“Impact assessment, risk management, adaptation strategy and long-term decision-making in coastal areas depend on future projections of sea level and crucially its low probability, high impact, upper limit – the so-called ‘worst-case scenario’, which is outside the range provided by the AR5 IPCC report,” Svetlana Jevrejeva of the UK’s National Oceanography Centre told environmentalresearchweb. “Infrastructure needs to survive the worst-case situation.”

More than 600 million people worldwide live less than 10 m above sea level, with 150 milllion dwelling within 1 m of high tide. Nations and organizations have used different values for projected sea-level rise in their contingency planning, for example the Netherlands’ Delta Commission plumped for 1.1 m, the US Army Corps of Engineers employed 1.5 m to design civil works, and the UK Climate Impacts Programme chose 1.9 m.

The IPCC report found it difficult to assess how much ice loss there would be from Greenland and Antarctica, and so its contribution to sea-level rise was unclear. The fifth assessment suggests that only the collapse of the marine-based sectors of the Antarctic ice sheet could cause sea-level rise to be substantially higher than the likely range for the 21st century. Since the report, modelling has indicated that West Antarctica’s Pine Island Glacier is probably already undergoing an unstable retreat; other glaciers are likely to follow suit.

Along with Aslak Grinsted from the University of Copenhagen, Denmark, and John Moore from Beijing Normal University, China, and the University of Lapland, Finland, Jevrejeva took a different tack to the IPCC for projecting sea-level rise.

“We applied a novel approach – hybrid methods combining process-based models with expert elucidation for factors related to the large ice sheets where our understanding is weakest and models are not mature enough,” said Jevrejeva.“We are a strong team with comprehensive and balanced expertise in oceanography, glaciology, sea-level and modelling.”

In this way, the researchers made probability density functions for global mean sea-level rise by 2100, and took the sea-level rise with a less than 5% probability as the upper limit. The experts consulted considered that ice sheets would contribute 84 cm to sea-level rise by 2100.

“We regard expert judgement as a useful tool to assess the uncertainty ranges, because the ice sheet experts know which physical processes (e.g. calving, ice sheet-ocean interaction) are insufficiently represented in their ice sheet models,” writes the team in Environmental Research Letters (ERL).

The researchers also assembled an upper limit from process-based models for individual contributions to sea-level rise such as thermal expansion, glacier surface mass balance, surface mass balance and dynamic changes for Greenland and Antarctica, and changes in land water storage. This gave a value of 1.9 m.

“These simple calculations demonstrate that using the highest estimates from process-based models for individual sea level components…provides support for the upper limit of sea-level projection by 2100 of 180 cm,” writes the team.

What is certain, according to the researchers, is that sea-level rise will continue beyond 2100 and that a sea-level rise with only a 5% probability for 2100 is far more likely to have occurred by 2200 because of the century-scale response times of the ice-ocean system.

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