Greenland ice sheet probably more stable than we thought
- 24 Jan 2013, 14:30
- Roz Pidcock
Sepp Kipfstuhl
The ice covering Greenland may be less sensitive to rising
temperatures than previously thought, according to a new study
which examines the effect of past warming on the ice sheet. But
it's not all good news - it could mean ice in Antarctica is more
vulnerable. Here's a look at how scientists found all this out, and
what it means.
The new research, published today in
Nature, is the first time scientists have managed to retrieve a
complete ice core from the Greenland ice sheet. From this, they
were able to reconstruct a record of temperature changes over
Greenland going more than 100,000 years back in time.
The results surprised the scientists for a couple of reasons.
First, it had been thought that the temperature during the last
interglacial period - between about 115,000 and 130,000 years ago -
was just a few degrees Celsius higher than now. The new study
suggests a bigger difference - more like eight degrees higher.
Secondly, the ice cores indicated only a "modest ice sheet
response" from the Greenland ice sheet to such warming - which
means that Greenland melt didn't contribute as much to past sea
level rise as previously thought.
Ice cores and air bubbles
Over a period of four years, a team of
300 scientists and students from 14 different countries, led by
Professor
Dorthe Dahl-Jensen of the Niels Bohr Institute in Denmark,
drilled right through the 2.5 km Greenland ice sheet. Earlier
attempts to do this have been unsuccessful, but this time the
result was a set of ice cylinders - each one 10.2 cm in
diameter and 3.5 m long - formed of layer upon layer
of compressed snow.
Like tree rings, the layers of snow and bubbles of gas trapped
in the ice tell scientists about climatic conditions when they
formed. The ratio of particular forms of oxygen and nitrogen reveal
information about ambient temperature, greenhouse gas levels and
moisture levels.
Lead scientist Dorthe Dahl-Jensen with a section of the ice
core. Credit: Sepp Kipfstuhl
Warmer than expected
Before this study, scientists estimated the last interglacial
period - known as the Eemian - was between
three to five degrees Celsius warmer than present day. The
Eemian interglacial was caused by a change in the earth's orbit,
altering the amount of solar energy delivered to earth.
But the new ice core suggests temperatures over Greenland peaked
at eight degrees Celsius above the average for the past millenium -
considerably more than previously thought.
In response to this warming, ice mass in Greenland started to
shrink at about six cm per year - but it did not disappear,
according to the study. In fact, the data show the ice sheet
decreased in thickness by about 400 metres, or less than 25 per
cent of its volume. Uncertainty around the measurements, however,
means that the loss in thickness could have been as little as 150 m
or as much as 650 m.
As Dahl-Jensen explains, this reduction in volume is less than
the scientists expected.
"The good news from this study is that
the Greenland ice sheet is not as sensitive to temperature
increases … in warm climate periods like the Eemian as we
thought"
When ice on land melts and runs into the ocean, it causes sea
level to rise. So the implication of the new results is that the
Greenland ice sheet contributed less to global sea level rise
during the Eemian than scientists previously thought - only about
two meters.
Not all good news
While it seems like good news that the Greenland ice sheet might
not melt as much as previously thought when temperatures rise, it's
only half the picture. Scientists estimate sea levels during the
Eemian rose to be about
four to six metres higher than today. If Greenland's ice sheet
only contributed less than half - as the new study suggests - it
suggests that ice melt from the other large body of land ice, the
Antarctic ice sheet, was responsible for the rest.
As Professor Jim White, the lead US scientist on the project
explains:
"When we calculated how much ice melt
from Greenland was contributing to global sea rise in the Eemian,
we knew a large part of the sea rise back then must have come from
Antarctica"
He continues:
"A lot of us were leaning in that
direction for some time, but we now have evidence that confirms
that the West Antarctic ice sheet was a dynamic and crucial player
in global sea level rise during the last interglacial period"
Present day
In summer, about half of the surface of the Greenland ice sheet
melts naturally in the warmer temperatures. But in the last ten
years, Greenland has experienced intense surface melting similar to
the kind seen during the Eemian, say the scientists. Satellite
measurements from July 2012 showed
97 per cent of the surface of Greenland melted - more than at
any time in the last 30 years of satellite measurements. This could
be set to continue, as Dahl-Jensen explains:
"The warming that is predicted to occur
over the next 50-100 years will potentially have Eemian-like
climatic conditions".
So looking at what happened during the Eemian can help
scientists better understand how present day ice sheets may
respond. Previously, the IPCC suggested in its
4th Assessment Report that the contribution
to sea level rise from the Greenland ice sheet is likely to be more
significant than the Antarctic ice sheet, but that could change if
ice flows from West Antarctic accelerate.
Scientists
are concerned that rising atmospheric and
oceanic temperatures around the West Antarctic ice sheet are
threatening to melt the floating ice shelves that keep the ice
sheet's interior in place. A collapse of the ice shelves and the
Western Antarctic ice sheet could raise global sea levels by more
than three metres. The possibility that this happened during the
Eemian - and could happen again -
cannot be discarded, say
scientists.
So not only is this new study a scientific first in terms of
analysing a complete Greenland ice core, if the Eemian is
considered an analog for current and future warming, the new
research may also help refine predictions about ice sheet responses
and future sea level rise.