The search for the world's missing carbon "sinks" is leading to findings that "go against every ecologist's text book", a conference was told last week.

Paul Curran, professor of geography at Southampton University, said scientists working for the Tiger (Terrestrial Initiative in Global Environmental Research) programme have discovered that mature trees in the tropical rainforest, which were believed not to absorb carbon because they are no longer growing, may in fact be absorbing a very large amount. The quantity may even account for more than half the missing carbon sink scientists are searching for.

Models of the global carbon cycle describe factors such as carbon emission via the burning of fossil fuels and carbon absorption by the oceans. But they fail to account for more than two billion tonnes of carbon each year. These are absorbed by sinks that have not yet been discovered.

Scientists from Edinburgh University, working on a section of the Tiger programme, have been measuring carbon flux from towers erected in rainforests of differing ages. Some towers were placed in mature rainforests, which are assumed to give out as much carbon as they absorb, in order to calibrate the measurements.

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But the scientists, led by John Grace, discovered that the trees were absorbing large amounts of carbondioxide. This may account for half the missing carbon.

Tiger's work on measuring the ages of the forests was described at a conference held by the Royal Geographical Society last week. Professor Curran, leader of another arm of the carbon cycle project, has been using remote sensing pictures of the earth to measure vegetation growth. Using pictures from a satellite instrument called the AVHRR he can estimate the age of regenerating tropical forests and make more precise assessments of their age using archives from other satellites, stretching back 22 years. This will show when a forest was cut down.

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Researchers then correlate information on the age of vegetation with the amount of carbon it absorbs. "The discovery goes against every ecologist's text book -- people believe that mature tropical forests are not growing," Professor Curran told the conference. "Yet a number of people have recorded it."