They say this is because these "carbon sinks" are themselves the product of temporary changes in land use.

They believe the entire land-based carbon sink could ultimately disappear.

The findings are important for a world keen to find natural methods of absorbing CO2.

The study, reported in the journal Nature, is especially relevant for the meeting in the Moroccan city of Marrakech of signatories to the Kyoto Protocol, the international treaty on tackling climate change.

Government ministers have taken charge of the negotiations at the meeting, of about 160 countries, which is due to end on 9 November with agreement on the protocol's detailed working.

Recent sink

Sinks soak up some of the CO2 emitted into the atmosphere by human activities (many climatologists believe CO2 and other gases are intensifying the climate's natural variability).

But exactly how much carbon they absorb is unknown. Scientists believe the land and the oceans together absorb about half the CO2 given off by the burning of fossil fuels.

The 30 authors of the Nature report say: "Atmospheric CO2 and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s.

"This recent sink can be largely attributed to northern extra-tropical areas, and is roughly split between America and Eurasia.

"Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation."

In North America, China and Europe, the authors say, the key factors were probably the regrowth of forests, often after farmland was abandoned in the 1980s and 1990s. A reduction in the frequency of fires also contributed.

Regional differences

Other factors probably include changes in foliage, plant litter and soil microbes. These in turn are affected by changes in photosynthesis, respiration, fire and insect outbreaks, influenced by climate fluctuations such as El Nino.

Growing trees absorb net quantities of CO2, and the higher levels of CO2 and nitrogen in the atmosphere are themselves stimulating tree and plant growth.

But the researchers expect these effects to reach saturation point and cease to have an effect.

They found big regional variations in the strength of sinks. Much of Siberia, for example, has warmed by about 0.5 degrees Celsius a decade since the 1960s.

An increase in wildfires and insect damage appears to have changed it from a sink into a temporary source of CO2.

In a possible pointer to future changes, the authors say: "Globally, there appears to be a net release of carbon to the atmosphere during warm and dry years, and a net uptake during cooler years."

The lead author is Professor David Schimel, of the Max Planck Institute for Biogeochemistry in Jena, Germany.

He said: "Although carbon sinks have a role to play in absorbing excess CO2, it is possible that the net global terrestrial carbon sink may disappear altogether in the future."