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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 2100.

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", whereas the 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 why.

Ice2sea

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 rise.

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 glaciers.

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 scenario.

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.

Vaughan explains:

"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.

A major 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.

Greenland _both

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 study out 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 Southern Ocean.

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 tells us:

"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."

Remaining uncertainties

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 last night.

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.


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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 scenario.

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