Last year tied with 2005 as the warmest on record, according to US agencies, but is likely to be overtaken by the next year with a strong El Nino weather event, experts said.

A gradual build-up of greenhouse gases from human activities is heating the planet but natural events such as El Nino, which every few years warms the surface of the eastern and central Pacific Ocean, can have a far bigger immediate impact. 

'It will take an El Nino year to break the record, so possibly the next one,' said professor Phil Jones of the Climatic Research Unit at the University of East Anglia in Britain.

On Wednesday, the US National Climatic Data Center (NCDC) and Nasa's Goddard Institute for Space Studies said 2010 tied with 2005 as the warmest year since reliable data started in 1880, capping a decade of record temperatures.    

Last year started with an El Nino, as did 2005 and 1998 which is rated the warmest year by the UN's World Meteorological Organization (WMO).    

The WMO is likely to give a view of 2010's ranking in late January, after compiling temperature data that is also due from Jones' unit alongside NCDC and NASA. El Nino can disrupt world weather, with effects on everything from food to energy prices.
   
Knut Alfsen, research director at the Center for International Climate and Environmental Research in Oslo, said  greenhouse gases from human activities caused the Earth to absorb more energy than it radiated into space.

'Most of that energy goes into the oceans -- so a record depends on the behaviour of the oceans, typically an El Nino or La Nina event,' he said.

A current La Nina, widely seen as a factor causing floods in Australia that have killed at least 19 people, is a natural cooling of the Pacific that mirrors El Nino.

Other natural events that affect temperatures include variations in the sun's output, shifts in ocean currents or big volcanic eruptions such as that of Mount Pinatubo in the Philippines in 1991, whose ash dimmed sunlight worldwide.   

Jones said that warming from 1979 to 2010 had been 0.51 degree Celsius (0.9 Fahrenheit), or 0.016 C a year. But annual natural swings were far bigger -- in the largest 20th century shift, 1977 was 0.29 degree C warmer than 1976.  - Reuters