Scientists warn of up to 70 cm of sea level rise by 2100, but is this better or worse than we thought?
- 15 May 2013, 13:50
- Roz Pidcock
From Tuvalu to Alaska, some communities are already feeling the
effects of rising sea levels - but knowing just how much melting
ice is contributing to sea level rise, and what we can expect in
the future is more difficult. A major EU project has just released
new projections - and it says sea levels could rise nearly 70 cm by
Today's media have reported the
new projections but seem confused over whether they're better
or worse than expected. The
Times says the risk from rising seas is "worse than feared",
New Scientist claims "it's not as bad as we thought".
As it turns out it could be seen as a bit of both - here's
Four years ago, the Ice2sea
project launched with the aim of improving scientists' projections
of how much melting sea ice will contribute to global sea level
Two years previously, the
IPCC 4th assessment report gave a best estimate of sea level
rise of around 40 cm by 2100, but said the biggest uncertainty was
the contribution to sea level rise from ice sheets and
As ice sheets and glaciers melt, water that was previously held
on land is added to the ocean, causing sea levels to rise. Head of
the Ice2sea project, Professor David Vaughan of the British
Antarctic Survey, explained at the launch of the project's final
report last night:
"The last IPCC report highlighted
particular uncertainties in projections of sea level rise related
to the Greenland and Antarctic ice sheets, which hold a
considerable amount of water … In a sense, the estimates were
incomplete - the full dynamics of ice sheets were missing."
Sea level rise
Since the last IPCC report, there have been major advances in
the tools available to scientists to monitor ice loss from sheets
and glaciers - and in scientists' understanding of processes
driving ice loss.
Last night at the event in London, Vaughan presented the outcome
of all this new knowledge -
updated projections of sea level rise.
The team's best estimate is that under a business as usual
medium emissions scenario, melting ice sheets and glaciers will
contribute between 3.5 and 36.8 cm to sea level rise by 2100.
When you add that to sea level rise caused by water expanding as
it warms - a process called thermal expansion - total sea level
rise is likely to be between 16.5 and 69 cm.
The scientists worked out there's a
one in 20 risk the contribution from land ice could be as much
as 85 cm, which could conceivably push total sea level rise to over
a metre by 2100. Even though the likelihood is small, presenting
the highest risk helps coastal planners prepare for the worst case
Projections in context
The Ice2sea estimate of total sea level rise - with the updated
contribution of ice sheets and glaciers - is slightly higher than
the range proposed by the IPCC in 2007 of 18 to 59 cm, which covers
all emissions scenarios.
"The range is not too different from the
last IPCC report but [the process is] more thorough, defensible and
we can be confident it contains the key processes."
On the other hand, the Ice2sea projections are lower than some
other estimates, which project up to 1.5 m by 2100. But the
higher estimates involve a somewhat simplified approach. Rather
than simulating all the processes leading to ice loss, they look at
the past relationship between warming and sea level rise and extend
it forward to 2100.
The IPCC's next report - due later this year - collects all the
scientific evidence since 2007. A
leaked draft of the report projects sea levels will rise by
between 29 and 82 centimeters by 2100 - significantly higher than
the last report, although the estimate may change in the final
version. Again, the IPCC range covers all emissions scenarios, not
just the medium emissions scenario Ice2sea uses.
Understanding ice sheets
The key to how scientists get to these projections is
understanding the mechanisms leading to ice sheet melt - and then
building computer models to simulate those processes.
analysis of satellite data last year as part of the Ice2sea
project showed that from 1992 to 2011 both ice sheets lost mass -
Greenland about twice as fast as Antarctica. Together, the ice
sheets are losing more than
three times as much ice now as they were in the 1990s.
In Greenland, rising air temperatures are causing an increasing
amount of the ice sheet's
surface to melt, as the image below shows.
Number of melt days on the Greenland ice sheet on average
(1979-2007) and last year (2012). Source:
National Snow and Ice Data Centre
But glaciers are an important part of the picture too. A
last week looked at how meltwater can loosen up the ice
so that glaciers flow quicker, discharging more ice to the
ocean. Meltwater can also cause the ice to
fracture - a process known as calving.
Another major factor is that warming oceans cause thinning of
ice shelves - sheets of ice that extend from the land out into the
As well as looking at ice sheet melt, the Ice2sea project has
contributed to a global inventory of more than 200,000 glaciers
across the world. Dr Tamsin Edwards from the University of Bristol
"The difference with the Ice2sea project
is that it took a really broad approach by pinning down the
contribution from all forms of land ice - which hadn't really been
done before in such a consistent way."
There are still uncertainties about the Ice2sea projections
because scientists have only been able to invetsigate in detail the
processes governing ice loss since satellites became available in
the late 1970s.
The Ice2sea projections also centre on the IPCC's medium
emissions scenario, A1B. If emissions are higher up to 2100 then
the sea level rise projections could also be higher, Vaughan said
But the scientists are clear that enormous progress has been
made in the last few years and the tools the project has developed
to monitor and predict ice loss will mean projections of sea level
rise - globally and regionally - continue to be refined.
Update 15th May 15.45
We've updated the post to make it clearer that the IPCC
projection range covers all emission scenarios, whereas the Ice2sea
projections are based on the A1B business as usual medium emissions