One trillion trees to save the biosphere : Good idea or fantasy ?


Last July was released by a group of Swiss scientists, a study suggesting that planting on a global scale about 1200 billion trees could help to efficiently reduce CO2 concentrations in the atmosphere. In the increasing context of GHG emissions and natural or intended wild fires, is there a consensus on this study and what is at stake ?

Planting trees to fight biodiversity loss is not a recent practice and appears to be a logical and nature based solution to also fight climate change nowadays. Nevertheless, it seems that the peer review hasn’t abounded in that direction. Indeed, the study suggesting that increasing the global forest area by a third would absorb up to two-thirds of the anthropogenic emissions emitted since the industrial revolution, has attracted many critics. Despite the 10 criteria of evaluation, the Swiss scientists have over-estimated the potential of tree planting. 

To begin with, this is not the 300 GtC (Gitatonnes of Carbon) that humans have emitted since the industrial revolution but 600 GtC, of which 55% have been absorbed by oceans and land ecosystems while the rest remains in the atmosphere. This is what we call the airborne fraction and this is one of the most important existing scaling factors to predict future climate’s changes. The natural absorption process is unfortunately less and less efficient because of positive feedbacks loops, which are self-sufficient and accelerate warming trends by disrupting naturals equilibirum.

Furthermore, replanting trees will not only have positive effects on climate and biodiversity. Depending on where the trees are planted and what kind of trees are planted, biophysical feedbacks occur and especially after afforestation process in snow-covered regions. This is the issue of albedo and evapotranspiration, which can counteract the cooling effect from CO2 uptake.
Finally, the sequestration potential estimated (205 GtC) is well above limits and after considering those aspects it has been finally found that the CO2 storage potential would be 5 times lower than in the initial study : 42 GtC and not 205 GtC.

From another angle, carbon offsets also lead some people to think it is a desirable solution. Lot of people think carbon offset cancels all the effects of the emissions generated. It is actually the communication strategy of Air France, which has announced it was going to compensate 100% of their CO2 emissions coming from internal flights. But trees are not going to directly absorb the equivalent of CO2, we don’t know what kind of trees they want to plant and neither do we know for how long. Considering the 6% annual growth of air travel and the contraction of available water, if all heavy emitters business sectors start to compensate a part of their activity, how big is going to be the necessary lands to keep that target ? Will sectors be in competition with one another, but also with agricultural activities ?
Ultimately, “offsets also risk giving the dangerous illusion of a “fix” that will allow our billowing emissions to just continue to grow”, explains UN Environment.

European forests are also in danger because of multiple factors. Under the effect of droughts, heat waves and insect attacks, spruces, firs, beeches and Scots pines from a large northeast quarter wither away. Most of our forest species are not adapted to climate change according to the forestry expert Philippe Gourmain. Beside climate, Germany, Czech Republic and North of Austria are facing an unprecedented invasion of bark beetles, the largest ever observed, which, after two hot and dry summers, are pullulating and putting million cubic meters of wood in danger. Those bark beetles dig tunnels and lay their larvae, which then infest the trees and lead them to a certain death. Each generation increases the total population by 40%, forcing trees to be removed as quick as possible from forests and mitigate the propagation. This year was observed a flight of the 3rd generation of bark beetles, while there are normally only 2 generations in the classical years.
Therefore, it is about time to conceive a massive plan to finance operational research on new forestry practices, associating local species and species from meridional regions, but also be prepared to several decades of deep change in our forested landscapes.

Even though, planting trees is not the best solution we have (not least because it can compete with agriculture which is already in competition with other sectors), this practice stays and information campaigns at both local and international levels are vital. Local autonomy is a key to protect more effectively the existing local ecosystems but discussions and action plans on a broader scale are cornerstones for such actions. Recent events could have been an actuating lever in forest management practices, but unfortunately they are still facing the unfortunate weighting of the criteria still dominated by competitiveness and therefore economic benefits, which are, undoubtedly, not the best friends of nature conservation.

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A propos de Albéric BARRET

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