The U.N. weather office says world temperatures maintained a long-term upward trend and Arctic sea ice shrank to record low volumes this year.
The report by the International Meteorological Organization, released in Geneva and at the U.N. climate talks on Tuesday, provided a bleak backdrop to negotiators seeking ways to limit pollution blamed for global warming.
The report says 2011 was tied for the 10th hottest year since records began in 1850.
It says the 13 hottest years on the books all occurred in the last 15 years.
The extent of Arctic sea ice in 2011 was the second lowest on record, and its volume was the lowest.
The report came on the second day of the two-week conference in South Africa among 192 parties seeking agreement on future action to curb climate change.