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 Sunday. 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 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 human-caused warming to just 0.9 degrees Fahrenheit (a half degree Celsius) from now, instead of the globally agreed-upon goal of 1.8 degrees Fahrenheit (1 degree Celsius).

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 four inches (0.1 meters) less.
• Half as many animals with back bones and plants would lose the majority of their habitats.
• There would be substantially fewer heat waves, downpours and droughts.
• The West Antarctic ice sheet might not kick into irreversible melting.
• It may 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 Natalie Mahowald, Cornell University climate scientist and lead author on the report.

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 Ove Hoegh-Guldberg, one of the lead authors on the report and 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.

In 2010, international negotiators adopted a goal of limiting warming to two degrees Celsius (3.6 degrees Fahrenheit) since pre-industrial times. It’s called the two degree goal.

In 2015, when the nations of the world agreed to the historic Paris climate agreement, they set dual goals: two degrees Celsius and a more demanding target of 1.5 degrees Celsius from pre-industrial times. The 1.5 was at the urging of vulnerable countries that called two degrees a death sentence.

The world has already warmed one degree Celsius since pre-industrial times, so the talk is really about the difference of another half-degree Celsius or 0.9 degrees Fahrenheit from now.

“There is no definitive way to limit global temperature rise to 1.5 above pre-industrial levels,” the U.N.-requested report said.

More than 90 scientists wrote the report, which is based on more than 6,000 peer reviews.

“Global warming is likely to reach 1.5 degrees Celsius between 2030 and 2052 if it continues to increase at the current rate,” the report states.

Deep in the report, scientists say less than two percent of 529 of their calculated possible future scenarios kept warming below the 1.5 goal without the temperature going above that and somehow coming back down in the future.

The pledges nations made in the Paris agreement in 2015 are “clearly insufficient to limit warming to 1.5 in any way,” said Joerj Roeglj, one of hte study’s lead authors from Imperial College in London.

“I just don’t see the possibility of doing the one and a half” and even two degrees looks unlikely, said Gregg Marland, Appalachian State University environmental scientist who isn’t part of the U.N. panel, but has tracked global emissions for decades for the U.S. Energy Department.

He likened the report to an academic exercise wondering what would happen if a frog had wings.

Yet, report authors said they remain optimistic.

Limiting warming to the lower goal is “not impossible but will require unprecedented changes,” said U.N. panel chief Hoesung Lee in a news conference where scientists repeatedly declined to spell out just how feasible that goal is.

They said it is up to governments to decide whether those unprecedented changes are acted upon.

“We have a monumental task in front of us, but it is not impossible,” Mahowald said. “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.

Rapidly switching away from fossil fuels like coal, oil and gas to do this could be more expensive than the less ambitious goal, but it would clean the air of other pollutants. That would have the side benefit of avoiding more than 100 million premature deaths through this century, the report said.

“Climate-related risks to health, livelihoods, food security, water supply, human security and economic growth are projected to increase with global warming” the report said, adding that the world’s poor are more likely to get hit hardest.

Michael Oppenheimer, Princeton University climate scientist, said extreme weather, especially heat waves, will be deadlier if the lower goal is passed.

Meeting the tougher-to-reach goal “could result in around 420 million fewer people being frequently exposed to extreme heat waves, and about 65 million fewer people being exposed to exceptional heat waves,” the report said.

The deadly heat waves that hit India and Pakistan in 2015 will become practically yearly events if the world reaches the hotter of the two goals, the report said.

Coral and other ecosystems are also at risk.

The report said warmer water coral reefs “will largely disappear.”

The outcome will determine whether “my grandchildren would get to see beautiful coral reefs,” Oppenheimer said.

For scientists there is a bit of “wishful thinking” that the report will spur governments and people to act quickly and strongly, said Hans-Otto Portner, one of the panel’s leaders.

“If action is not taken it will take the planet into an unprecedented climate future,” he said.