The Real Science Behind Rising Sea Levels?

In a sobering warning, a high-profile study led by US climatologist James Hansen suggests that sea levels could rise by several meters by the end of this century. This prediction is based on the two largest contributors to the observed rise since 1900: thermal expansion of the oceans and the loss of ice from glaciers.

The study, while not specifically naming individual scientists, involved the GFZ Helmholtz Center for Geosciences in Potsdam and the GEOMAR Helmholtz Center for Ocean Research Kiel. They are currently building a research infrastructure called SAFAtor for ocean monitoring.

The Intergovernmental Panel on Climate Change (IPCC) has also weighed in on the matter. They forecast a sea-level rise of 52-98 cm by 2100 if greenhouse emissions continue to grow, or of 28-61 cm if emissions are strongly curbed.

However, more studies are needed to determine whether the ice-sheet disintegration modelled by DeConto and Pollard will indeed lead to rises of the order of 15 m over the coming four centuries. The West Antarctic ice sheet, which some studies suggest is potentially in a state of unstoppable retreat, could contribute significantly to this rise.

The threshold at which Greenland's ice sheet would collapse is unknown, but it could be as little as 1°C above pre-industrial average temperatures or as high as 4°C. If warming air temperatures cause surface melting to outstrip snow accumulation, Greenland's ice sheet could add an extra 7 m to sea levels over a millennium or more.

Seas have already risen by more than 20 cm since 1880, and since 1993, sea level has been rising faster at about 3 mm per year (30 cm per century). This trend is expected to continue, with the majority of the future rise likely to come from increased ocean expansion, glacier melt, and surface melting from the Greenland ice sheet.

Under the RCP8.5 scenario, a new study forecasts a rise of about 80 cm by 2100. However, this could be almost totally averted with lower emissions. The new study suggests that ice-sheet collapse could begin seriously affecting sea level as early as 2050 under high greenhouse emissions scenarios.

Looking further into the future, the IPCC has projected a rise of as much as 7 m by 2500. However, they have also warned that the available ice-sheet models may underestimate Antarctica's future contribution. During the most recent interglacial period (129,000 to 116,000 years ago), sea level was 5-10 m higher than today.

With present-day temperatures already almost 1°C above pre-industrial temperatures, it is possible we could cross this threshold this century, particularly for high-emission scenarios. Beyond 2100, seas will continue to rise for many centuries, perhaps even millennia. It is a grim forecast that underscores the urgent need for action to curb greenhouse gas emissions.