OK, so maybe they aren't quite that black-and-white about it, but scientists whose work is described in today's issue of Nature suggest that trying to plant our way out of global warming is going to have some so-far-underappreciated side effects.

Planting trees to absorb carbon and reduce the threat of climate change could cause a range of new environmental problems, researchers warn.

In a global study published today (23 December) in Science, the researchers say tree plantations can dramatically reduce water availability, remove nutrients from soil and increase its salinity. They say better planning is needed to assess the environmental costs and benefits of planting trees to mitigate climate change.

"The purpose of our paper isn't to suggest that plantations are bad," says lead author Robert Jackson of Duke University, United States.

"Rather, we are saying it is important to think carefully about where and what people plant, particularly where water resources are scarce."

Climate change is partly caused by increased concentrations of the gas carbon dioxide in the atmosphere. Planting trees can help limit climate change because, as trees grow, they absorb large amounts of carbon from the atmosphere, and store it in their tissues. Tree planting is set to increase in developing countries as 'carbon trading' initiatives such as the Clean Development Mechanism of the Kyoto Protocol get underway. But young trees use up considerably more water than crops or pastures. This can decrease the flow of water in nearby streams or dry them out completely. By absorbing more nutrients than other types of vegetation, trees can also change the chemical make-up of soil.

"They take up 'good' nutrients, such as nitrogen, calcium, and potassium, and leave behind others, such as sodium, that can increase salinity," explains Jackson. Trees can also deplete shallow underground supplies of freshwater, drawing up deeper salty water and potentially affecting local drinking water supplies, as has happened in parts of Argentina. But Jackson points out that tree plantations have also benefited regions such as Africa's Sahel, where trees can improve water quality.

Plantations also help reduce erosion and nutrient and pesticide runoff when croplands are replaced with trees. He adds that wetter regions in developing countries may be the best place for new plantations.