Afforestation and Reforestation: Barking up the wrong tree?

Reforestation (replanting trees in deforested areas) and afforestation (planting trees in previously non-forested areas) does not sound like anything new. I’m sure your familiar with the ‘3 tree promise’ that has heavily featured as part of toilet paper adverts in the past. However, what makes reforestation and afforestation a geoengineering method is the scale and intent to remove carbon from the atmosphere.

The 3 tree promise (Velvet, 2016)



How many trees?

It is widely agreed that net-zero carbon emissions will have to be reached to meet the 2^oC or 1.5 ^oC target by 2100. In terms of personal carbon emissions, there’s a handy online calculator. This works out a rough estimate of the number of trees required to offset your personal carbon emissions, based on your lifestyle. Even with my vegetarian student lifestyle, I got 3.5 trees….per month. How do you compare?  

My results for my personal emissions. Check yours at Carbonify!

Assuming there’s around 7.46 billion people, with my lifestyle we would need to plant 26.11 billion trees each month. Putting this into context, with 390 billion trees in the Amazon Rainforest, we would have to plant the equivalent of the Amazon every 15 months to offset carbon emissions, if everyone lived as I do.  

Other issues?

Aside from the huge amount of trees required, initially the idea of planting trees is pretty straightforward. Particularly as it is fairly easy to quantify and allows us to ‘offset’ our actions. However, Abiodan et al. (2012) modelled impacts of large-scale afforestation in Nigeria, finding there would be more rainfall in afforested areas and less rainfall in non-afforested areas, along with increased extreme events such as droughts. Not only in Nigeria, but also in the surrounding countries. This is due to changes in evapotranspiration affecting local climates, as in the diagram below. Similarly, Dickinson et al. (2013) found afforestation led to net warming in dry regions of China, where the lower albedo and changes in the heat and radiation transport cause local warming.

Impacts of the forest on rainfall. The forest increases 'recycled rain' through evapotranspiration, because the trees transfer water from the soil to the atmosphere. Increasing rainfall in forested areas influences the wider climate system, leading to decreases elsewhere (Aragao, 2012)



Personally, I don’t think trees are the answer to meeting the climate targets. I’m certainly not against replacing trees we’ve already cut down, and there are plenty of those, but as a long term solution I feel the impacts won’t be large enough.