Planting Trees Helps Fight Global Warming, but Only in the Tropics

In November, a discussion arose over tree planting as a viable method to offsets carbon emissions. A new study, which used computer modeling to calculate the impact of forests on climate change, concludes that yes, trees can help... if they're planted in the right places. According to a BBC article,

Dr [Govindasamy] Bala [of Lawrence Livermore National Laboratory] and colleague Ken Caldeira, from the Carnegie Institution of Washington, used a computer model to determine the impact which forests in different parts of the planet would have on temperature.

Their analysis indicates that three key factors are involved:

  • forests can cool the planet by absorbing the greenhouse gas carbon dioxide during photosynthesis
  • they can also cool the planet by evaporating water to the atmosphere and increasing cloudiness; a deck of white clouds reflects incoming solar radiation straight back out into space
  • trees can also have a warming effect because they are dark and absorb a lot of sunlight, holding heat near ground level
"Our study shows that tropical forests are very beneficial to the climate because they take up carbon and increase cloudiness, which in turn helps cool the planet," explained Dr Bala.

The further you move from the equator, though, these gains are eroded; and the team's modelling predicts that planting more trees in mid- and high-latitude locations could lead to a net warming of a few degrees by the year 2100.

"The darkening of the surface by new forest canopies in the high-latitude boreal regions allows absorption of more sunlight that helps to warm the surface," Dr Bala said.

Though we're guessing the debate has just begun on this issue, this study will certainly have an impact on services offering to offset carbon emissions by planting trees. We'll also be interested to see if this changes the dynamics between companies harvesting timber and fiber from northern forests, and the activists that keep an eye on them. It shouldn't -- forests provide many other services besides sequestering carbon. We'll be watching to see if anyone tries to use this study to declare that forests must be cut to lessen global warming. ::BBC News via Technocrat
UPDATE: As a commenter below pointed out, I made the mistake of equating carbon emissions with the warming effect. Trees do sequester carbon dioxide everywhere... but are more likely to trap heat in in upper latitudes. My apologies.

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