Lehman et el (2007) in their seminal study on “terra preta del Indio” (biochar) notes that if the complete planets organic waste converted into “flash” pyrolytic oil, (all the waste organics are converted into oil) this would provide enough energy to replace fossil fuels. Fossil fuels yield about 2ppm of carbon dioxide added “permanently” to the atmosphere each year. If, instead, the agricultural waste is slowly pyrolysed at 600°C, one half of the carbon can be preserved as near indestructible biochar , the other half is distributed between oil and syngas. Thus, in theory one ppm of carbon dioxide can be removed every year from the atmosphere, as long as the pyrolysate is used instead of fossil fuels, and hydrogen and/or algal derived fuels make up the shortfall. (all of this assumes that energy demand remains stable). Thus it would take about a century, after a seven year "lead - in" period during which fossil fuels are replaced (assuming absolute universal compliance) to return atmosphere carbon dioxide concentrations to pre - industrial levels. This is too slow, as the polar ice is melting far too rapidly. However if the biomass is heated rapidly, all the carbon can be volatilised into "flash pyrolysate" which could be injected back into abandoned oil reservoirs, only 50 years would be needed to remove all the post industrial CO2 into the atmosphere. However this would be economically prohibitive. It will still be necessary to bring much marginal and arid land under cultivation (quickly) to produce enough biochar or pyrolysate to remove enough carbon dioxide rapidly, should the polar “tipping point” be approached. 
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