Six things to know about Antarctic ice
- 12 Nov 2012, 15:30
- Roz Pidcock
When scientists talk about ice and climate change, it's often
about how quickly it's disappearing. So recent
news stories about Antarctic sea ice growing may come as a
surprise.
The amount of ice in the ocean around Antarctica is indeed
increasing, but this is only part of what's going on in the
Antarctic as a whole. We've put together six things you should know
about climate change and Antarctic ice.
1. Antarctic waters are warming faster than the global
average
Along with the rest
of the world, the Antarctic is warming up. The Southern Ocean,
which surrounds the continent of Antarctica, has been warming
faster than the rest of the world's oceans since the 1950s, at a
rate of
0.17 degrees Celsius compared to a global average of 0.1
degrees. The increased rate of warming is mainly due to the way large
weather systems transport heat to the poles.
2. Despite rapid warming, there's more Antarctic sea
ice
Despite rapidly warming water, the amount of ice that floats on
the Southern Ocean around Antarctica - known as sea ice - is
slightly increasing. On 26 September this year, the US National Snow and Ice Data Centre
(NSIDC) confirmed that Antarctic sea ice reached a record
extent - a measure of sea ice cover - of 19.44 million square
kilometres.
3. Scientists believe the wind is driving sea ice
increase
New research released in the journal Nature Geoscience
yesterday tracked Antarctic sea ice between 1992 and 2010. It
suggests that trends in sea ice are closely linked to local winds.
The main driver of the overall increase in sea ice is the changing
strength of cold winds, which blow ice away from the coast and
allow the water in between to freeze. (Although there are some
differences across the continent.)
The new study says that the wind change may be down to natural
climate variability, but that there is evidence that the southern
hemisphere ozone
hole also plays a part.
4. Most of the ice in Antarctica is on land
The new research talks about the ice floating on top of the
Southern Ocean. But most of the ice in the Antarctic is actually
contained within a vast expanse that covers the land mass and
extends into the surrounding ocean - known as the Antarctic ice
sheet.
Measurements from the Gravity Recovery and Climate Experiment
(GRACE) satellite
since 2002 have shown that the mass of the Antarctic ice sheet is
decreasing
at an average rate of 100 cubic kilometres every year - the size of
a small UK city. Scientists have compared different techniques for
measuring this ice loss and found good
agreement between them.
The graph below, using the GRACE data, shows the rate of ice loss
from the Antarctic ice sheet since 2002, relative to the 2002 to
2011 average.
Source: NASA/University of California, Irvine
5. What's going on in the Arctic is quite different
because it's mainly ocean
While the Antarctic is a land mass surrounded by ocean, the
Arctic is an ocean with land around the outside. The different
geographies mean that the forces determining sea ice distribution
are different. In the Arctic, floating sea ice melts as a direct
result of warming water and the wind does not play such a major
role. In September, Arctic sea ice reached its lowest
extent since satellite records began in 1979.
6. The world is losing ice overall
Although Antarctic sea ice is growing in extent, the loss of sea
ice in the Arctic outweighs this small gain. The graph below from
climate science blogger Tamino shows that globally,
sea ice has been decreasing steadily since 1979. The zero point on
the left hand side of the graph is the average sea ice extent for
the period 1979 to 2011.
Source:
Tamino. Data is from US National Snow and Ice
Data Center.
Together with the loss of ice mass from the Greenland and
Antarctic ice sheets, this means that the earth is losing ice
overall. A loss in ice cover means that the earth's surface
reflects less solar energy, reinforcing atmospheric warming.
Melting ice sheets add freshwater to the oceans, contributing to sea
level rise.
Focusing on the whole picture
Some media outlets have played down the consequences of climate
change for declining global ice cover by focussing on the gains in
Antarctic sea ice. While this has led to some confusing and at
times apparently contradictory media coverage, there are some very
accessible internet resources to keep up to date with the latest
changes in polar ice cover. These include the National Snow and Ice
Data Center (NSIDC) website,
and Cryosphere
Today.
The information about ice that scientists have to work with is
increasing. For example, just last month a team of scientists
produced the first
three-dimensional map of the underside of Antarctic sea ice
using a robot submarine.
Maps like these will help scientists better understand how
climate change may be affecting the thickness of sea ice, as well
as the area it covers. Only by considering sea and land ice in both
hemispheres can we get a full picture of how the planet is
responding to global warming.