Melting Greenland ice may threaten Northeast
A melting of the Greenland Ice Sheet this century may drive more
water than previously thought toward the already threatened coastlines of New York,
Boston, Halifax, and other cities in the northeastern United States and Canada, new
research shows.
Weiqing Han
The study finds that if Greenland ice melts at moderate to high rates, ocean circulation by
2100 may shift and cause sea levels off the northeast coast of North America to rise by
about 12 to 20 inches more than other coastal areas. The research
builds on recent reports that have found that sea level rise associated with global warming
could adversely affect North America, and its findings suggest that the situation is even
more urgent than previously believed.
“If the Greenland melt continues to accelerate, we could see significant impacts this
century on the northeast U.S. coast from the resulting sea level rise,” says Aixue Hu, a
scientist with the National Center for Atmospheric Research in Boulder and lead author of the paper. ”Major northeastern cities are directly in the path of the greatest rise.”
Hu’s paper will be published on 29 May in Geophysical Research Letters, a journal of the
American Geophysical Union. A previous study in Nature Geoscience in March
warned that warmer water temperatures could shift ocean currents in a way that would
raise sea levels off the Northeast by about 8 inches more than the average global sea
level rise. But it did not include the additional impact of Greenland ice, which at
moderate to high melt rates would further accelerate changes in ocean circulation and
drive an additional 4 to 12 inches of water rise toward heavily populated areas
in northeastern North America on top of average global sea level rise. More remote areas
in extreme northeastern Canada and Greenland could see even higher sea level rise.
This visualization, based on new computer modeling, shows that sea level rise may be an additional 10 centimeters (4 inches) higher by populated areas in northeastern North America than previously thought. Extreme northeastern North America and Greenland may experience even higher sea level rise. (Graphic courtesy Geophysical Research Letters, modified by UCAR.) Click to view larger version.
The paper is co-authored by three other scientists, including Weiqing Han, an assistant professor of atmospheric and oceanic sciences at the University of Colorado.
Scientists have been cautious about estimating average sea level rise this century in part
because of complex processes within ice sheets. The 2007 assessment of the
Intergovernmental Panel on Climate Change projected that sea levels worldwide could
rise by an average of 7 to 23 inches this century, but many researchers
believe the rise will be greater because of dynamic factors in ice sheets that appear to have
accelerated the melting rate in recent years.
To assess the impact of Greenland ice melt on ocean circulation, Hu and his coauthors
used the Community Climate System Model, an NCAR-based computer model that
simulates global climate. They considered three scenarios: the melt rate continuing to
increase by 7 percent per year, as has been the case in recent years, or the melt rate
slowing down to an increase of either 1 or 3 percent per year.
If Greenland’s melt rate slows down to a 3-percent annual increase, the study team’s
computer simulations indicate that the runoff from its ice sheet could alter ocean
circulation in a way that would direct about one foot of water rise toward the
northeast coast of North America by 2100. This would be on top of the average global sea
level rise expected as a result of global warming. Although the study team did not try to
estimate that mean global sea level rise, their simulations indicated that melt from
Greenland alone under the 3 percent scenario could raise sea levels by an average of 21 inches.
If the annual increase in the melt rate dropped to 1 percent, the runoff would not raise
northeastern sea levels by more than the 8 inches found in the earlier study in Nature
Geoscience. But if the melt rate continued at its present 7-percent increase per year
through 2050 and then leveled off, the study suggests that the northeast coast could see
as much as 51 cm (20 in) of sea level rise above a global average that could be several
feet. However, Hu cautioned that other modeling studies have indicated that the 7
percent scenario is unlikely.
In addition to sea level rise, Hu and his co-authors found that, if the Greenland melt rate
were to defy expectations and continue its 7 percent increase, this would drain enough
fresh water into the North Atlantic to weaken the oceanic circulation that pumps warm
water to the Arctic. Ironically, this weakening of the meridional overturning circulation
would help the Arctic avoid some of the warmed ocean impacts of global warming and
lead to at least the temporary recovery of Arctic sea ice by the end of the century.
The northeast coast of North America is especially vulnerable to the effects of Greenland
ice melt because of the way a north-south oceanic flow, known as the meridional
overturning circulation, acts like a conveyor belt transporting water through the Atlantic
Ocean. The circulation carries warm Atlantic water from the tropics to the north, where it
cools and descends to create a dense, deep layer of cold water flowing south. As a result,
sea level is currently about 28 inches lower in the North Atlantic than the North
Pacific, which lacks such a dense layer.
If the melting of the Greenland Ice Sheet were to increase by 3 percent or 7 percent
yearly, the additional fresh water could partially disrupt the northward conveyor belt.
This would reduce the accumulation of deep, dense water. Instead, the deep water would
be slightly warmer, expanding and elevating the surface across portions of the North
Atlantic.
“The oceans will not rise uniformly as the world warms,” says NCAR scientist Gerald
Meehl, a co-author of the paper. “Ocean dynamics will push water in certain directions, so
some locations will experience sea level rise that is larger than the global average.”
The research was funded by the U.S. Department of Energy and by the National Science
Foundation. It was conducted by scientists at NCAR, the University of Colorado at
Boulder, and Florida State University.
American Geophysical Union/National Center for Atmospheric Research
May 27, 2009
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