Even if humanity beats the odds and caps global warming at 1.5 degrees Celsius above preindustrial levels, seas will rise for centuries to come and swamp cities home to half a billion people, researchers warned yesterday.
In a world that heats up another half-degree above that benchmark, an additional 200 million of today's urban dwellers would regularly find themselves knee-deep in sea water and more vulnerable to devastating storm surges, they reported in Environmental Research Letters.
Worst hit in any scenario will be Asia, which accounts for nine of the 10 mega-cities at highest risk.
Land home to more than half the populations of Bangladesh and Vietnam fall below the long-term high tide line, even in a 2 degrees Celsius world.
Built-up areas in China, India and Indonesia would also face devastation, the study said.
Most projections for sea level rise and the threat it poses to shoreline cities run to the end of the century and range from half-a-meter to less than twice that, depending how quickly carbon pollution is reduced.
But oceans will continue to swell for hundreds of years beyond 2100 – fed by melting ice sheets, heat trapped in the ocean and the dynamics of warming water – no matter how aggressively greenhouse gas emissions are drawn down, the findings show.
"Roughly 5 percent of the world's population live on land below where the high tide level is expected to rise based on carbon dioxide that human activity has already added to the atmosphere," said the study's lead author Ben Strauss, CEO and chief scientist of Climate Central.
Today's concentration of CO2 – which lingers for hundreds of years – is 50 percent higher than in 1800, and Earth's average surface temperature has already risen 1.1 degrees.
That's enough to eventually push up sea levels nearly 2 meters, whether it takes two centuries or 10, Strauss said.
The 1.5 degrees warming limit enshrined in the Paris Agreement that nations will try to keep in play at the COP26 climate summit in Glasgow next month translates into nearly 3 meters over the long haul.
Unless engineers figure out how to quickly remove massive amounts of CO2 from the atmosphere, that amount of sea level rise is not a matter of "if" but "when," according to the study.
These are the optimistic scenarios.
"The headline finding for me is the stark difference between a 1.5 degrees world after sharp pollution cuts versus a world after 3 degrees or 4 degrees of warming," Strauss said.
"At Glasgow and for the rest of this decade, we have the chance to help or to betray a hundred generations to come."
National carbon-cutting pledges under the 2015 Paris treaty would, if honored, still see Earth warm 2.7 degrees Celsius by 2100.
If efforts to reign in greenhouse gases falter, temperatures could rise 4 degrees or more above mid-19th century levels.