Climate change could be worse than models have predicted as Australian researchers have found forests are already at the limit of the amount of carbon they can extract from the air.
In the study, published by 'Nature', scientists from Western Sydney University pumped carbon from a commercial source into the air above a forest of 90-year-old trees.
For four years they kept carbon levels 38pc higher than normal while they tracked the movement of carbon through the ecosystem.
They found that while mature trees could consume an additional 12pc of carbon at elevated levels, it was not retained, with the majority of extra carbon emitted back into the atmosphere.
"Global calculations assume mature forests will store extra CO2 as concentrations go up, but our results are implying that mature forests can't keep doing that into the future," said Professor Belinda Medlyn, who led the study.
The poor quality of the soil in which the trees grow was a key factor in their failure to store more carbon, Prof Medlyn said.
She added: "The soil doesn't have a lot of nutrients in it. It seems what they've done when they've been given extra carbon is just to use that to go looking for extra nutrients."
This increases soil respiration, which accounts for the majority of the carbon being sent back into the atmosphere.
While studies show younger trees are proficient at storing extra carbon, models based on the assumption that mature trees will do the same could be undermined.
Prof Medlyn said older trees do store a lot of carbon and should be protected but "they won't keep on taking up more carbon in the future". (© Daily Telegraph, London)