"Annual net emissions of carbon dioxide (CO2), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO2-C equivalent (CO2-Ce) per year (12% of current anthropogenic CO2-Ce emissions; 1 Pg=1 Gt), and total net emissions over the course of a century by 130 Pg CO2-Ce, without endangering food security, habitat or soil conservation."
Woolf, D., Amonette, J. E., Street-Perrott, F. A., Lehmann, J., Joseph, S., 2010. Sustainable biochar to mitigate global climate change. Nature Communications 1, 56. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2964457/
CITED in Assessment Report from the GCEP Workshop on Energy Supply with Negative Carbon Emissions Jennifer L. Milne 1 and Christopher B. Field, Stanford University, http://gcep.stanford.edu/pdfs/rfpp/Report%20from%20GCEP%20Workshop%20on%...