Global sea levels could rise more than expected: Study

Global sea levels could rise far more than predicted, due to accelerating melting in Greenland and Antarctica, according to a study.

Melting ice sheets in Greenland and the Antarctic, and subsequent sea level rise (SLR) this will cause, is widely recognised as posing a significant threat to coastal communities and ecosystems, said researchers led by the University of Bristol in the UK.

Strategies and measures to mitigate and plan for the potential impacts are reliant on scientific projections of future SLR -- conventionally provided using numerical modelling.

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Such projections remain challenging due to ongoing uncertainty regarding the evolution of the Greenland and Antarctic ice sheets, particularly in response to climate change, according to the study published in the journal Proceedings of the National Academy of Sciences.

Using a technique called structured expert judgment (SEJ), researchers asked 22 ice sheet experts to estimate plausible ranges for future sea level rise due to the projected melting of each of the Greenland, West Antarctic and East Antarctic ice sheets under low and high future global temperature rise scenarios.

"SEJ provides a formal approach for estimating uncertain quantities based on current scientific understanding, and can be useful for estimating quantities that are difficult to model," said Professor Jonathan Bamber from the University of Bristol.

"Projections of total global SLR using this method yielded a small but meaningful probability of SLR exceeding two metres by the year 2100 under the high temperature scenario, roughly equivalent to 'business as usual', well above the 'likely' upper limit presented in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)," Bamber said.

The findings suggest that coastal communities should therefore not rule out the possibility of 21st-century SLR in excess of two metres when developing adaptation strategies. 

"Such a rise in global sea level could result in land loss of 1.79 million square kilometre, including critical regions of food production, and potential displacement of up to 187 million people. A SLR of this magnitude would clearly have profound consequences for humanity," Bamber said.

The SEJ process provided an opportunity for experts to discuss their scientific rationales for the quantitative judgments they make on uncertainties relating to future ice sheet contributions to sea level.

This unique approach also served to identify some poorly understood but potentially critical processes, such as "marine ice cliff instability", which may act in future as significant tipping points in ice sheet response to temperature rise.