Warming ocean is biggest driver of Antarctic ice shelf melt, says new study
- 13 Jun 2013, 21:45
- Roz Pidcock
Larsen Ice Shelf, NASA
Of earth's two vast ice sheets, Antarctica is perhaps the more
mysterious. From the icy surface to the ocean below, there are
several different ways the ice sheet is shrinking. How these
processes compare is key to knowing how fast melting ice sheets are
raising sea levels worldwide - and a
new study out today may help to shed a bit more light.
Ice sheets and sea level rise
There is huge scientific interest in monitoring how the ice
sheets are changing because when ice on land melts it drains into
the ocean, causing sea levels to rise.
Last week, we reported on
major review of how scientists' understanding of ice sheet melt
has advanced since the last Intergovernmental Panel on Climate
Change (IPCC) report in 2007.
But while Greenland is relatively well understood, at the other
end of the planet in Antarctica the picture is a little less clear,
the report concludes.
Antarctic ice shelves
To understand how most of the ice is lost from Antarctica, a
new study just published in the journal Science looks to the
ice shelves that surround 75 per cent of the continent.
Ice shelves are floating extensions of land ice that act as
buttresses, stopping ice flowing from the interior straight out
into the ocean.
Ice shelves line 75 per cent of the Antarctic coastline. The
biggest ones are the Ross and Ronne-Filchner ice shelves, marked
here in red and dark blue. Source: National
Snow and Ice Data Centre
Traditionally, scientists thought large chunks of solid ice
breaking off the ice shelves was the main source of ice loss from
Antarctica - a process known as iceberg calving.
But there's another way. As the ocean below the ice shelves
warms, the ice melts from the bottom up - something scientists call
basal melting. With melting from the top and bottom, some ice
shelves are getting noticeably thinner, says the new study.
The paper looked at both processes of ice loss across 99.5 per
cent of Antarctica's ice shelves between 2003 and 2008. Overall,
the scientists found basal melting caused 55 per cent of ice loss,
although they saw quite a lot of variation between regions.
This makes bottom-up ice shelf melt the largest source of ice
shelf loss in Antarctica, the paper suggests. Previous studies have
estimated the contribution to be more like 30 per cent, or even as
low as 10 per cent.
Calling the new findings "a game changer", lead author on the
paper, Professor Eric Rignot from the University of California
Irvine, explains what this means for scientists' understanding of
the ocean surrounding Antarctica - the Southern Ocean. He tells
"[T]he role of the Southern Ocean
in controlling the evolution of ice shelves, thereby the evolution
of the ice sheet as a whole, is more significant
than estimated previously. This places more
emphasis on understanding the evolution of the state of the
The scientists found smaller ice shelves melted more than large
ones. Giant ice shelves like the Ross, Filchner and Ronne occupy
two thirds of the total ice shelf area but accounted for just 15
per cent of melting. Half of the meltwater came from 10 smaller ice
shelves occupying just eight per cent of the total area.
An uncertain ice sheet
If Antarctica melted completely, scientists estimate it could
add about 58
metres to global sea level. At the moment, satellite data
indicates the Antarctic ice sheet is losing ice but only at a rate
of about 0.2
mm per year - more than three times slower than
That's partly because different parts of the ice sheet are
changing in different ways. Even though ice shelves in West
Antarctica and the Antarctic Peninsula are melting and breaking
apart, this doesn't contribute directly to sea level rise as they
are already floating on water.
But their thinning accelerates ice flow from the many glaciers
in the continent's interior, which does raise sea levels. Warmer
air overlying the ice also causes melting on the surface.
Nearly half of the ice shelves In East Antarctica are thinning
too, according to the new paper. But
satellite data indicates the mass of ice in East Antarctica is
growing overall thanks to an increase in snowfall.
But scientists warn there are still lots of
uncertainties about these estimates. So monitoring the
contribution of different processes - and how they're likely to
change in the future - is critical for producing reliable
projections of sea level rise.
Rignot et al., (2013) Ice shelf around
Updated 14th June 09:55 GMT to
include a quote from lead author Professor Eric