IPCC Expert Meeting on Geoengineering

Annex 4: Poster Abstracts
Biochar for Climate Change Mitigation: Prospects and Limitations
Johannes Lehmann*, Kelli Roberts, Thea Whitman, Dominic Woolf
Cornell University, Ithaca, NY 14853, USA
Biochar has received significant attention as a rapid climate-change mitigation strategy. Biochar is a charcoal-type
substance that shares properties with biomass-derived pyrogenic carbon which is ubiquitous in soils (E. Krull et al., 2008;
Johannes Lehmann et al., 2008). In a biochar system, biomass of a wide variety of feedstocks, including wood, grass,
animal manures, crop residues, and other appropriate byproducts, is pyrolysed to biochar, which is added to soil. In many
applications, this conversion is used to generate usable energy from the volatile liquids and gases, such as hydrogen, biooil,
ethanol, electricity or heat. Several anthropogenic and natural analogs can be found that allow some evaluation of
biochar amendments to soils for the long term. Notably, so-called Terra Preta soils in the Amazon have provided incentive
to evaluate biochar for soil improvement as these soils received biochar-type materials several thousand years ago and
retained their fertility.
However, dedicated biochar research and development only started to any significant extent in 2006. The stability of
biochar has been calculated to mean residence times of several hundred to a few thousand years (Johannes Lehmann et al.,
2009; Zimmerman, 2010; Spokas, 2010). While the greater stability of biochar relative to the uncharred biomass is the
basis for the emission reductions through utilization of biochar as a soil amendment rather than combusting it for energy to
offset fossil fuels, it is not always sufficient for achieving net emission reductions. Life-cycle assessment demonstrates that
projects can have positive or negative net emission balances primarily depending on the source of the feedstock (Roberts et
al., 2010). In bioenergy systems, using the biochar as a soil amendment shows greater emission reductions compared to
the use of combustion only if the soil productivity increases or if other greenhouse gas emissions are either offset or
reduced (Roberts et al., 2010; Woolf et al., 2010; Hammond et al., 2011).
On a global scale, this may result in greater emission reductions through biochar systems than combustion for bioenergy
(Figure A.4.4). The proportion of stable carbon in biochar provides about 50% of the total emission reductions of a biochar
system if it is integrated in a bioenergy project. If either no bioenergy is generated or bioenergy from cookstoves with low
burning efficiencies are replaced by biochar-cookstoves, the importance of biochar stability may increase to over 80% or
fall below 30% (Whitman et al., 2011). The emission reductions using waste materials or crop residues as feedstocks vary
between 0.7-3.8 t CO 2e t-1 feedstock (JL Gaunt and J. Lehmann, 2008; J Gaunt and Cowie, 2009; Roberts et al., 2010;
Hammond et al., 2011). In a modeled cookstove system, an improved combustion stove provided similar emission
reductions of 3.5 t CO 2e yr-1 per household compared to 3.69-4.3 t CO 2e yr-1 per household for an improved pyrolytic
stove with biochar additions to soil (Whitman et al., 2011). Hammond et al. (2011) report life-cycle emissions abatement of
1.4-1.9 t CO 2e MWh-1 for a variety of biochar-bioenergy systems in the UK in comparison to a generation of additional
emissions of 0.05-0.3 t CO 2e MWh-1 for other bioenergy systems.
The global technical potential is likely not much greater than 1 Pg CO 2-Ce yr-1 if only biomass resources are used that do
not compete with food crops and other existing uses of biomass, do not require land use change or clearing of natural
vegetation, and do not remove crop residues to an extent that would negatively impact soil health (Woolf et al., 2010). The
economic viability largely depends on the price for the biomass feedstock, and can vary significantly between projects. In
many current projects, the costs may be offset by the value of increased crop yields due to biochar, but adoption may still
benefit from financial support through a price on carbon.
References
Gaunt J., and A. Cowie, 2009: Biochar, greenhouse gas accounting and emissions trading. In: Biochar for environmental
management: science and technology. D.J. Lehmann, S. Joseph, (eds.), Earthscan, London, UK. pp.318–340, (ISBN:
9781844076581).
Gaunt J.L., and J. Lehmann, 2008: Energy balance and emissions associated with biochar sequestration and pyrolysis
bioenergy production. Environmental Science & Technology 42, 4152–4158.
Hammond J., S. Shackley, Saran Sohi, and P. Brownsort, 2011: Prospective life cycle carbon abatement for pyrolysis biochar
systems in the UK. Energy Policy 39, 2646–2655. (DOI: 10.1016/j.enpol.2011.02.033).
<strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 52