Melting of glacial ice will probably raise sea level around the globe,
but how fast this melting will happen is uncertain. In the case of the
Greenland Ice Sheet, the more temperatures increase, the faster the ice
will melt, according to computer model experiments by geoscientists.
"Although lots of people have thought about sea level rise from the
ice sheets, we don't really know how fast that will happen," said
Patrick Applegate, research associate, Penn State's Earth and
Environmental Systems Institute.
If all the ice in the Greenland Ice Sheet melts, global sea level
would rise by about 24 feet. In the last 100 years, sea level in the New
York City area has only increased by about one foot. However, storm
surges from hurricanes stack on top of this long-term increase, so sea
level rise will allow future hurricanes to flood places where people are
not ready for or used to flooding. A vivid example occurred during
Hurricane Sandy when parts of the New York City subway tunnel system
flooded.
Greenland might be especially vulnerable to melting because that area
of Earth sees about 50 percent more warming than the global average.
Arctic sea ice, when it exists, reflects the sun's energy back through
the atmosphere, but when the sea ice melts and there is open water, the
water absorbs the sun's energy and reradiates it back into the air as
heat. Arctic sea ice coverage has decreased over the last few decades,
and that decrease will probably continue in the future, leading to
accelerated temperature rise over Greenland. Floating ice does not add
to sea level, but the Greenland Ice Sheet rests on bedrock that is above
sea level.
Feedbacks in the climate system cause accelerated temperature rise
over the Arctic. Other feedbacks in the Greenland Ice Sheet that
contribute to melting include height-melting feedback. A warm year in
Greenland causes more melt around the edges of the ice sheet, lowering
the surface. The atmosphere is warmer at lower altitudes, so the now
lower surface experiences even more melting. This process can lead to
accelerated ice melt and sea level rise.
Another form of feedback occurs because ice sheets are large masses
that want to spread. This spreading can either help preserve the ice
sheet by allowing it to adjust to increased temperature or accelerate
ice melting by moving ice to lower, warmer, places.
"Many studies of sea level rise don't take into account feedbacks
that could cause rapid sea level rise," said Applegate. "We wanted to
look at the effects of those feedbacks."
The researchers looked at two models of the Greenland ice sheet that
include some of the important feedbacks. The first model is a
three-dimensional ice sheet model. The second model looks at a transect
across the island and was developed by Byron Parizek, associate
professor of geosciences and mathematics, Penn State Dubois. To run both
models, Robert Nicholas, research associate, EESI, estimated how much
warming might take place over Greenland using results from global
climate models.
Both the three-dimensional and transect models showed that the time
necessary for ice mass loss from the Greenland ice sheet decreases
steeply with increases in temperature. Shorter time scales -- faster
melting -- imply faster sea level rise. The interplay between the
height-melting feedback and ice flow causes this acceleration.
"Our analysis suggests that the benefits of reducing greenhouse gas
emissions, in terms of avoided sea level rise from the Greenland Ice
Sheet, may be greatest if emissions reductions begin before large
temperature increases have been realized," the researchers state in a
recent issue of Climate Dynamics.
Currently, about a billion people live in areas that would be flooded by a three-foot sea level rise.
"If we are going to do something to mitigate sea-level rise, we need
to do it earlier rather than later," said Applegate. "The longer we
wait, the more rapidly the changes will take place and the more
difficult it will be to change."
This story is taken from Science Daily
