

Many carbon offset schemes rely on planting new trees to counteract rising carbon dioxide levels and the climate change they cause. But new research shows that these schemes only work if trees are planted in the tropics. Plant elsewhere, and you’ll only be adding to global warming.

Plant a tree and save the planet. If it sounds too good to be true, it’s usually because it is.

Temperate forests actually keep the world warmer by absorbing solar radiation into their shaded undergrowthIt is now crystal clear that modern global warming is a man-made phenomenon. But with this acceptance comes guilt, and the quest to find ways of mitigating our energy-hungry lifestyles

On the surface, carbon offset schemes appear to offer a win-win solution. People can assuage their guilt over yet another business flight, or drive to the shops, by paying for trees to be planted or investing in renewable technology.

Trees act as carbon sinks, sucking the gas in from the air and shunting the carbon atoms across into the plants’ own molecules. So plant enough trees, and the emissions you are responsible for will effectively be negated. You can whistle a jaunty tune and slap a carbon-neutral sticker on your car.

That’s the theory anyway. But Govindasamy Bala and colleagues form the Lawrence Livermore National Laboratory have found that it’s not just what you plant that matters, it’s where.

They ran complex simulations of how the planet’s climate would change if trees in different parts of the world were removed or restored. Unexpectedly, they found that overall, deforestation cools the planet down, and adding new trees in some regions may actually fuel global warming.

Carbon offset schemes are a poor alternative to not flying at all.In a simulation where all the world’s trees were removed, the global temperature fell by about 0.3 degrees Celsius.

Why should this be? After all, trees soak up carbon dioxide and store carbon in their bodies – this keeps the planet cool. They release water vapour into the air, which forms clouds that reflect solar radiation away form the earth, again resulting in cooling.

But forests are also dark and by absorbing the energy from sunlight, they heat the planet too. According to Bala’s simulation, this heating effect outweighs the cooling ones.

When Bala looked at the effect of deforestation in specific areas, a clearer picture emerged. The tropical rainforests are doing their bit in fighting global warming by forming clouds and absorbing carbon dioxide. Their loss led to a rise in global temperature.

In contrast, the temperate and polar forests aren’t pulling their weight. These verdant slackers heat the planet themselves by absorbing solar radiation. Without them, the underlying snow would reflect more of the sun’s energy into space and we’d get a cooler planet.

Carbon offset schemes only fight global warming if trees are planted in the tropics.These experiments suggest that tree-planting will only help to restrain global warming as planned if it happens in the tropics. In other parts of the world, it could even do more harm than good. When it comes to carbon offset schemes, the devil’s in the details.

Bala and his co-workers are modest on their work and are quick to point out that it is based on a single simulation. And they are careful to quickly stem the inevitable backlash from anti-environmental groups, who may well perversely suggest that this data warrants declaring war on trees.

Forests clearly have value beyond their influence on temperature. They harbour a great richness of life, keep the soil together and stop the oceans from acidifying by storing carbon dioxide – the list goes on. Deforestation is clearly not a solution to global warming, but wanton re-forestation won’t do any good idea.

Bala’s study gives pause for thought to those of us who seek to placate our environmental consciences by paying into carbon offset schemes.

At the very least, the details of any schemes should be checked carefully. Even better, serious thought should be given to preventive measures, like reducing car or plane use, rather than cures.


Reference: Bala, Caldeira, Wickett, Phillips, Lobell, Delire & Mirin. 2007. Combined climate and carbon-cycle effects of large-scale deforestation. PNAS 104: 6550-6555.

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5 Responses

  1. Ed Hi,

    If you look closely at their research it indicates that the warming effect of trees planted in temperate and boreal zones only commences once a mature (closed) canopy is established after 40 or more years. If the trees are harvested prior to this (and ideally used as timber) and the ground replanted with saplings, the forest never develops into the dense canopy that has the described warming effect.
    We take their work seriously and are now selecting species such as ash that have thin shiny, reflective leaves to increase the albedo of our forests.
    Global models like this need to treated carefully before we make policy judgements based on them. As you say the devil is in the detail.
    Lots more on this here:

  2. Hi Ru,

    Thanks for the really interesting reply. I completely agree that this is one study which needs to be considered carefully. Which is why I’ve tried to be cautious in the article, and highlighted the fact that the authors are taking the same attitude.

    There are of course other pros and cons of carbon offset schemes and I would encourage any readers to check these out for themselves.

  3. Yes Ed,

    There are so many pros and cons regarding offsets that its hard to know what’s best. The voluntary market in carbon is still in its infancy and needs to evolve and improve in many ways. Our position is pretty simple. We want to develop ways to take advantage of the immense capabilities of trees to absorb CO2.
    Keep up the good work.

  4. The conclusions drawn from this study are completely bogus;

    For a more accurate analysis visit:

  5. Have to agree with the other posters here about this science. It is nice to use computer modeling to attempt to see into the future but I know testing by the Arbor day has shown trees moderate the climate. Certainly changing our energy usage is one of the pieces as is reforestation which is good for so many of our current ecological woes.

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