Now comes another scary prediction: If carbon dioxide emissions from fossil fuels continue unabated, the vast West Antarctic ice sheet could begin to disintegrate, causing the sea to rise by five to six feet by the end of the century, destroying coastal cities and low-lying island nations and creating environmental devastation within the lifetimes of children born today.

The startling new finding was published Wednesday in the journal Nature by two experts in ice-sheet behavior: Robert DeConto of the University of Massachusetts, Amherst, and David Pollard of Pennsylvania State University. It paints a grimmer picture than the one presented only three years ago by a United Nations panel that forecast a maximum sea level rise of three feet by 2100. But that projection assumed only a minimal contribution from the massive ice sheets of Greenland and Antarctica. And things could get worse in the centuries to come — the melting from Antarctica alone, not counting other factors like thermal expansion, could cause the seas to rise by nearly 50 feet by 2500, drowning many cities.

But the report also contains what passes for good news nowadays: The collapse of Antarctica is not inevitable, it says, and could be prevented with an aggressive global effort to keep greenhouse gases at or below the levels called for in Paris, where leaders embraced a goal of holding warming “well below” an increase of two degrees Celsius compared with preindustrial levels.

But the pledges made by individual countries will not come close to meeting those targets, which means a great deal more work lies ahead for all nations — particularly big emitters like China, the European Union and the United States — to avoid trigger points at which the big ice sheets will begin to melt.