A warmer sun, combined with other natural factors, could increase global temperatures over the next five years by more than scientists previously predicted, according to new research.

The U.S. study assesses the impact of four natural factors on climate change: man-made CO2 emissions; heating from the sun; volcanic activity and a phenomenon called El Nino which takes place in the Pacific Ocean every few years and causes global temperatures to warm.

The hottest year on record so far is 1998. Then, a very active sun was just one of the reasons temperatures across the world soared.

However, the relatively cool years since have prompted some to suggest global warming is not happening.

The new study, carried out by Judith Lean, of the U.S. Naval Research Laboratory, and David Rind, of Nasa's Goddard Institute for Space Studies, says the opposite may be true.

Gareth Jones from the Met Office told Sky News Online: "Over the next five years, different factors will affect our climate.

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