Why does everything have to be some complicated? One thing seemed so sure: If you plant trees, you take CO2 out of the atmosphere and reduce global warming. Trees are currently planted everywhere on a massive scale by governments and private donors. That is good. But apparently, the effect on climate change is not so easy to assess. Trees don’t only reduce global warming, they also contribute to it through several mechanisms:
- Tree leaves are dark and therefore absorb more sunlight than for example grass. Scientifically speaking, they reduce the surface albedo. This effect is greater in some regions than others depending on the soil and the kind of tree.
- Trees emit volatile organic compounds (VOC) like isoprene and also significant amounts of methane that can also contribute to global warming.
Nature.com has published an article about the topic:
[Isoprene] can react with nitrogen oxides in the air to form ozone — a potent climate-warming gas when it resides in the lower atmosphere. Isoprene can also lengthen the lifetime of atmospheric methane — another greenhouse gas. Yet isoprene can have a cooling influence, too, by helping to produce aerosol particles that block incoming sunlight.
The effect is not always and everywhere the same, however:
Most scientists agree, however, that tropical forests are clear climate coolers: trees there grow relatively fast and transpire massive amounts of water that forms clouds, two effects that help to cool the climate.
So the possible contribution of tree planting schemes to global cooling seems to depend on a complicated set of factors with respect to the local environment and climate and the types of trees planet.
What shall we make out of that? Dr. Nadine Unger wrote a piece in the NY Times called “To Save the Planet, Don’t Plant Trees” (title apparently not chosen by her). 30 scientists responded with a rebuttal. My conclusion is this: 1) Planting trees is good and definitely contributes to a healthy environment. 2) more research is needed to make tree planting schemes as efficient and possible and 3) we should compare their contribution with alternative ways to fight climate change.
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