Oceans are likely to rise as much as 1.3 metres by 2100 if Earth's surface warms another 3.5 degrees Celsius, scientists warned Friday.
By 2300, when ice sheets covering West Antarctica and Greenland will have shed trillions of tonnes in mass, sea levels could go up by more than five metres under that temperature scenario, redrawing the planet's coastlines, they reported in a peer-reviewed survey of more than 100 leading experts.
About ten percent of the world's population, or 770 million people, today live on land less than five metres above the high tide line.
Even if the Paris climate treaty goal of capping global warming below 2C is met -- a very big "if" -- the ocean watermark could go up two metres by 2300, according to a study in the journal Climate Atmospheric Science.
Earth's average surface temperature has risen just over one degree Celsius since the pre-industrial era, a widely used benchmark for measuring global warming.
"It is clear now that previous sea-level rise estimates have been too low," co-author Stefan Rahmstorf, head of Earth system analysis at the Potsdam Institute for Climate Impact Research (PIK), told AFP.
The new projections for both the 2100 and 2300 horizons are significantly higher than those from the UN Intergovernmental Panel on Climate Change (IPCC), including a special report on oceans it released in September.
"The IPCC tends to be very cautious and conservative, which is why it had to correct itself upwards already several times," Rahmstorf said.
Sea-level projections in the IPCC's landmark 2014 Assessment Report were 60 percent above those in the previous edition, he noted. A new Assessment will be finalised by the end of next year.
While less visible than climate-enhanced hurricanes or persistent drought, sea level rise may ultimately prove the most devastating of global warming impacts.
“A global sea-level rise by several metres would be detrimental for many coastal cities such as Miami, New York, Alexandria, Venice, Bangkok, just to name a few well-known examples. Some may have to be abandoned altogether as they cannot be defended,” Rahmstorf told The Guardian.
Indeed, it is the extra centimetres of ocean water that make storm surges from ever-stronger tropical cyclones so much more deadly and destructive, experts say.
Benjamin Horton, acting chair of the Nanyang Technical University's Asian School of the Environment in Singapore, led the survey to give "policymakers an overview of the state of the science", a statement said.
Across the 20th century, sea level rise was caused mainly by melting glaciers and the expansion of ocean water as it warms.
But over the last two decades the main driver has become the melting and disintegrating of Earth's two ice sheets.
Greenland and West Antarctica are shedding at least six times more ice today than during the 1990s. From 1992 through 2017 they lost some 6.4 trillion tonnes in mass.
Over the last decade, the sea level has gone up about four millimetres per year. Moving into the 22nd century, however, the waterline could rise ten times faster, even under an optimistic greenhouse gas emissions scenario, the IPCC has said.
The Greenland and West Antarctic ice sheets hold enough frozen water to lift oceans about 13 metres. East Antarctica, which is more stable, holds another 50 metres' worth.
“Like in the Covid pandemic, timing is critical to prevent devastation. If you wait until you already have a serious problem, then it is too late. Unlike with corona, sea-level rise cannot be stopped for many centuries or even millennia once ice sheets have been destabilised past their tipping points,” Rahmstorf told The Guardian.
(With inputs from AFP)