The assessment of the global "cryosphere" -- parts of the Earth covered in ice and snow for at least some of the year -- urges upcoming climate talks to commit to keeping warming below 1.5C.
"Because of what we have learned about the cryosphere since the Paris Agreement was signed in 2015, 1.5C is not merely preferable to 2C. It is the only option," the report argues.
The Paris Agreement called for warming to be held below 2C, but the United Nations this week said global climate commitments are vastly off-track for that target.
Thursday's report from the non-profit International Cryosphere Climate Initiative says recent research shows only dramatic emissions reductions can prevent irreversible consequences for areas from the Arctic to the Himalayas.
At 2C warming, they warn, melting ice sheets will cause "extensive, potentially rapid, irreversible sea-level rise".
The world will also see major ice loss from the world's glaciers, "with some disappearing entirely".
Sea ice may be the worst affected part of the cryosphere, it added, with the Arctic Ocean becoming ice-free every summer.
Open Arctic waters would absorb more heat from the 24-hour summer sun, in turn speeding the thawing of permafrost and the melting of the Greenland ice sheet, raising sea levels.
Thawing permafrost presents another dire risk because it releases carbon dioxide and methane, emissions that help boost temperatures.
At 2C warming, "these emissions are irreversibly set in motion and will not slow for one-to-two centuries", the report warns.
The Earth's polar seas would not be spared either.
The carbon they would absorb at 2C warming would cause "year-round, essentially permanent corrosive ocean acidification conditions", the report says.
This would endanger shell-building marine life, and all the food chains that depend on them, including cod, salmon and lobsters.
The report's findings, which have been reviewed by dozens of scientists internationally, come days after the UN warned the world is "failing to get a grip on the climate crisis".
Current national climate plans from nearly 200 countries would put 2030 carbon emissions just two percent below 2019 levels.
That is nowhere near the 43 percent reduction needed to keep the 1.5C target in reach, the UN said.
Evidence of the effect of climate change on the cryosphere is already clear, the report's authors warn.
They cite everything from the glacial lake burst in India last month, which killed at least 70 people, to all-time lows for sea ice around Antarctica.
"None of these tragic events surprised us," a foreword from seven scientists said.
"This continued rise in carbon dioxide is unacceptable. The melting point of ice pays no attention to rhetoric, only to our actions."
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