Instructors who include a module on climate geoengineering may wish to include a section on a carbon dioxide removal scheme called “biochar.” Biochar is charcoal produced by medium- or high- temperature heating of biomass with little or no oxygen to drive off volatile gasses (a process called pyrolysis), leaving a more stable carbon, called char, behind. Biochar is touted as a potential “negative carbon” process, because it can sequester up to 40% of the total carbon from the biomass for centuries. Moreover, biochar is an excellent soil amendment, because it’s very effective at retaining both water and water-soluble nutrients, can reduce fertilizer requirements. Moreover, the pyrolysis process produces bio-oils with fuel value that is generally 50 – 70% that of petroleum bases fuels and can be used as boiler fuel or upgraded to renewable transportation fuels. However, some commentators have expressed serious concern that a large-scale biochar program would result in “land grabs” for biochar feedstocks that could threaten the interests of vulnerable populations, as well as adverse environmental impacts associated with land clearance under some scenarios.

A study from a few years ago by Woolf et al. in the journal Nature Communications would be an excellent reading for students because it focuses on the trade-offs between biochar production and other environmental considerations that can stimulate student discussion about broader considerations of the balancing of interests in the climate policy making realm. Through a series of scenarios, the study seeks to the maximum amount of biochar that can be produced sustainably or “maximum sustainable technical potential” (MSTP). The MTSP is defined as “the portion of the global biomass resource that can be harvested … without endangering food security, habitat, or soil conservation” when converted to biochar.

Among the conclusions of the study:

1. Sustainable global implementation of biochar could offset a maximum of 12% of current carbon dioxide equivalent emissions, with a total estimate of 130 Pg CO2-Ce over the course of a century
   1. By contrast, conversion of all sustainably obtained biomass to bioenergy could only offset 10% of current anthropogenic emissions. The advantage of biochar over bioenergy is greatest in areas where biomass crops would be grow in poor soils, while biomass combustion is a superior strategy for climate mitigation in areas with fertile soils, where biomass energy production is optional to offset coal combustion.
2. Use of land clearance strategies to produce biomass feedstock can result in “unacceptably high” carbon-payback times, e.g. 10 years for conversion of temperate grasslands, 50 years for clearance of rainforests, and an astounding 325 years for clearance of rainforest on peatlands;
3. Half of avoided emissions associated with biochar production would constitute sequestration of carbon dioxide, 30% from energy production that could displace fossil fuels, and 30 from avoided methane and nitrous oxide emissions;
4. Simultaneous large-scale production of bioenergy and biochar are not feasible.

Some of the class discussion questions that might be pertinent to this reading:

• Is it realistic to assume that a ramped-up biochar program would avoid unsustainable practices in the manner contemplated in this study?
• Is it appropriate to classify biochar as a “climate geoengineering” approach. Why or why not?;
• What would be the appropriate level of funding for biochar research programs given its mitigation potential?