However, long-term sequestration "would require high biochar stability," they wrote. "Estimates of biochar half‐life vary greatly from 10 years to more than 100 years. The type of feedstock also contributes to stability, with wood being more stable than grasses and manure."
In addition to long-term stability, questions have been raised about the impact of biochar on soil conservation, biodiversity and water use. As an example, the authors pointed to research showing that negative effects on soil fertility can occur if the pH of the biochar and the soil are not well matched.
According to the authors, biochar systems can be net negative if the biochar is made from waste biomass, sustainably harvested crop residues or crops grown on abandoned land that has not reverted to forest. On the other hand, biochar production that relies on forest ecosystems may result in a net increase in greenhouse gas emissions, they cautioned.
Even large agricultural systems can be net negative. The GCEP report cited research by Jose Moreira of the University of Sao Paulo. Using computer models, Moreira found that from 1975 to 2007, ethanol production from sugar cane in Brazil resulted in a net-negative capture of 1.5 metric tons of CO2 per cubic meter of ethanol produced. "In this model, the system took 18 years to recoup carbon emissions, with most reductions coming from soil replenishment from root growth and replacement of gasoline with ethanol," the GCEP authors wrote. However, questions remain about the long-term effects of ethanol combustion on climate.
The report also explored the possibility of sequestering carbon in the ocean, with a particular focus on the problem of ocean acidification, which is destroying coral reefs around the world. Ocean acidification results from the increased uptake of atmospheric CO
|Contact: Mark Shwartz|