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In the high latitudes of the Northern
Hemisphere, snow typically covers the land surface for nine months each
year. The snow serves as a reservoir of water, and a reflector of the
Sun’s energy, but recent decades have witnessed significant changes in
snow cover extent.
The maps on this page show June snow cover extent anomalies for every
third year from 1967 through 2012. Each June’s snow cover is compared
to the 1971–2000 mean. Above-average extent appears in shades of blue,
and below-average extent appears in shades of orange. Toward the
beginning of the series, above-average extents predominate. Toward the
end of the series, below-average extents predominate.
The graph shows June snow cover in millions of square kilometers from
1967 through 2012, and the overall decline in snow cover is consistent
with the changes shown in the maps. The graph and maps are based on data
from the
Rutgers University Global Snow Lab.
The snow-cover study authors, Chris Derksen and Ross Brown, found an
overall decline in snow cover from 1967 through 2012, and also detected
an acceleration of snow loss after the year 2003. Between June 2008 and
June 2012, North America experienced three record-low snow cover
extents. In Eurasia, each successive June from 2008 to 2012 set a new
record for the lowest snow cover extent yet recorded for that month.
Previous research identified a link between rising air temperatures
and shrinking snow cover, so Derksen and Brown were not surprised to see
an overall loss of snow, “But we were surprised at the continued broken
records of June Arctic snow cover extent over the past five years,”
says Brown. “Arctic spring snow cover typically fluctuates over cycles
of about three to four years so you don’t expect to see sequences of
decreasing snow cover persisting through these natural cycles.”
As with
sea ice, declining snow cover extent means decreasing
albedo.
The overall “whiteness” of an object determines how much sunlight it
reflects back into space. Snow has very high albedo, reflecting up to 90
percent of the sunlight it receives. As snow cover declines, dark soils
and vegetation absorb more of the Sun’s energy. The
Geophysical Research Letters study pointed out that declining snow cover raises ground temperatures and increases the thickness of the
active layer—the
uppermost layer of permafrost that thaws each summer. When organic
material in thawing permafrost decomposes, it can release methane, a
potent greenhouse gas when released to the atmosphere.
Anticipating future changes in Arctic snow cover poses challenges for
researchers. “Changes in fall snow cover are complicated because the
longer open-water season provides additional moisture for increasing
snowfall,” Brown says. “As for future snow cover, climate is strongly
influenced by interannual variability over periods of five to ten years,
so that time range is hard to predict. But global climate models show
the rate of Arctic snow cover decline speeding up over the long term.”
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