As the summer solstice
approaches in the Northern Hemisphere, long hours of sunlight warm the Arctic
and melt snow and sea ice. Sea ice retreat in June is typical, but the first
half of June 2012 brought unusually rapid ice loss, the National Snow and Ice Data
Center (NSIDC) reported on June
19.
One area of rapid ice
retreat was the Beaufort Sea, north of Alaska. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite
collected these images on May 13, 2012 (top), and June 16, 2012 (bottom). By
mid-June, the open-water area off the coast had expanded substantially and snow
had melted on land.
The rapid melt north of
Alaska was part of a larger phenomenon. Sea ice across the entire Arctic
reached record-low levels for this time of year, NSIDC stated, slightly below
the previous record set in June 2010. It was also lower than the extent in June
2007; Arctic sea ice reached its lowest extent ever recorded by satellite in
September 2007.
In the first half of June
2012, the Beaufort Sea was a “hotspot” of rapid retreat, driven by a
high-pressure pattern over the region that kept skies clear at the very time of
year when sunlight lasts the longest. In addition, larger-scale climate
patterns in early June 2012 favored ice retreat along the coastlines of Alaska
and Siberia. As of June 18, temperatures were above freezing over much of the
sea ice in the Arctic, and snow had melted earlier than normal, leading to
warming on land.
On June 19, 2012, NSIDC
reported: “Recent ice loss rates have been 100,000 to 150,000 square kilometers
(38,600 to 57,900 square miles) per day, which is more than double the climatological
rate.” (For comparison, the area of the state of Illinois is roughly 150,000
square kilometers.)
The early onset of the
spring melt and the sunny skies around the solstice increased the likelihood of
heightened melt rates throughout the rest of the summer, largely by reducing albedo: the proportion of solar
energy reflected back into space. If an object reflects all the energy it
receives, it has an albedo of 1.0. Sea ice has high albedo because of its
bright appearance. But when it starts to melt, its albedo drops from roughly
0.9 to 0.7, causing the ice to absorb more energy. Increased energy absorption
leads to increased melt, which exposes ocean water. Thanks to its dark
appearance, ocean water has an albedo of less than 0.1. Long, sunny days pour
energy into the water, and it retains the heat throughout the summer. In
September, when the Sun is low on the horizon, the heated ocean water continues
melting sea ice.
1. References
N
هیچ نظری موجود نیست:
ارسال یک نظر