LONDON, Feb 18 (Reuters) - Tropical trees have grown bigger over the past 40 years and now absorb 20 percent of fossil fuel emissions from the atmosphere, highlighting the need to preserve threatened forests, British researchers said on Wednesday.

Using data collected from nearly 250,000 trees in the world's tropical forests over the past 40 years, their study found that tropical forests across the world remove 4.8 billion tonnes of carbon dioxide emissions each year.

"To get an idea of the value of the sink, the removal of nearly 5 billion tonnes of carbon dioxide from the atmosphere by intact tropical forests, based on realistic prices for a tonne of carbon, should be valued at around £13 billion per year," said Lee White, Gabon's chief climate change scientist, who co-led the study, said in a statement.

The researchers do not know exactly why trees are getting bigger and mopping up more carbon but they suspect that extra carbon dioxide in the atmosphere may be acting like a fertiliser.

While nature has provided a free subsidy for dealing with carbon emissions, it is one that won't last forever because trees can only grow so much bigger, said Simon Lewis, an ecologist at the University of Leeds who led the study.

"The trees are growing just a bit bigger but they make a big difference because there are so many trees and half their mass is carbon," Lewis said in a telephone interview.

"Our study gives us another reason why it is really important to conserve tropical rain forests."

The U.N. Intergovernmental Panel on Climate Change estimates that human activity produces 32 billion tonnes of carbon dioxide worldwide each year, but only about 15 billion tonnes actually stays in the atmosphere and affects climate change.

Human-produced greenhouse gases are blamed for warming temperatures, which experts say will spark heat waves, droughts, more powerful storms, species extinctions and higher sea levels.

Knowing what exactly what happens to the carbon dioxide pumped into the atmosphere will help researchers better understand future climate change, Lewis added.

The team analysed data on 250,000 tree records collected from the world's tropical forests over a 40-year period and found that the total mass of trees -- which is mostly in their trunks -- was getting bigger on average.

As a result, tropical forests absorb more carbon emissions and now make up about half the world's land carbon sink, the researchers said in the journal Nature.

"This is all about what is happening with the trees but we still don't know what is happening with the soils," said Lewis, who noted that oceans absorb about 8 billion tonnes of carbon dioxide each year.

(Reporting by Michael Kahn; Editing by Will Dunham and Richard Williams)

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