Planting trees?

Another way that was proposed to mitigate climate change is to 'simply' plant more trees. Carbon dioxide circulates between different reservoirs. Forests currently cover roughly 1/3 of the world’s land surface and are the most significant terrestrial carbon store (Eliasch 2008). Trees sequester CO₂ through photosynthesis and store it in their biomass. This is the reason why one geo-engineering approach suggest to plant more trees to compensate for anthropogenic CO₂ emissions. But can that really work?

When examining this approach it falls into place that CO₂ reduction is not the only impact of planting new forests. Other biophysical effects like changes in evapotranspiration, cloud formation and albedo also have to be taken into account (Bala et al. 2007, Betts 2000, Gibbard et al. 2005).

Albedo effects are especially significant in boreal forest regions. Planting trees leads to a decrease in albedo most notably in winter, because bare ground, lower vegetation structures and snow-covered ground have a higher albedo than forests. Afforestation therefore has a warming effect at high latitudes and is therefore counterproductive,  because the albedo effect outweighs the cooling effect though carbon sequestration (Bala et al. 2007, Betts 2000, Gibbard et al. 2005).

Tropical forests at low latitudes in contrast have higher rates of evapotranspiration and for this reason more clouds are formed. In addition to the carbon induced cooling effect, evapotranspiration leads to a cooling and cloud formation enhances albedo. Thus the net effect of afforestation at low latitudes is cooling (Bala et al. 2007, Betts 2000, Gibbard et al. 2005).

Temperate forests take an almost neutral position in this geo-engineering method. According to Bala et al. (2007) warming effects due to a albedo descrease and cooling effects carbon sequestration even out.

The following graphic taken from Bonan (2008) illustrates the environmental impacts of the three mentioned forest types (A-C) and their geographical distribution:

There is still a need for further research on the quantity of effects and on comparison of different models, but so far the conclusion can be drawn that forestation CAN have a positive impact on mitigating climate change, IF applied in the right place. Afforestation on a large scale at low latitudes can have a cooling effect, while it would only increase warming at higher latitudes.

Literature:

• Bala, G., K. Caldeira, M. Wickett, T. J. Phillips, D. Lobell, C. Delire, and A. Mirin (2007): Combined climate and carbon cycle effects of global deforestation, Proceedings of the National Academy of Sciences, 104(16), 6550-6555. Available online: http://caos.iisc.ernet.in/faculty/gbala/pdf_files/Bala_etal_PNAS2007.pdf

• Betts R.A. (2000): Offset of the potential carbon sink from boreal forestation by decreases in surface albedo, Nature, 408, 6809, 187-190 Available online: http://www.nature.com/nature/journal/v408/n6809/full/408187a0.html

• BonanBetts, G.B. (2008): ‘Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests’, Science, 320, 1444 - 1449. Available online: http://www.sciencemag.org/content/320/5882/1444.full

• Eliasch, J. (2008): Climate Change: financing global forests: the Eliasch review, London: Earthscan. Online version: http://www.official-documents.gov.uk/document/other/9780108507632/9780108507632.pdf

• Gibbard, S. et al. (2005): Climate effects of global land cover change, Geophysical Research Letters, 32, 23. Available online: http://www.agu.org/pubs/crossref/2005/2005GL024550.shtml