Hopes that planting trees could absorb significant amounts of carbon dioxide (CO2) and so help to slow down global warming look set to be dashed.

As delegates prepare to head for the German city of Bonn for the next round of climate negotiations starting on 25 October, scientists working for the UN's Intergovernmental Panel on Climate Change (IPCC) say tree-planting will simply buy the world a little time.

Their research effectively scuppers the hopes of countries like the US, the biggest emitter of greenhouse gases, that they can plant their way out of trouble by creating new forests to soak up their CO2.

The IPCC says these planned forests, called "carbon sinks", will soon become saturated with carbon and start returning most of it to the atmosphere.

'Saturation curve'

This will have the effect of temporarily accelerating global warming, not slowing it down.

Peter Cox, of the UK's Hadley Centre for Climate Prediction and Research, says the IPCC's conclusions are correct, and we are on a "saturation curve".

"This is not something that may or may not happen as the world warms. It is more or less inevitable."

Will Steffen, of Sweden's Royal Academy of Sciences, says the problem of sink saturation was barely known even a couple of years ago, when the forests were assigned an important role under the Kyoto Protocol, the international agreement on tackling climate change.

"New forests are temporary reservoirs that can buy valuable time to reduce industrial emissions, not permanent offsets to these emissions."

False optimism

Annual CO2 emissions from human activity add just over six billion tonnes of carbon to the atmosphere, of which about a third is absorbed by the world's forests.

Governments thought this meant that more trees would simply mean more carbon absorption, leaving them more leeway to carry on polluting.

Absorption is one thing, respiration quite another

But it now seems the carbon sinks are a recent and temporary phenomenon.

The problem is that although trees absorb CO2 during photosynthesis, they also release it back into the air when plant matter breaks down the sugars they have made. This process is called respiration.

Crucially, respiration increases in response to temperature rises, which are triggered by the rising levels of CO2. Many scientists believe that respiration may be about to accelerate, turning the forests from sinks to sources of carbon.

Long-term consequences

They failed to recognise that this could happen because, although CO2 take-up is instantaneous, the warming that triggers respiration has a built-in delay of about 50 years, mainly because of the oceans' thermal inertia.

So planting more trees could soon prove a quick way of speeding up climate change, not of moderating it.

Bob Scholes, of the South African Government's research agency, CSIR, says it could be a costly mistake.

"The carbon cycle has a very long equilibrium time. The consequences of actions taken now will persist for many centuries."

The new research is published in New Scientist magazine.