With global climate talks kicking off in Marrakech, Morocco on Monday, a new study provides a sobering warning about what may happen to coastal mega-cities if decisive global emissions cuts are not made soon.

Based on a scenario in which countries fail to sharply rein in emissions of global warming pollutants, coastal cities are likely to see the fastest rate of sea level rise in human history before the end of the current century, the study found.

This damaging scenario is not just limited to a future generation in the year 2100 but has already begun.

What's more striking is that the study shows that more than more than 90 percent of the world's coastal areas will see more than the global average sea level rise.

The study paints a particularly dark scenario for the densely populated cities of South and Southeast Asia, where low-lying coastal cities could be eaten away by the sea, displacing millions.

The study, published Monday in the journal Proceedings of the National Academy of Sciences, found that if global warming pushes past 2 degrees Celsius, or 3.6 degrees Fahrenheit, above preindustrial levels, about 80 percent of the global coastline may see more sea level rise than the global average.

The study is the first to make specific sea level rise projections for 136 coastal cities starting with 2 degrees Celsius of warming and above, according to lead author Svetlana Jevrejeva of The National Oceanography Center in Liverpool.

Jevrejeva and her colleagues found that 2 degrees of warming would yield an average global ocean rise of 0.6 feet. But in the sprawling city of Lagos, Nigeria, for example, that much warming would likely cause 0.7 feet of sea level rise with a worst-case-scenario of 1.1 feet.

Two degrees Celsius is also defined as the upper limit to global warming under the Paris Climate Agreement, which went into effect on Nov. 4, but that limit is likely to be reached by 2045, based on emissions trends.

"If the Paris Agreement fails and the worst-case scenario comes to pass, South Florida and the boot of Louisiana would not likely survive this century. Many more places, from Boston to Shanghai, would be gravely threatened,” said Ben Strauss, a sea level rise researcher at the nonprofit group Climate Central who is unaffiliated with the new study.

A worker constructs Jakarta's giant sea wall on August 16, 2016. Image: Pacific Press/Sipa USA

The meeting in Marrakech is aimed at speeding up emissions cuts and generating more ambitious targets so that the Paris goal is met.

This study asks what would happen to global sea levels if we blow past the target.

If the climate were to warm by 5 degrees Celsius, or 9 degrees Fahrenheit, above preindustrial levels by 2100 — which is roughly the path we're on now — New York City could see more than a meter, or about 3.6 feet, of sea level rise with an even higher upper limit, when factoring in sources of uncertainty.

"If warming continues above 2 degrees Celsius, then, by 2100, sea level will be rising faster than at any time during human civilization," the study says.

Here are some of the heavily populated, rapidly growing cities with at least 2.9 feet of sea level rise expected by 2100, according to the study:

Dakar, Senegal

Guangzhou, China

Ho Chi Minh City, Vietnam

Lagos, Nigeria

Manila, Philippines

Qingdao, China

A separate study published in February found that current rates of sea level rise are likely unprecedented in at least the past 2,800 years.

This week's findings makes clear that many cities in South and Southeast Asia are at particular risk of flooding, including rapidly developing mega-cities.

Because of land subsidence, or sinking, groundwater depletion and urban development policies that incentivize growth in low-lying areas, these cities and others are likely to see far higher amounts and faster rates of sea level rise than the global average, particularly after the middle of this century.

"The coastal communities of rapidly expanding cities in the developing world, and vulnerable tropical coastal ecosystems will have a very limited time to adapt to sea level rises after the 2-degree threshold is likely to be reached," Jevrejeva said in an email to Mashable.

"We show that warming above 2 degrees will be extremely damaging for coastal communities and coastal megacities."

5,000 computer model simulations

For the study, scientists ran more than two-dozen computer models about 5,000 times to create probability-based projections of future sea level rise. The researchers took into account many factors that can cause regional variations in sea level rise, such as ocean current changes and where sea and land-based ice melts. However, large uncertainties remain.

Sheldon Glacier with Mount Barre in the background, seen from Ryder Bay near Rothera Research Station, Adelaide Island, Antarctica. Image: Rex Features via AP

The methods largely follow past studies that have also generated future sea level rise scenarios, though this differs somewhat, says Robert Kopp, a sea level rise researcher at Rutgers University.

"The ‘twist’ is that they try to cast the projections in terms of global mean temperature rather than time," Kopp, who was not involved in the new research, told Mashable in an email.

"To be honest, I’m not sure the way they’ve cast it makes sense," Kopp said, because it matters a great deal how long the climate stays at a particular temperature. The longer the planet were to remain above 2 degrees Celsius, for example, the more likely it would be that the ice sheets in Greenland and Antarctica would melt more extensively.

"... Sea level is a lagging response" to warming, Kopp says, "so the question of how high sea level would be at 2 degrees Celsius depends strongly on how long it’s at 2 degrees Celsius." Kopp said the new study may be of limited use because it doesn't consider that delay.

Even with this caveat, the study clearly spells out how damaging sea level rise could be within our current lifetimes, let alone in 2100.