The Independent says we’re headed for “catastrophic" sea level rise: Here’s what you should know about melting ice sheets
- 07 Jan 2013, 17:15
- Roz Pidcock
Based on a new study, today's Independent claims scientific
experts believe melting ice sheets contribute more to sea level
rise than previously thought. But the idea that past sea level rise
predictions have underestimated melting ice sheets is already well
known in the scientific community. Here's your guide to how
scientists' understanding of the ice sheets has improved in recent
years and where the new study fits in.
According to today's
"Glaciologists fear they may have
seriously underestimated the potential for melting ice sheets to
contribute to catastrophic sea level rise in coming decades".
The study doesn't use the term "catastrophic" - so the
Independent has rather sensationalised the paper's findings. But
that point aside, let's look at the research that sparked the
Earth scientists from the University of Bristol carried out a
survey of 26 leading climate scientists, asking them to give
detailed judgements on how rising global temperatures will affect
the Greenland and Antarctic ice sheets in the coming century.
study, published in the journal Nature Climate Change, uses a
technique that "mathematically pools" expert opinion. It's been
used before in engineering medicine and the natural sciences - but
not in assessing the weight of opinion on earth's melting ice
The collective opinion of the scientists surveyed is that
melting ice sheets are likely to contribute an average of 29 cm to
sea level rise by 2100. Some predict even more, with a five per
cent chance it could be as much as 85 cm. Together with other
factors causing sea level rise, such as the
expansion of water as it warms, the experts suggest this could
conceivably push total sea level rise to over a metre by 2100.
The Greenland and Antarctic ice sheets hold 99.5 per cent of the
earth's glacier ice and are the largest potential source of
future sea level rise.
As lead author of the new research, Professor Jonathan Bamber,
"[Our research] shows glaciologists
believe there is a one in 20 chance of sea levels rising by a metre
or more by 2100, and a metre in sea level rise is really very
Were past estimates too low?
Every five or six years, the Intergovernmental Panel on Climate
Change (IPCC) produces an
in-depth assessment of the state of research in all areas of
climate science, including sea level rise. The IPCC published its
last assessment in 2007 (AR4) and the next one (AR5) is due early next year.
Scientists know that rising
global temperatures are causing ice sheets to melt, but
estimating their contribution to sea level rise is more difficult.
At the time of AR4, scientists were uncertain about the mechanisms
causing ice loss - so much so that they couldn't be sure whether
the Antarctic was
gaining or losing ice overall.
But with several years' more data and substantial technical
improvements, scientific knowledge has moved on. A recent
analysis of 19 years of satellite data shows that melting ice
sheets have caused sea level to rise by about 11 millimetres since
1992, which is towards the upper end of the IPCC's 2007
Satellite measurements show both ice sheets together
contributed a total of 11 mm to sea level rise between 1992 and
2011. Source: Shepherd
et al., (2012)
The IPCC's coming AR5 updates the scientific evidence since
2007. A blogger
leaked a draft of the report online
in December and although
it's not the finalised version, it's clear that the new preliminary
sea level rise projections are significantly higher than in
The draft report indicates sea level is likely to rise by
between 29 and 82 centimeters by the end of the century, compared
to 18-59 centimeters in the 2007 report. The main reason for this
is that the new figures take into account the contribution to sea
level rise from ice sheets.
Weather or climate change?
But this isn't the end of the story. While there's no doubt that
rising global temperatures caused by
human activity are contributing to melting ice sheets,
scientists are still uncertain about how much of the recent changes
could be due to natural fluctuations in the climate system, as the
new paper notes.
What's more, the physical processes that influence ice loss
differ across the ice sheets. For example, Western Antarctica and
the Antarctic Peninsula have both lost
ice since 1992, whereas Eastern Antarctica gained ice because
of increased snowfall.
In the face of such uncertainty, it appears the AR5 draft takes
a conservative position. It estimates that, on average, ice sheets
will contribute 11 cm to sea level by 2100 - less than the 29 cm
suggested by the group of experts in the new study. The AR5 report
"Larger values cannot be excluded, but current scientific
understanding is insufficient for evaluating their probability"
So does the new survey of climate scientists add anything new?
It provides an analysis of expert judgement rather than predictions
of sea level rise based on new data - but it gives us some useful
extra information. As Bamber says in the paper:
"[This study] is not a substitute for
improved process understanding; nor is it intended to remove
uncertainty, but rather to quantify it, given limitations in
It's not news within the scientific community that previous IPCC
projections of sea level rise were probably too low. At the time,
the AR4 report recognised ice sheet melt as the most significant
remaining uncertainty. Nevertheless, the new study highlights how
far the science has progressed and is another way of evaluating
where uncertainties still remain in a rapidly developing, but still