Feel bad

There are skeptics about the whole carbon offset deal, according to this 2005 article (again proving that I am not trendy, because I hadn't even heard of carbon offsets until last week).

Brides and grooms do it. Transatlantic travellers do it. And you might even be getting it for Christmas. Neutralising your carbon emissions is becoming the must-do activity for the eco-conscious citizen. But now an international team of scientists has raised an unexpected objection: some tree-planting projects may, they suggest, be doing more harm than good....

Researchers have found that planting trees to soak up carbon can have detrimental knock on effects. "I believe we haven't thought through the consequences of this," says team-member Robert Jackson at Duke University in North Carolina, "I think the policy could backfire on us, but it will take decades to play out."

His team pooled more than 500 separate yearly observations from studies from five continents which compared planted areas with plots nearby that did not have trees. They report in Science that the plantations had a drastic effect on stream flow. By sucking water out of the ground and evaporating it from their leaves the trees reduced flow by half. And 13% of streams dried up for at least a year. This would have effects downstream where less water would be available for plants and animals.

The team found that nutrients in the soil were also affected by tree planting. Calcium, magnesium and potassium were all depleted while sodium was enriched, meaning that plantation soil was more salty on average. All of these changes would affect the range of plant species.

Dr Jackson says the two most common plantation species are pines and eucalyptus trees. These fast-growing species rapidly suck CO2 out of the atmosphere, but they result in monoculture forests which support a meagre range of biodiversity. Dr Jackson stresses that planting trees is not a bad thing per se, but schemes that are not well thought through can be environmentally harmful.

The last point is the important one. Carbon offsets per se are not bad, but you need to plant the right trees to achieve the optimum effect.

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