The findings in the new report are sobering enough -- that the world's scientists agree global warming is "unequivocal" and irreversible: Manmade greenhouse gases are shooting up -- driving the rise in earth's temperature and sea level, and the decline in earth's snow cover.
But there's a massive unknown worrying the scientists: Sea levels could rise in the coming decades faster than anyone thought.
Ominous news in the fourth unanimous assessment in 20 years by the IPCC [Intergovernmental Panel on Climate Change], just finalized at a plenary session in Paris, sets the stage: The human-induced warming has now reached down more than a mile to normally frigid deep ocean currents -- currents that for millions of years have acted as a massive cooling system for the planet.
Water expands as it heats up, so scientists can now calculate that sea level will rise up between seven inches and nearly two feet before the end of the century.
But there's also an enormous wild card: It's the Greenland Ice Sheet, two miles thick at its center and containing enough ice to raise the world's oceans 23 feet.
It's melting so fast lately that the scientists in Paris couldn't settle on any predictions for it.
"We don't know what the likelihood is that part of those ice sheets might suddenly destabilize," climate scientist Richard Somerville, standing not far from the Eiffel Tower, told ABC News.
Somerville, one of the grandfathers of accurate global warming prediction, had just emerged from a week of intense IPCC consensus-seeking among scientists and public officials from 113 nations.
He described the slow, inclusive and painstaking IPCC assessment process as unprecedented in the history of the world's science, and "the gold standard" for climate prediction.
Back in the United States, NASA-Goddard glaciologist Waleed Abdalati, who has been watching Greenland for years, confirms the possibility that Greenland could well hold unpleasant surprises.
"I would bet that the rate at which Greenland contributes to the rising sea is going to increase in the near future," he told ABC News.
He and his glaciologist colleagues have never seen anything like what's been happening in Greenland.
"We've seen widespread glacier acceleration in many parts of the ice sheet," he told us. "It doesn't take that much to have significant impact on coastal regions. They're flat and low."
It's not just a future threat.
Even without an accelerating collapse of Greenland's ice sheet, there is already a humanitarian and ecological catastrophe bearing down on coasts and island nations worldwide.
Inhabitants in exotic and beautiful low-lying atolls and reef-based islands -- many millions of people all told -- are expected to become refugees, with no homes to go back to, in the next few decades.
At the United Nations, experts in humanitarian aid and assistance are focusing on this imminent upheaval that most people in the world have given little attention -- but which is all to real too those eyeing the fast creeping waterlines.
Indonesia's environment minister has now announced that scientific studies estimate about 2,000 of the country's lush tropical islands could disappear by 2030 -- just over 20 years from now.
And that's just based on the sea level rise scientists can calculate.
Even as scientists and engineers around the globe scramble to figure out ways to mitigate global warming (to stop it from reaching much above the additional two degrees Fahrenheit scientists say Earth will experience by about 2050) they are also trying to get the word out to people everywhere that humanity must at the same time learn to adapt to the irreversible changes it is now too late to prevent.
"Everybody is going to have to learn to adjust and adapt to climate change because it's going to be warmer no matter what," Princeton University climate scientist Michael Oppenheimer told ABC News.
"But at the same time," he said, "we have to combine that with a program for sharply reducing emissions, because adaptation alone can never be enough."
The scientists in Paris did agree -- unanimously -- that that even if humanity soon makes drastic cuts in greenhouse emissions, sea level -- partly because of long slow delays in its global circulation of heat -- will keep rising for centuries.
What humanity may be able to control, they say, is how much and how fast.