Bio-char: Learning from our ancestors?

Sticking with the theme of biomass based geoengineering techniques, next on the list is bio-char. Essentially, bio-char is charcoal that is applied to land. It is made by decomposing biomass at high temperatures in oxygen-deprived conditions (pyrolysis). The bio-char is then mixed with soil to act as a fertiliser and increase the productivity of land, replicating techniques of ancient civilisations. This removes carbon dioxide from the atmosphere through photosynthesis and stores the carbon in the soil which remains there for hundreds or thousands of years. The video below demonstrates this process.

Woolfe et al. (2010) calculated 12% of current anthropogenic emissions could be offset using bio-char without negatively impacting agriculture, biodiversity or food production. This is because crop residues and invasive plants can be used rather than converting large areas of land to purposely grow biomass. Energy production can also be added into the pyrolysis stage. If the carbon dioxide is captured and stored, as it is with BECCS, it provides another carbon negative energy source. The diagram shows the positive impacts of bio-char through the production process.

Positive impacts of biomass production (Downie et al., 2012)

Although it also uses biomass, the issues raised are surprisingly different to those with BECCS, because it doesn’t necessarily require purpose grown biomass. Ennis et al. (2012) investigate the specific impacts of bio-char on soil microbiology. They identify the long lifetime of biochar in soils could have negative side effects if it is not implemented properly because any issues would be long-lasting. Furthermore, Williamson (2016) highlight the issue of albedo. The darker colour of charcoal will lower the albedo of soil, increasing the absorption of heat. If this is applied to a significant proportion of the land’s soil this could have an overall warming effect. However, with a proper evaluation of the proposed sites the likelihood of these issues could be reduced. 

Personally, the idea of bio-char appeals much more to me than BECCS. It is a low-tech process, previously used by ancient civilisations that is unlikely to have a large impact on global food, economy or biodiversity if it is implemented in the right areas.