Melting ice (stock image).
Credit: © Vladimir Melnik / Fotolia
A
new study from climate scientists Robert DeConto at the University of
Massachusetts Amherst and David Pollard at Pennsylvania State University
suggests that the most recent estimates by the Intergovernmental Panel on
Climate Change for future sea-level rise over the next 100 years could be too
low by almost a factor of two. Details appear in the current issue of Nature.
DeConto
says, "This could spell disaster for many low-lying cities. For example,
Boston could see more than 1.5 meters [about 5 feet] of sea-level rise in the
next 100 years. But the good news is that an aggressive reduction in emissions
will limit the risk of major Antarctic ice sheet retreat."
With
mechanisms that were previously known but never incorporated in a model like
this before, added to their ice-sheet model to consider the effects of surface
melt water on the break-up of ice shelves and the collapse of vertical ice
cliffs, the authors find that Antarctica has the potential to contribute
greater than 1 meter (39 inches) of sea-level rise by the year 2100, and greater
than 15 meters (49 feet) by 2500 if atmospheric emissions continue unabated. In
this worst case scenario, atmospheric warming (rather than ocean warming) will
soon become the dominant driver of ice loss.
The
revised estimate for sea-level rise comes from including new processes in the
3-dimensional ice sheet model, and testing them against past episodes of high
sea-levels and ice retreat.
The
researchers find that "ocean-driven melt is an important driver of
Antarctic ice shelf retreat where warm water is in contact with shelves, but in
high greenhouse-gas emissions scenarios, atmospheric warming soon overtakes the
ocean as the dominant driver of Antarctic ice loss." Further, they find
that if substantial amounts of ice are lost, the long thermal memory of the
ocean that will inhibit the ice sheet's recovery for thousands of years after
greenhouse-gas emissions are curtailed.
DeConto
and Pollard's study was motivated by reconstructions of sea level rise during
past warm periods including the previous inter-glacial (around 125,000 years
ago) and earlier warm intervals like the Pliocene (around 3 million years ago).
These high sea levels, ranging from a few meters to 20 meters above today,
imply that the Antarctic Ice Sheet is highly sensitive to climate warming.
"So,
at a time in the past when global average temperatures were only slightly
warmer than today," says DeConto, "sea levels were much higher.
Melting of the smaller Greenland Ice Sheet can only explain a fraction of this
sea-level rise, most which must have been caused by retreat on
Antarctica."
To
investigate this, DeConto and Pollard developed a new ice sheet-climate model
that includes "previously under-appreciated processes" that emphasize
the importance of future atmospheric warming around Antarctica.
They
explain that, "to date, research into Antarctic ice sheet vulnerability
has focused on the role of the ocean, melting floating ice shelves from below.
The ice shelves that fringe the land-based ice hold back the flow of inland ice
to the ocean. However, it is often overlooked that the major ice shelves in the
Ross and Weddell Seas and the many smaller shelves and ice tongues buttressing
outlet glaciers are also vulnerable to atmospheric warming."
They
add, "Today, summer temperatures approach or just exceed 0 degrees C. on
many shelves, and due to their flat surfaces near sea level, little atmospheric
warming would be needed to dramatically increase the areal extent of surface
melting and summer rainfall."
"If
protective ice shelves were suddenly lost in the vast areas around the
Antarctic margin where reverse-sloping bedrock (where the bed on which the ice
sheet sits deepens toward the continental interior, rather than toward the
ocean) is more than 1,000 meters deep, exposed grounding line ice cliffs would
quickly succumb to structural failure as is happening in the few places where
such conditions exist today," the researchers point out.
https://www.sciencedaily.com
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