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

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.