The world runs a growing risk of triggering accelerating and potentially unstoppable sea level rise from the Antarctic ice sheet if greenhouse gas emissions are not strictly curtailed. However, this fate can be avoided if the Paris Agreement's targets are met, according to two new studies published Wednesday.

Why it matters: At stake is the viability of coastal megacities like Shanghai, Manila and New York City, as well as entire nations like the low-lying Maldives. The severity of sea level rise depends largely on the pace and extent of ice melt from the world's two largest ice sheets: Antarctica and Greenland.

Background: The Paris Agreement, reached in 2015, calls for limiting warming to "well below" 2°C (3.6°F) and for countries to work toward keeping warming to no more than 1.5°C (2.7°F) above preindustrial levels by 2100.

• But the globe has already warmed by about 1.2°C compared to the preindustrial era, and countries' Paris pledges to date would put us on course for about 3°C (5.4°F) of warming by 2100.

How it works: There is already evidence that warming is destabilizing parts of Antarctica.

• Floating ice shelves buttressing parts of the West Antarctic Ice Sheet are thinning as warm ocean water sneaks beneath them, while warming air temperatures eat away at them from above.
• As these shelves relinquish their grip, they speed up the flow of inland ice into the sea, like removing a doorstop.
• Climate scientist Rob DeConto of the University of Massachusetts, Amherst, an author of one of the new studies, and others have shown there is also the possibility that massive, unstable ice cliffs can form at the edge of glaciers — and suddenly collapse.

The big picture: The studies, both published in the journal Nature, show that future sea level rise rates are closely tied to how quickly greenhouse gas emissions are cut in the near-term.

• Both papers agree if warming is limited to the Paris targets — a big "if" given recent emissions trends — sea level rise from Antarctica will proceed at about the same pace it is today through 2100.
• By that time, the continent will have contributed about nine centimeters to global sea level rise.

But if warming follows the 3° warming scenario or higher, sea level rise could begin to accelerate irreversibly as soon as 2060, the DeConto-led work shows, and spike to 15 to 34 centimeters, or 13.4 inches by 2100.

• Much greater amounts of sea level rise would occur beyond 2100, on the order of several meters, if emissions stay high well into this century, the study shows.
• "That's a totally, world-altering kind of environmental change and catastrophe," DeConto tells Axios.

What they did: DeConto and his colleagues used hundreds of simulations from computer models of ice sheet behavior in order to recreate historical and recent ice loss and sea level rise, and make future projections.

• Researchers tossed out projections from simulations that were inconsistent with observations and historical data. They also incorporated 16 years of detailed satellite imagery to help make the modeling as precise as possible.
• They limited the ice cliff instability to observations from glaciers in Greenland, but DeConto noted that it remains a "wild card" that could significantly speed up Antarctic ice loss.
• The other study, led by Tamsin Edwards of Kings College in the U.K., looked at land ice loss worldwide, and found limiting global warming to 1.5°C would cut in half the land ice contribution to sea level rise when compared to the current pace.

Yes, but: The study Edwards led does not include the ice shelf or cliff instabilities that could drastically increase Antarctic melt.

• It warns that ice loss from that continent could be up to five times higher, which would boost the land ice loss contribution to sea level rise from between 13 to 25 centimeters to up to a half a meter by 2100.
• Due in large part to the instability of ice shelves and low elevation of inland areas of western Antarctica, that part of the ice sheet constitutes one of the many dreaded climate change "tipping points."
• "We're not finding really any way of slowing things down" once the ice sheet starts melting rapidly, DeConto said.

The intrigue: DeConto and his colleagues found runaway ice loss would occur even if technologies to pull carbon dioxide out of the atmosphere, which are still in their infancy, were to be employed later this century.

What they're saying: "This study is a stark reminder that the fuse is short for deep and sustained cuts to greenhouse gas emissions," study coauthor Andrea Dutton of the University of Wisconsin-Madison, said via email.

• Dutton noted the rate of sea level rise may jump quickly and sharply, including between 2050 and 2070 if emissions stay high, overwhelming the efforts of coastal residents to cope with increased flooding.

The bottom line: The new studies show that limiting global warming through emissions cuts now avoids expensive, world-altering outcomes in the future.

This story was updated with more information about Antarctic ice cliff instability.