by the United States Geological Survey, have been declining since the 1980s at a more rapid pace than at any time in the last millennium.

To put the recent observations in a historical context, [Gregory] Pederson and his colleagues examined tree rings to reconstruct winter snow accumulation in the mountains that drain to the Colorado, Columbia and Missouri river basins -- which collectively provide 60 percent to 80 percent of the water needs for more than 70 million people. . . .

Tree rings indicate snowpack levels in two ways: Lower-elevation trees like Douglas fir and ponderosa pine produce thicker growth rings during large snowpack years because more water is available.  Higher-elevation trees like hemlock and subalpine larch show the opposite -- heavier snowfall that takes longer to melt means the growing season is shorter, so the growth rings are thinner. . . .

After analyzing hundreds of thousands of tree rings, the researchers found only two instances of sustained low snowpack in the northern Rockies comparable to the 20th century from about 1300 to 1330 and about 1511 to 1530. However, those dips were not as severe as current declines.

"You have a pretty severe early 20th century and especially a post-1980s decline that is really isn't matched most anywhere in the paleo-climate record," Pederson said. . . .

The contrast between the study's findings and this year's record snowpack is a prime example of the difference between a single weather event and climate, Pederson said. Weather is the state of the atmosphere at a given time and place, whereas climate is the average condition over multiple decades.

"The actual definition of a 'climate normal' is a 30-year moving average," Pederson said. "So when you hear people say, 'Well, snowpack this year is 300 percent of normal,' it's really anomalous to the last 30 years. ... But we're extremely anomalously low compared to the last century and millennium."

From the New York Times and Greenwire, “1,000-Year Record Shows Unusual Snowpack Declines,” reporting a study published in Science.

See also this discussion by NASA reporting high levels of snowpack in 2011 in five western states:

By July in a typical year, the snow that covers the slopes of the Rocky Mountains has given way to grasses and wildflowers, leaving only the peaks capped in white. But 2011 has not been a typical year. As this image of the Uinta Mountains in northeast Utah shows, winter’s snow is lingering into summer. On June 26, the snowpack on the southern face of the range was 849 percent above average. The northern face had 892 percent more snow than average.

The image was taken by the Landsat 5 satellite on July 15, 2011. . . .

The Uintas are typical of conditions throughout the northern Rockies and northwestern United States. From Montana to California, the extent of lingering snow is “exceptionally unusual,” said the National Climate and Water Center, who reports that snowmelt is usually complete by July.

A cool spring is part of the reason that the western mountains still hold snow in July. Washington and Oregon experienced the coldest April to June on record, and other western states experienced temperatures below or much below normal. With low temperatures, mountain snow didn’t melt quickly.

The other reason the snowpack is lasting into summer is that there was more snow to melt. The winter and spring of 2010-2011 brought far more snow than average, leading to a record mountain snowpack in at least five states. A healthy snowpack is a boon to western states most of time. Snow stores water for use in the dry summer months. However, if the snow melts quickly, the runoff could cause floods. Already, melting snow contributed to flooding in the Missouri River basin. As the snow melts in other basins, it could swell rivers with unseasonal floods.

Despite the record snow in some western states, the snow extent in North America as a whole was below average in the spring (March to May) for the eighth year in a row. The extent was low because less snow fell in central and northwestern Canada and Alaska than average.


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