What could be wrong with planting trees? The benefits are obvious; they stabilise soils, soak up excess water, take carbon dioxide out of the atmosphere and provide a home for many plants and animals. Trees have never been more popular. The UK Forestry Commission provides grants to encourage landowners to plant trees, while giving trees has become a fashionable wedding present for the ecologically minded couple.
Well, ironically, it turns out that planting trees could add to global warming.
Tree roots do a great job of keeping soil firmly on the ground and out of the wind's clutches. A swirling dustbowl can be transformed into a leafy glade by planting a few trees. The problem is that some of those dust clouds play an important part in soaking up carbon dioxide.
Satellite photos show huge dust storms billowing out over the oceans from dry regions such as North Africa and central Asia. During stormy periods, tonnes of dust are scooped up by the wind and deposited as a thin film over the ocean surface. The supply of this dust fuels oceanic life. Dr Andy Ridgwell, an environmental scientist from the University of East Anglia, has spent the past few years studying dust, and his results are beginning to show that, sometimes, dust can be a good thing.
"Microscopic marine plants (phytoplankton) rely on the iron in dust to provide a vital part of their nutrient supply," says Ridgwell. These minute organisms are also greedy gobblers of atmospheric carbon dioxide. "They use the carbon to make their bodies and when they die, they sink to the bottom of the ocean, taking the carbon with them," explains Ridgwell.
Phytoplankton respond to changes in their dust supply. Experiments involving spreading dissolved iron solution over the Pacific ocean have shown that iron is an essential ingredient for phytoplankton survival. "Fertilising parts of the equatorial Pacific with iron caused such an increase in the number of phytoplankton that the ocean changed colour," he says. Equally, a reduction in dust supply may well diminish the number of phytoplankton.
Ridgwell decided to calculate how much carbon dioxide phytoplankton guzzled compared to trees. With the aid of a computer model, he showed that even a small reduction in dust supply to the ocean could reduce the ocean's ability to soak up carbon dioxide.
"For instance, reducing the dust supply to the world ocean by 20% lessens phytoplankton production sufficiently to decrease the uptake of atmospheric carbon dioxide by the ocean by up to 150m tonnes of carbon per year. This is equivalent to the carbon dioxide emissions from the entirety of the UK, including John Prescott's two Jags!" says Ridgwell. His calculations also showed that the extra carbon dioxide absorbed by the newly planted trees may not be enough to outweigh the loss caused by the reduction in phytoplankton.
But is any significant reduction in dust likely to happen in the future? The Chinese government has plans for a massive re-forestation programme to combat soil erosion and dust storms in the drier northern provinces of the country, particularly around the capital, Beijing. Dust from this part of Asia is carried east and much of it deposited in the Pacific Ocean. If the tree-planting programme in China is successful, and the dust supply reduced, then the net result may be that less carbon dioxide gets locked away in the ocean. Planting trees in such a sensitive area could have the bizarre effect of adding to global warming.
The findings from Ridgwell's research become all the more compelling when you consider his past as an environmental protester. "I lived in a tree for a year to try to stop it being chopped down, as part of an anti-road protest in the mid-90s," he says. "Of course I'm not trying to deforest the earth with my research. This work demonstrates the complexity of the system and the importance of not tinkering with it without fully understanding the consequences. Hence the need to focus on big cuts in carbon dioxide emissions rather than monkeying about with the land surface."
Further support that forest expansion will not help in the battle against global warming came from a paper published in Nature last week. An American academic, Professor Robert Jackson, and colleagues, have shown that when wet, native grassland areas are invaded by trees, carbon is lost from the soil. "We are studying why the soil carbon disappears, but one theory is that trees do a lot more of their growing above ground compared to grasses, so less carbon goes directly into the soil from trees," says Jackson. In wet areas of the world, the gain from trees absorbing carbon dioxide above ground seems to be outweighed by the loss of carbon from the soil below ground.
"By deliberately engineering more carbon dioxide uptake on land, we might damage the ocean sink," says Ridgwell. Countries that plan to combat global warming by planting trees may have to think again. Solutions to environmental problems are often more complex than they first appear, and understanding the Earth's climate is an enormous challenge.
The greenest wedding present may be a dusty field rather than a leafy woodland.
