We all know that plants absorb CO2 during photosynthesis. Consequently, there have been many geo-engineering ideas that propose we embrace this natural phenomenon to aid our battle against global warming. In addition to the fertilization of marine phytoplankton as mentioned in the previous post, there are also several terrestrial biomass methods that work to draw down CO2 in this way, namely afforestation, reforestation and biochar.
Firstly I am going to look at afforestation and reforestation:
Increasing terrestrial biomass has great potential for absorbing CO2 and sequestering carbon. Depending on who you ask, anthropogenic deforestation has been causing increases in atmospheric carbon dioxide concentrations for thousands of years (Ruddiman, 2003). It makes sense, therefore, that reforestation and afforestation would help to mediate the current CO2 problem.
But what to what capacity can these two options really help?
Firstly, reforestation (restocking of forest and woodlands that have been depleted) has the potential to reduce CO2 concentration by 40-70 ppm by 2100 (Candell and Raupach, 2008) - this figure is by no means conservative! - these figures assume that complete reforestation is possible, it is not. Should we implement afforestation also, the possibilities are limited owing to competing land-use requirements such as food production, urbanisation and conservation etc (Candell and Raupach, 2008). Already (up until 2000) anthropogenic fossil fuel combustion has caused an increase of 90ppm, exceeding the potential of reforestation. It is clear that reforestation falls short of being able to truly mitigate atmospheric CO2 currently, let alone if atmospheric CO2 continues to climb. Additionally, this method is by no means a quick fix! - I doesn't take a genius to work out that we can't grow entire forests over night!
What are the impacts? - (Zomer et al., 2008)
9% of land surface available for af/re-forestation, ~700 Mha of land, in terms of being biophysically suitable. However, more than half of this is cropland posing high socioeconomic cost and compromise of food security. Such a conversion of land-use would also see a displacement of tenant farmers due to low labour requirements of forestry activities, loss of land access rights to indigenous peoples.
Af/re-forestation will also likely have hydrological impacts. Generally it has been shown that this would cause and increase in 'green vapour' flow and a reduction in runoff. The extent of this impact varies regionally, however, afforestation in 27% of suitable areas would likely result in a 80-100% decrease in runoff, whilst 50% of suitable areas are vulnerable to a significant reduction of less than 60% (Trabucco et al., 2008). Careful consideration is required as af/reforestation in terms of hydrology, would likely have impacts on water resources, water management, biodiversity, downstream food security. Water resources are a particularly important consideration in-light of the escalating concerns over water scarcity (Trabucco et al., 2008).
Biochar
A pre-Columbian agricultural tool used in the Amazon basin, biochar or terra preta was applied to increase soil fertility. Its persistance in this region for thousands of years is indicative of its potential of a truly long term sink of carbon. Biochar essentially describes a man-made charcoal. This can be produced through the burning of biomass in low an low or no oxygen environment, this process is known as pyrolysis. Can be used not only as a means of trapping carbon, but also increases soil fertility as it was originally intended for historically.
Capacity to reduce CO2?
By 2100, it seems possible that this technology could sequester 400 billion tonnes of carbon, lowering atmospheric concentrations CO2 by 37 ppm, though this number is considered to be at the high end of estimates (Kleiner 2009).
Impacts? - (Kleiner 2009)
There are concerns that promotion of this CRD method could result in significant land-use changes in order to make room for the necessary plantations. Similar responses have been the case with encouragement of biofuel production and the destruction of virgin forests for palm-oil plantations.
Concerns with biochar also lie in its true ability to sequester carbon. In its application to soils to increase fertility, there are concerns that this will in fact increase microbial breakdown in the humus. This works in opposition to the primary purpose of biochar, as it would accelerate release of CO2 from the soil. The extent of this is unknown and more work needs to be done to truly undertand the level of stability of biochar in soil.
Conclusion
Each of these technologies are unable to solely solve global warming. The speed at which reforestation can occur for example, and the conflicts of land use in all cases mean that these methods are restricted in timescale and extent. However, I think that each could have a significant role in mitigating high atmospheric CO2 levels, in combination with other methods. Consideration of the compromises that would have to be made socioeconomically in balance with the benefits would ned to be carefully considered.
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