💾 Archived View for gmi.noulin.net › mobileNews › 1618.gmi captured on 2023-01-29 at 07:57:48. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2021-12-05)
-=-=-=-=-=-=-
2009-11-23 12:08:15
By Richard Black
Environment correspondent, BBC News website
The East Antarctic ice sheet has been losing mass for the last three years,
according to an analysis of data from a gravity-measuring satellite mission.
The scientists involved say they are "surprised" by the finding, because the
giant East Antarctic sheet, unlike the west, has been thought to be stable.
Other scientists say ice loss could not yet be pinned on climate change, and
uncertainties in the data are large.
The US-based team reports its findings in the journal Nature Geoscience.
The data comes from Nasa's Gravity Recovery and Climate Experiment (Grace)
mission.
It energises me as a scientist, but I'm not convinced that as yet it should
energise anyone else
Professor Richard Alley
Grace has previously shown that the smaller West Antarctic and Greenland ice
sheets are losing mass.
These two bodies of ice contain enough water to raise sea levels by about six
to seven metres (20ft) each if they melted completely.
Melting the East Antarctic sheet would raise sea levels by much more - about
50-60m.
But scientists have generally discounted the possibility of it happening
because the region is so cold.
The Grace measurements suggest there was no net ice loss between 2002 and 2006.
But since then, East Antarctica has been losing 57 billion tonnes (Gt) per
year.
"We felt surprised to see this change in East Antarctica," study leader Jianli
Chen from the Centre for Space Research at the University of Texas in Austin
told BBC News.
The loss still looks small by contrast with West Antarctica, which is losing
132Gt per year, and with Greenland, where a recent analysis combining Grace
data with other measurements indicated an annual figure of 273Gt.
Previous Grace analyses - and those from other satellites - had given an
inconclusive picture for the giant ice body.
The twin Grace satellites fly in close formation, detecting minute changes in
the Earth's gravity through the marginal changes this causes in their relative
positions.
Eastern energy
Measuring Antarctic ice loss is a tricky issue because the continent itself is
rising and deforming.
Its ice cover was significantly thicker during the last Ice Age; as the ice
melted, the weight pressing down on the rock abated, and the rock is
"isostatically rebounding".
Readings from satellite missions have to be adjusted to allow for this rebound
- and that is one source of uncertainty when trying to assess the significance
of the new research, according to Richard Alley, one of the world's leading
glaciologists.
"The first thing is that lots of this is dependent on the isostatic [rebound]
model, and (recent work has) cast some doubt on the istostatic models that
people are using," commented the Penn State University researcher (who was not
involved in the paper).
"And then you get into the age-old question of 'is it climate or is it weather?
'
"So it energises me as a scientist, but I'm not convinced that as yet it should
energise anyone else."
Rising potential
The Grace data gives a picture of where ice is being lost across the continent;
and these areas are mainly on the coast.
It is not clear what physical processes could be driving any loss of mass here,
although it is not simply melting due to high air temperatures, because
temperatures are well below zero.
One clue could lie in research published last year by Leigh Stearns and
colleagues, showing that lakes under the ice sheet can periodically overflow,
with the liquid water then acting as a lubricant to speed glaciers on their way
towards the sea.
Commenting on the new research, Dr Stearns told BBC News: "In these coastal
regions the ice loss could be driven by some interaction with the oceans or
some weather patterns, or it could be a sub-glacial lake that drained and
caused some thinning - so it might not be climate-related.
"It's easy to jump to the conclusion that it's exceptional because it's the
first time we've recorded it, but we do need a baseline of how things have been
in the past so we do need to be cautious," said the University of Kansas
researcher.
"Nevertheless, it awakens us to the fact that the East Antarctic sheet is more
dynamic than we thought, and we do need to pay attention to it because its
potential for sea level rise is so much greater than in West Antarctica or
Greenland."
Dr Chen said that one of his team was currently conducting airborne surveys of
one of the regions where mass loss had been detected, hoping to shed some light
on the mechanisms involved.
Richard.Black-INTERNET@bbc.co.uk