— Everybody knows that trees are good for the climate, because they absorb greenhouse gas carbon dioxide, right? Maybe not, says a team of US and French climate experts. They say that whether or not trees help fight global warming depends on where they are.
If they are in the tropics, then investing in an "adopt a tree" scheme may be worthwhile. "Tropical forests have a net cooling effect because they take up carbon and increase cloudiness, which helps cool the planet," explains Govindasamy Bala of Lawrence Livermore National Laboratory, US.
Bala and his colleagues used computer models to predict what global temperatures would be in 2100 if all forests had been removed from the planet in 2000. Bala says their study differs from previous ones because their models do not only include the carbon-storing effects of trees. They also account for the release of water vapour into the atmosphere by trees, which promotes cloud formation, and the extra heat absorbed from the Sun by foliage, which is usually darker than the ground it grows in.
The tropics receive more sunlight, so they have more energy to convert atmospheric carbon dioxide into biomass. That is why tropical forests are better at storing carbon than their counterparts at mid-to-high latitudes.
At these latitudes, the warming effect of their dark colour outweighs the cooling effect of the CO2 they store. Bala's team say trees there can actually contribute to warming the planet.
The climate warming due to the absorption of heat by leaves more than offsets the cooling effect from carbon uptake, says Bala. The team found that if they removed forests from the planet in 2000, global temperatures in 2100 were 0.2°C cooler than if forests were left intact.
"Forests are, of course, very valuable in many other respects: they provide timber and preserve ecosystems and biodiversity," Bala adds. The findings were presented at the American Geophysical Society annual meeting in San Francisco, California, US, on 15 December.
The bottom line is that planting forests in boreal regions may not be the solution to climate change, says Christine Delire, at the Université Montpellier II, France, who worked with Bala.
She points out that the shrubs in the Arctic are already moving further north because of warming climates. If trees did the same, it would increase warming, she says, if the model is accurate.
So what of Wangari Maathai's celebrated billion tree campaign? Maathai, who won the 2004 Nobel Peace Prize, launched her programme with the UN Environment Programme earlier in 2006. The objective is to plant at least one billion trees worldwide during 2007. (Read our interview with Wangari Maathai.)
"The value of our study is that it provides guidance on where the trees should be planted if the main purpose is to slow down global warming," says Bala.
UNEP seem aware of this. At the launch of the Billion Tree Campaign, Dennis Garrity, director general of the World Agrofrestry Centre said: "Planting trees is great, although using appropriate scientific knowledge to plant the right tree in the right place is even greater.