Researchers warned Tuesday that even if humanity manages to limit global warming to 1.5 degrees Celsius above preindustrial levels the seas will rise for centuries and flood cities home to half-a billion people.
They reported in Environmental Research Letters that 200 million urban dwellers today would find themselves regularly knee-deep in water and more susceptible to storm surges in a world heated up half-degree beyond that benchmark.
Asia will be the most vulnerable in any scenario, with nine of the ten megacities at greatest risk.
Long-term high tides would cause land that is home to more people than half of the population in Bangladesh and Vietnam to fall below the line. This applies even in a 2C world. Devastation would also befall areas that have been built up in India, China, and Indonesia.
Most projections of sea level rise and the threat to coastal cities by it run until the end century. They range from half-a-meter up to less than twice that depending on how fast carbon pollution is reduced.
The findings reveal that oceans will continue to rise for hundreds of more years than 2100, regardless of how aggressively greenhouse gases are reduced.
It's not 'if', but 'when'
"About 5 percent of the world’s population lives on land below the level at which the high tide level is predicted to rise based upon carbon dioxide that human activities have already added to the atmosphere," Ben Strauss (CEO and chief scientist at Climate Central), told AFP.
The concentration of CO2 in the air today is 50 percent higher than it was in 1800. Earth's average surface temperature has also risen by 1.1 C.
This is enough to eventually raise sea levels by nearly 2m (more than 6ft), regardless of whether it takes 10 Strauss or two centuries.
1.5 C is the limit on global warming that was enshrined by the Paris Agreement. This will be a goal of the COP26 climate summit in Glasgow next week. It could translate into almost 3 meters long-term.
According to the study, sea level rise will not be a matter "if" it isn't possible for engineers to rapidly remove large amounts of CO2 from the atmosphere.
These are the most optimistic scenarios.
Strauss stated, "The headline finding is the stark contrast between a 1.5 C world following sharp pollution cuts and a world after 3 C/4 C of warming."
"At Glasgow, and for the remainder of this decade we have the opportunity to help or betray a hundred generations."
Time to buy
If the 2015 Paris treaty is honored, national carbon-cutting promises would still lead to Earth warming by 2.7 C by 2100. Temperatures could rise by 4 C or more if efforts to curb greenhouse gases fail.
This amount of warming would increase the global oceans' depth by 6-9 meters over the long-term. It would force cities that currently house nearly a billion people to build on higher ground or mount huge defenses against future sea levels rise.
China alone would see 200 million people move to areas below the 3C level. The threat is not just long-term. Without massive sea walls, Chinese cities could be unlivable in 80 years.
Strauss stated that although 1.5 C of warming will still cause devastating sea-level rise, the hotter alternatives are much worse.
"We are in poor shape, but it's never too late to make a difference. The impact we can make is immense."
Scientists warn that higher temperatures can lead to irreversible melting of ice sheets and the release of natural CO2 and methane from permafrost.
We can also limit global warming as low and as quickly as possible, which gives us more time to adapt.
Strauss stated that it is almost certain that seas rise slower in a 1.5 C- or 2 C warmer world.
The study was conducted by researchers from Princeton University and Potsdam Institute for Climate Impact Research, Germany.