As sea levels rise, U.N. scientists issue a stark warning.

Preventing an extra single degree of heat could make a life-or-death difference in the next few decades for multitudes of people and ecosystems on this fast-warming planet, an international panel of scientists reported. But they provide little hope the world will rise to the challenge.

The Nobel Prize-winning Intergovernmental Panel on Climate Change issued its gloomy report Sunday at a meeting in Incheon, South Korea.

In the 728-page document, the U.N. organization detailed how Earth’s weather, health and ecosystems would be in better shape if the world’s leaders could somehow limit future humancaused warming to just 0.9 degrees Fahrenheit from now, instead of the globally agreed-upon goal of 1.8 degrees F. Among other things:

Half as many people would suffer from lack of water.

There would be fewer deaths and illnesses from heat, smog and infectious diseases.

Seas would rise nearly 4 inches less.

There would be substantially fewer heat waves, downpours and droughts.

The West Antarctic ice sheet might not kick into irreversible melting.

And it just may be enough to save most of the world’s coral reefs from dying.

“For some people, this is a life-or-death situation without a doubt,” said Cornell University climate scientist Natalie Mahowald, a lead author of the report. More than 90 scientists contributed to the report, which is based on more than 6,000 peer reviews.

Limiting warming to 0.9 degrees from now means the world can keep “a semblance” of the ecosystems we have. Adding another 0.9 degrees on top of that — the looser global goal — essentially means a different and more challenging Earth for people and species, said another of the report’s lead authors, Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, Australia.

But meeting the more ambitious goal of slightly less warming would require immediate, draconian cuts in emissions of heat-trapping gases and dramatic changes in the energy field. While the U.N. panel says technically that’s possible, it saw little chance of the needed adjustments happening.

Limiting warming to the lower goal is “not impossible but will require unprecedented changes,” U.N. panel chief Hoesung Lee said in a news conference.

“We have a monumental task in front of us, but it is not impossible,” Mahowald said earlier. “This is our chance to decide what the world is going to look like.”

To limit warming to the lower temperature goal, the world needs “rapid and far-reaching” changes in energy systems, land use, city and industrial design, transportation and building use, the report said. Annual carbon dioxide pollution levels that are still rising now would have to drop by about half by 2030 and then be near zero by 2050. Emissions of other greenhouse gases, such as methane, also will have to drop.