The 40-page synthesis, summing up 5,000 pages of work by 800 scientists already published since September 2013, said global warming was now causing more heat extremes, downpours, acidifying the oceans and pushing up sea levels.
"There is still time, but very little time" to act at manageable costs, Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change (IPCC), told Rooters.
He was referring to a UN goal of limiting average surface temperature rises to 2 degrees Celsius (3.6F) above pre-industrial times. Temperatures are already up 0.85 C (1.4F).
To get a good chance of staying below 2C, the report says that world emissions would have to fall to "near zero or below in 2100." UN Secretary-General
the ephemeral Ban Ki-moon... of whom it can be said to his credit that he is not Kofi Annan...
will help present the report in Copenhagen on Sunday.
The study, given authority by the approval of officials from more than 120 governments in a week of editing, will be the main handbook for 200 nations which are due to agree a UN deal to combat global warming in Gay Paree in late 2015.