Afforestation is often praised as a measure to combat climate change. But scientific studies show that this is too simplistic. The effect of trees on the temperature of the air depends on where they are planted.

In principle, all trees absorb CO2 by their growth, which does indeed counteract climate change. However, this effect is very slight, compared to the large amounts of greenhouse gases emitted by vehicles, industry, farming, etc. So afforestation cannot substitute for reducing the emissions. Furthermore, trees also have other important effects on the local climate.

At high latitudes, where snow still lies in the spring, trees protrude above the snow and absorb a relatively large amount of sunlight, due to their dark surface. By contrast, a completely snow-covered area reflects much more sunlight. So forests make the local climate warmer in these climate zones. Afforestation at high latitudes has relatively little effect on the rest of the world. By contrast, afforestation in the tropics tends to have a cooling effect (even apart from the effect of CO2 storage). This is because trees evaporate a great deal of water through their leaves. This consumes latent heat of vaporization, just like people sweating on a hot day. So the cooling effect of trees in the tropics is considerably larger than one would expect from the CO2 storage effect alone.

These physical mechanisms (masking of snow at high latitudes and latent heat of vaporization in the tropics) have been ignored in political climate conventions to date – all that counts is CO2 storage. Unfortunately, the tropics have been logged increasingly in recent decades, while the forested area at high latitudes is increasing. Both factors reinforce global warming.

Publications

  • Bathiany, S., Claussen, M., Brovkin, V., Raddatz, T. & Gayler, V., 2010: Combined biogeophysical and biogeochemical effects of large-scale forest cover changes in the MPI Earth System Model. , 7 (5), 1383-1399.

http://www.biogeosciences.net/7/1383/2010/bg-7-1383-2010.html