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Carbon Replacement Forestry

Can we plant more trees as an alternative to reducing the consumption of fossil fuels? Not according to the UN's Intergovernmental Panel on Climate Change (IPCC). They believe it won't work. This article explains why. The idea has also been debunked elsewhere. We have run an article on those who benefit from promoting this idea.(Trading CO2 Emissions for Trees: Who Benefits?)

That Sinking Feeling
by Fred Pearce,

It sounded like a good idea, but planting trees to absorb CO2 is no substitute for cutting fossil fuel emissions

NOT FOR THE FIRST TIME, the human race may be about to take a dangerous short cut. Next week, governments from around the world will meet in Bonn, the former German capital, to complete plans for creating forests to soak up the greenhouse gas carbon dioxide. They see planting trees as a partial alternative to cutting emissions of the gas from power stations and vehicle exhausts.

Unfortunately, just two weeks ago, the UN's Intergovernmental Panel on Climate Change (IPCC) put the finishing touches to a report that shows this strategy to be based on a dangerous delusion. In reality, say its scientists, planned new forests, called "carbon sinks", will swiftly become saturated with carbon and begin returning most of their carbon to the atmosphere, temporarily accelerating global warming. Peter Cox of the Hadley Centre, part of  Britain's Meteorological Office, shares the UN panel's conclusions. "This is not something that may or may not happen as the world warms--it is more or less inevitable," he says.

The result will be no overall reduction in CO2 levels. Despite this, the US and other major CO2 producers will cite the "sink" process as justification for their continuing tardiness in cutting CO2 emissions.

The US Environmental Protection Agency would not comment when approached by New Scientist. But other governments are concerned. Britain's environment department says: "The UK emphasises that the main action should be reducing actual emissions. Sinks are much less secure than carbon in fossil fuels left unburnt."

The discovery that forests are not a panacea for global warming only emerged after they were given a central role in the Kyoto Protocol, the treaty signed two years ago by most of the world's governments in a bid to stem the greenhouse effect. "Just a couple of years ago, the issue of sink saturation was barely known," says Will Steffen of Sweden's Royal Academy of Sciences, who chairs the International Geosphere-Biosphere Programme, which has pioneered research into the global carbon cycle. The first public warning came in the March issue of the IGBP newsletter. And this month, the IPCC incorporated its analysis into a forthcoming report on land use change and forestry.

Each year, CO2 emissions from human activity pour just over 6 billion tonnes of carbon into the atmosphere. Around a third is absorbed by the world's forests. The discovery of this large carbon sink encouraged policy makers to believe that CO2 pollution could be cut by planting more trees. But now it seems the sink is a recent phenomenon, and a temporary one. In fact, the suggestion that planting trees means less atmospheric CO2 ignores simple logic.

Before the large-scale development of industry, mature forests were in equilibrium with the atmosphere. Photosynthesis, the process that creates plant matter, absorbs CO2 from the air. But trees also release CO2 back into the air when plant matter breaks down the sugars they make during photosynthesis. This process is called respiration. Much the same happens in forest soils, which absorb carbon from trees and release CO2 as microorganisms break down plant matter. This equilibrium has been increasingly upset by the higher concentrations of CO2 in the atmosphere from burning fossil fuels. Usually, the low level of CO2 is what limits photosynthesis. Higher CO2 levels promote "CO2 fertilisation", accelerating both forest growth and the accumulation of carbon in forest soils. And as the forests grow faster, they absorb more CO2, helping to stave off climate change--for a while. 

Until a few months ago, researchers had assumed that as long as CO2 levels in the air went on rising, the forest sink would continue to grow. The IPCC's last assessment, published in 1996, concluded that forests would soak up around 290 billion t nnes of carbon next century, even without extra planting. But this now seems highly unlikely. Experts such as Bob Scholes of the South African government's research agency, CSIR, argue that CO2 fertilisation may already have peaked and that respiration may be about to accelerate. Early in the next century, forests planted to protect the planet from global warming could be contributing to it.

How did researchers get it so wrong? Scholes, a leading light in the IGBP's Global Carbon Project, says that the confusion was caused by a time-lag. CO2 fertilisation is an instantaneous process. But respiration increases in response to temperature rises triggered by the CO2. That warming has a built-in delay of about fifty years, caused largely by the thermal inertia of the oceans. So the extra outpouring of CO2 from the world's forests would not yet be apparent. "During this delay there is an apparent carbon sink," he says.

At the Hadley Centre, Cox has just finished modelling the likely future carbon cycle. He warns that we are on a "saturation curve", where extra CO2 has an ever-smaller effect on plant growth. Respiration, on the other hand, continues to increase with temperature. Soil respiration in particular goes up exponentially with temperature, at least for a time, says Cox. So if CO2 levels in the air continue to rise, fertilisation rates will flatten out while respiration rates soar. He predicts that by 2050, forests will have released much of what they have absorbed. The overall reduction in CO2 levels will therefore have been small.

"The timing is uncertain but we are pretty certain it will happen," he says. Wolfgang Cramer of the Potsdam Institute in Germany has recently reached a similar conclusion. Neither study is yet published.

The effect of accelerated respiration on the atmosphere could be even more dangerous if, as predicted by some scientists, the heat and drought caused by global warming degrade tropical forests at the same time.

This isn't to say planting trees is in itself a bad thing. Whether they are absorbing or releasing the gas, they will always be keeping some CO2 out of the atmosphere and providing other ecological benefits. But forests are an insecure way of storing carbon out of harm's way," says Steffen.

The real danger, he says, arises when countries use plans to plant forests as a justification for not cutting their CO2 emissions from burning fossil fuels. And next week, the politicians and climate negotiators meeting in Bonn will be doing just that. They are meeting to agree rules for implementing the 1997 Kyoto Protocol, which give the planting of forest sinks equal value to emissions cuts as a way to meeting its targets.   Many countries, including the US, which produces about a quarter of the world's CO2 emissions, are relying to a large degree on the supposed benefits of tree planting to meet their targets. And dozens of forestry companies are already planning to join the anticipated global market in certificated carbon sinks. Typical is a new project, announced by a Norwegian forestry company last month, to plant fast-growing pine and eucalyptus trees on 150 square kilometres of grassy plain in southwest Tanzania. The company, called Treefarms, promises that by 2010 the Kilombero Forest will store more than a million tonnes of carbon. But will it?

Such claims are based on models of CO2 accumulation that assume current rates of CO2 fertilisation will continue. But Scholes believes the carbon sink will start to decline within the next few decades. This would make the certificates for carbon stored in forests such as Kilombero worthless.

Ultimately, says Steffen, we will only save the world from catastrophic climate change by cutting emissions. "New forests are temporary reservoirs that can buy valuable time to reduce industrial emissions, not permanent offsets to these emissions." But the Kyoto Protocol does not reflect that--and nor will next week's negotiations. It could prove a devastating mistake.  "The carbon cycle has a very long equilibrium time," says Scholes. "The consequences of actions taken now will persist for many centuries.

Source: Fred Pearce, New Scientist Magazine


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