Climate change may have "serious, pervasive and irreversible" impacts on human society and nature, according to a draft U.N. report due for approval this week that says governments still have time to avert the worst.
Delegates from more than 100 governments and top scientists meet in Copenhagen on October 27-31 to edit the report, meant as the main guide for nations working on a U.N. deal to fight climate change at a summit in Paris in late 2015.
They will publish the study on November 2.
European Union leaders on Friday agreed to cut emissions by 40 percent below 1990 levels by 2030, in a shift from fossil fuels towards renewable energies, and urged other major emitters led by China and the United States to follow.
In a paragraph summing up the risks, the draft says that a continued rise in world greenhouse gas emissions is "increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems."
It adds that "a combination of adaptation and substantial, sustained reductions in greenhouse gas emissions can limit climate change risks."
Past reports have warned that warming could, for instance, trigger impacts irreversible on human time scales such as a runaway meltdown of Greenland's ice that would raise sea levels and swamp coasts from Florida to Bangladesh.
The IPCC says that it is at least 95 percent certain that human activities, led by the burning of fossil fuels, are the main cause of climate change since 1950, up from 90 percent in the previous assessment in 2007 and 66 percent in 2002.
Opinion polls indicate many people, especially in the United States, are unconvinced and suspect that natural variations in climate are to blame. That gap between public and scientific opinion is a big complication for work on the Paris accord.