Tree Planting - Help or Hindrance?

Carbon Offsetting involves paying an organisation to neutralise the climate impact of your own activities, thereby making those activities "carbon neutral" or "climate neutral". Some offset schemes focus on reducing future emissions by, for example, giving out low-energy lightbulbs in the developing world or buying renewable electricity credits. Others focus on sucking CO2 directly out of the atmosphere - usually by planting trees.

The planting of trees clearly has a positive effect in the fight against global warming: the destruction of rainforests across the tropics is a significant contributor to climate change, accounting for roughly a fifth of recent human produced CO2 emissions. Tropical forests hold nearly half of the carbon present in vegetation around thew world. When they're burned to clear land, the trees, soil and undergrowth release CO2. In addition to the CO2 from the fires, bacteria in the newly exposed soil may release more than twice the usual amount of another greenhouse gas, nitrous oxide, for at least two years. Brazil's National Institute for Amazon Research estimates that deforestation puts four times more carbon into the atmosphere than the nation's fossil-fuel burning does.

Rainforests also coll the climate on a more local level, their canopy helping to trap moisture and allow it to slowly evaporate, providing a natural air-conditioning effect. When the rainforest has been slashed and burned over large areas, hitter and dryer conditions often set in, although the exact strength of this relationship is difficult to quantify. Across eastern Brazil, where nearly 20% of the Amazon rainforest has been destroyed, 2005 saw the region's worst drought in a century, perhaps related to changes in the nearby Atlantic and to rain-suppressing smoke from fires as well as to the deforestation itself.

Yes, despite it being one of the more populist approaches to tackling global warming, tree-planting has also become one of the more controversial - especially in the context of carbon offsetting. It's true that trees soak up CO2 as they grow. Two or three dozen can be enough to absorb an entire household's emissions. However, at snow-prone higher altitudes, trees can actually accelerate climate change, according to a 2006 study led by Govindasamy Bala of the US Lawrence Livermore National Laboratory. That's because their CO2 absorbing benefit is outweighed by the impact of their dark colour, which in wintertime absorbs sunlight that might otherwise be reflected to space by bright snow cover atop barren ground.

In snow-free warmer climates, and especially in the tropics and subtropics, the dark colour of trees isn't a problem. But there are other catches. First, when a tree dies, much of its stored carbon returns to the atmosphere. So the offset will only be permanent if each tree planted is replaced by another - and so on. Second, there are quicker, cheaper and longer lasting ways to fight climate change, such as distributing low-energy technologies to displace fossil fuels.

For all these reasons, many offset schemes have switched from tree-planting to energy-saving. That said, unless you live in a snowy region, planting a few trees in your garden is still unlikely to be beneficial for the climate - at least for the crucial coming decades.

A separate, and altogether more pressing, approach is protecting the forests that are already standing. That's not so much because of the CO2 that mature forests absorb. It's because deforestation, and in particular the destruction of tropical rainforests, is one of the very largest sources of greenhouse emissions.

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