The easiest geo-engineering technique?

Geo-Engineering can be divided into to two main categories, solar geo-engineering and carbon dioxide removal (CDR) geo-engineering. The previous post on ocean acidification falls into the latter category. Next I wish to look at another form of CDR geo-engineering, one which I feel can have obvious impacts, is relatively cheap and easy to implement and the barriers to implement this are relatively small. This method is called afforestation/reforestation. Below, I will outline this method and why I feel it is a Carbon Dioxide Removal scheme that we as a planet should be introducing now because it has very few negative repercussions.

Afforestation/reforestation are two distinct terms and it is vital to explain the difference now. Caldeira et al. (2013) explain the differences clearly and concisely:

“Afforestation is the direct human-induced growth of forest on land that has not historically been forested. Reforestation is the direct human-induced conversion of nonforested land to forested land on land that had been previously converted from forest to other uses.”

The planting of new young trees in areas previously covered before human activity is an easy way to increase the storage of atmospheric carbon dioxide because trees take in CO₂ during the process of photosynthesis that takes place during the day. This means that forests are a natural carbon sink (store of carbon). Further to this, trees absorb more carbon dioxide during their early development phase for growth and this means that planting new forests would have a greater impact than existing forests. The planting of new trees is therefore seen to have quick impacts on the atmospheric carbon dioxide levels.

The processes of afforestation and reforestation have clear benefits. In the instance of reforestation, there are relatively few drawbacks as it is just the process of returning land to the habitat that existed there around 300 years ago. Because of this, it is unlikely that reforestation will have any unexpected or dramatic impacts on the planet as a whole which is a particular concern for many of the other geo-engineering schemes proposed. There is also clear understanding of the processes involved and therefore predictions on carbon uptake by trees is more reliably calculated. The ecological benefits for endangered species are noticeable as many of the species now endangered were put in this situation by human clearance of their habitats in the past. Reforestation could help many species recover.

The use of afforestation of new areas would also be beneficial. Research has found that afforestation at low latitudes in tropical climates could produce an added cooling benefit due to increased formation of low clouds that could increase the amount of solar radiation reflected back into space. One ambitious estimate suggested that the use of forest planting could restore all the carbon lost through human deforestation and potentially decrease atmospheric carbon dioxide levels by 40 to 70 ppm by 2100. This is a very optimistic estimate however with other research estimating that if the USA and European countries planted forests over 0.5% of their areas, around 40Mt CO₂ per year could be removed from the atmosphere which is just 0.2% of the annual world anthropogenic CO₂ emissions. Other papers have suggested that a feasible estimate for atmospheric carbon dioxide reductions would be 15-30ppm by the end of the century. Despite this, I still feel that the planting of forests could have other as yet unknown beneficial climate feedbacks that could aid cooling to restore the climate to pre-industrial revolution levels.

There are unfortunately some drawbacks to this method of geo-engineering as there are with all methods. Much of the land that would be used for reforestation was cut down to make way for crops and livestock and this land is still being used for this purpose today. Farmers are unlikely to want to make way and lose their livelihoods to plant some trees and this highlights the main reason why reforestation has not been employed more aggressively in the past. On top of this, the world is under ever growing food insecurity and the demand for food is endlessly increasing. It therefore seems implausible that it will be easy to implement reforestation schemes without harming other sectors.

Afforestation also has some drawbacks with regard to climate. Afforestation in higher latitude areas that are seasonally or permanently covered by snow would lower the albedo (reflectivity) of the area causing more solar radiation to absorbed, actually leading to increases in temperature despite reductions in carbon dioxide. This highlights that the use of afforestation needs to be more carefully studied compared to reforestation as there may be less well understood adverse impacts that alter the climate of a region possibly affecting other systems such as rainfall patterns, monsoons, flooding, etc. These impacts may also occur far away from the afforestation programme meaning larger scale research is necessary.

To end this post, it is useful to consider the extent to which afforestation and reforestation could be implemented. A 2010 paper studied how humans have altered the biomes (different habitat types) between 1700 and 2000. The map below (Figure 1) shows the different anthromes (human biomes) worldwide. The 2000 map shows that a large proportion of the land that used to be forested is now dominated by croplands and rangelands (livestock). This illustrates the potential for reforestation on a large scale. In 1700 over 90% of the world was classed as wild or semi-natural but in 2000, less than 50%of the land was classified in such a way. Personally, I am not advocating a total reforestation of all the land that was previously woodland – that would be completely unachievable in the current global situation with the high demand food but a gradual and progressive reforestation of cropland and rangelands as well as careful afforestation of suitable areas would have an influence on atmospheric CO₂ (albeit small) with relatively few serious drawbacks. What’s not to like?

Figure 1: Anthropogenic biomes (1900 and 2000)