Comparison of the probability distribution of Antarctic mass loss (left) and global total sea level rise (right) in 2100 compared to 1985–2005 for the RCP8.5 scenario for: IPCC AR5 (red), Antarctic mass loss DP16 (blue), including temperature dependent Antarctic mass loss DP16T (green). A sensitivity of global estimates to the γ parameter representing CMIP5 model ensemble uncertainty is also added: DP16T with γ = 1.64 (black) instead of 1 for the other curves. Graphic: Le Bars, et al., 2017 / Environmental Research Letters

25 April 2017 (University of Southampton) – Global sea levels could rise by more than three metres – over half a metre more than previously thought – this century alone, according to a new study co-authored by a University of Southampton scientist.

An international team including Sybren Drijfhout, Professor in Physical Oceanography and Climate Physics, looked at what might happen if carbon dioxide emissions continue unabated.

Using new projections of Antarctic mass loss and a revised statistical method, they concluded that a worst-case scenario of a 2.5 to three-metre sea level rise was possible by 2100.

Professor Drijfhout said: “It might be an unlikely scenario, but we can’t exclude the possibility of global sea levels rising by more than three metres by the year 2100.

“Unabated global warming will lead to sea-level rise of many metres – possibly more than ten metres – within a few centuries, seriously threatening many cities all over the world that are built in low-lying river deltas. This will also seriously affect the coastline of the UK.”

The research – published this month in Environmental Research Letters – is consistent with the National Oceanic and Atmospheric Administration’s (NOAA) recent adjustment of its possible future high-end sea-level rise from two to 2.5 metres.

However, the new study integrated different model estimates with a new statistical method, whereas the NOAA estimate relied on expert judgment.

Recent observation and modelling studies have shown the future melt of Antarctica might happen dramatically faster than previously thought.

Professor Drijfhout and scientists at the Royal Netherlands Meteorological Institute, which led the research, took this and other factors – including ocean warming, glacier melt, land water storage and Greenland ice sheet melt – into account to create their projection.

“This is the first time that robust statistical techniques have been used to develop a scenario like this, whereas previous high-end sea level projections have always been based on subjective expert judgment,” said Professor Drijfhout.

“It’s important for policy-makers and the general public to know what the consequences might be when carbon dioxide emissions are not decreased, especially as there is a severe time-lag between emission-reduction and the sea-level rise response.

“Also, the construction of artificial flood defences need to take account of low-probability events, including the possibility that the international community fails to take adequate measures in reducing measures.

“We should not forget that the Paris Agreement is only a declaration of intention, and that no adequate measures have yet been agreed to turn these intentions into policy.”

The team’s projection explicitly accounted for three scientific uncertainties – the speed at which the Antarctic ice sheet is going to melt, the speed at which the ocean is warming up, and the amount of emitted greenhouse gases over the 21st century.

Sea levels could rise by more than three metres, shows new study

ABSTRACT: The potential for break-up of Antarctic ice shelves by hydrofracturing and following ice cliff instability might be important for future ice dynamics. One recent study suggests that the Antarctic ice sheet could lose a lot more mass during the 21st century than previously thought. This increased mass-loss is found to strongly depend on the emission scenario and thereby on global temperature change. We investigate the impact of this new information on high-end global sea level rise projections by developing a probabilistic process-based method. It is shown that uncertainties in the projections increase when including the temperature dependence of Antarctic mass loss and the uncertainty in the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble. Including these new uncertainties we provide probability density functions for the high-end distribution of total global mean sea level in 2100 conditional on emission scenario. These projections provide a probabilistic context to previous extreme sea level scenarios developed for adaptation purposes.

A high-end sea level rise probabilistic projection including rapid Antarctic ice sheet mass loss



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