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Climate changers.
Trees have different effects on local weather depending on their location.

Careful Where You Plant That Tree

When it comes to climate change, not all trees are created equal. A tree growing in the tropics cools the planet, according to a new study, but the same tree contributes to global warming in the high latitudes. The findings could help eco-warriors better target their conservation efforts.

Trees affect local weather in two divergent ways. On one hand, trees are great carbon dioxide sinks, sucking the heat-trapping gas from the air. They also, through a process known as evapotranspiration, create clouds that bounce back warming rays (ScienceNOW, 12 March). But trees also absorb heat from the sun and prevent snow from reflecting ultraviolet rays, warming the regional climate. A team led by atmospheric scientist Govindasamy Bala of Lawrence Livermore National Laboratory in California wondered whether a tree's location would cause some of these effects to overpower others.

So the team clear-cut forests--using a computer simulation, of course. When the researchers compared their simulation to a standard model with no deforestation, they found that the impact of deforestation depended on location. In the northern latitudes, a tree's ability to trap heat and block snow reflection appears to overpower the cooling effects of its carbon dioxide consumption; without trees, these areas became 0.8° Celsius cooler by 2100 compared to the standard model. The opposite is true in the tropics, Bala says, which warmed by 0.7° when treeless over the century. And in temperate regions, the effects seem to balance out; the difference in temperature between the two models in these areas was just 0.04°, the team reports online this week in the Proceedings of the National Academy of Sciences.

The study confirms that reforestation efforts in the tropics could slow global warming, and it serves as an important warning against planting trees in boreal regions, says climate scientist Victor Brovkin of the Potsdam Institute for Climate Impact Research in Germany. Planting trees in temperate regions probably has no net effect on global warming, he says. As for the idea that we might be better off in a world without trees, Bala warns against underestimating the value of forests, which provide habitat for wildlife and help preserve Earth's biodiversity.

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