* Warming may have "irreversible impact"
* U.S. tornado season one of the worst on record
* Arctic sea ice volume at record low (Adds background paras 5-11)
By Emma Farge
GENEVA, March 23 (Reuters) - Human activity kept global temperatures close to a record high in 2011 despite the cooling influence of a powerful La Nina weather pattern, the World Meteorological Organization said on Friday.
On average, global temperatures in 2011 were lower than the record level hit the previous year but were still 0.40 degrees Centigrade above the 1961-1990 average and the 11th highest on record, the report said.
WMO Secretary-General Michel Jarraud warned that the consequences of global warming could be permanent. "The world is warming because of human activities and this is resulting in far-reaching and potentially irreversible impact on our Earth, atmosphere and oceans," he said.
La Nina, a natural weather phenomenon linked to heavy rains and flooding in the Asia-Pacific and South America and drought in Africa, was one of the strongest in the past 60 years and stayed active in the tropical Pacific until May 2011.
Further signs of man's contribution to climate change will put the biggest polluters in the spotlight after they agreed for the first time last December at U.N. climate talks in Durban to limit greenhouse gas emissions.
Critics of the agreement have said that the plan was too timid to slow global warming.
The WMO, part of the United Nations, said that elevated temperatures had contributed to extreme weather conditions such as intense droughts and flooding and droughts in east Africa and North America.
Global tropical cyclone activity was below average last year but the United States had one of the most destructive tornado seasons on record, the report said.
Another impact of high temperatures was shrinking Arctic sea ice and its extent was the second-lowest minimum on record and the volume was the lowest.
The WMO also said that the decade between 2001-1010 was the warmest ever recorded across all of the world's continents ahead of the release of its 'Decadal Global Climate Summary'.
2010 tied for the warmest year since data started in 1880, capping a decade of record high temperatures that shows mankind's greenhouse gas emissions are heating the planet, two U.S. agencies said. (Reporting by Emma Farge; Additional reporting by Nina Chestney in London; Editing by Susan Fenton)