PARIS: Old-growth forests remove carbon dioxide from the atmosphere, helping to curb the greenhouse gases that drive global warming, according to a study to be published tomorrow.

Many environmental policies are based on the assumption that only younger forests, mainly in the tropics, absorb significantly more Co2 than they release.

Partly as a result, primary forests in temperate and subarctic regions of the northern hemisphere are not protected by international treaties, and do not figure in climate change negotiations seeking ways to reward countries that protect carbon-absorbing woodlands within their borders.

Some 30 per cent of global forest area -- half old-growth -- is unmanaged primary forest.

"Old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral," lead researcher Sebastiaan Luyssaert, a professor at the University of Antwerp in Belgium, told AFP.

An international team led by Luyssaert analysed scores of databases set up to monitor the flow of carbon into and out of the world's vegetal ecosystems.

They calculated that primary forests in Canada, Russia and Alaska alone absorb about 1.3 gigatonnes of carbon per year, about ten per cent of the net global carbon exchange between the ecosystem and the atmosphere.

These forests need to be protected not just because they help to absorb carbon dioxide, but also because destroying them could release huge stores of greenhouse gases.

"Old-growth forests accumulate carbon for centuries and contain large quantities of it," Luyssaert said. If these pools of CO2 "are disturbed, much of this CO2 will move back into the atmosphere," he added.