Dear “Earth Talk”: What are the
implications of the increasing breakup of Antarctica’s large floating
ice shelves in recent years? — Gaertner Olivier, Brussels, Belgium
Ice shelves are thick plates of ice that float on the
ocean around much of Antarctica. Snow, glaciers and ice flows feed these
large plates in the colder months. In warmer periods, surface melting
creates standing water that leaks into cracks and speeds the breaking off
(calving) of icebergs, decreasing the continent’s mass in a natural
cycle as old as Antarctica itself. “Large icebergs calve off on a fairly regular
basis from the larger ice shelves in Antarctica,” says Dr. Ted
Scambos, a research associate at the National Snow and Ice Data Center.
“This is a part of their normal evolution.”
The only effect of such calving that scientists are
sure about is that it is changing the outline of Antarctica. The breakup of
the ice shelves, which account for about 2 percent of the continent’s
land mass, does not have any measurable effect on sea levels. “Since
an iceberg floats in ocean water and much of it is below the surface, it is
already displacing the same volume of water it will contribute when it
eventually melts,” Scambos says. But although such calving activity is not new, it has
increased over the last 30 years, with larger and larger chunks breaking
off from Antarctica, then floating free in the ocean and breaking up into
successively smaller pieces. One especially large iceberg, a chunk the size
and shape of New York’s Long Island and dubbed “B15A” by
researchers, broke off from Antarctica’s Ross ice shelf in 2000 and
just last April collided with the continent’s Drygalski ice tongue (a
long shelf of ice extending out to sea from the mainland). The iceberg
itself remained intact, but a city-sized chunk of the ice tongue broke off
and is now floating free. Most researchers suspect that recent increases in
calving are linked to warming surface air temperatures resulting from
human-induced climate change. British glaciologist David Vaughan says,
“There is no doubt that the climate on the Antarctic Peninsula has
warmed significantly over the last few decades. What we’re seeing now
are changes only just working through to glaciers and ice sheets.”
Scambos says that, as Antarctic summer temperatures continue to increase,
the process can be expected to become more widespread and may begin to
significantly increase sea levels around the world. Even a relatively small rise in sea level would make
some densely settled coastal areas uninhabitable. The Intergovernmental
Panel on Climate Change, an international group of climatologists, predicts
a global sea-level rise of less than 3 feet by 2100 but also warns that
global warming during that time may lead to irreversible changes in the
Earth’s glacial system and ultimately melt enough ice to raise sea
levels by many more feet in coming centuries. Some 200 million people
inhabiting low-lying areas in countries such as Vietnam, Bangladesh, China,
India and the Philippines could be displaced, leading to a major
international refugee crisis.
For more information: Intergovernmental Panel on
Climate Change, www.ipcc.ch, NASA’s iceberg collision page,
www.nasa.gov/vision/earth/lookingatearth/Iceberg_collides.html.
This article appears in Dec 15-21, 2005.
