I have several times been asked about the amount of carbon existing in excess in the atmosphere, and the possibilities to remove it within reasonable time. Therefore, I will provide the figures as far as I know them, and the reasoning behind them.
If you can't stand numbers, stop reading here.
The global annual net primary production (NPP) varies, but is estimated to be between 70 and 100 Gt per year, by different sources. This is the annual biomass growth in the plant cover of the Earth. For ease of calculation, let's take 80 Gt as the number. Say that 50% of this is thin roots and leaves, not usable for charring. Remains 40 Gt C that is theoretically available for charring each year.
Assume furthermore that efforts to create charcoal for carbon sequestration results in an annual sequestration of 2 Gt C as charcoal. The global forest product production 2004 was abut 9.5 Gt (FAO), with a carbon content that can be estimated to 3.8 Gt C, so we are speaking of a herculean, warlike efforts in charring, about half the size of the global forest industry.
However, char can be made from not only forest products, but also straw and husks from agriculture, as well as forest products that are useless to the industry, which gives us a large possibility to reach a goal of 2 Gt. Which is about 8.5% of the above figure of coarse biomass production.
The carbon dioxide "cloud" is presently about 475 Gt too large (counted as C, carbon). The number is the difference between the atmospheric content in pre-industrial time (280 ppm) and that of today. (Of this figure, about 33% is from deforestation). (Figures from Richard A Houghton, Woods Hole Research Center, one of the IPCC guys.)
Imagine that you could take this amount away and convert it into charcoal. That amount would add about 95 kg char per hectare agricultural land globally. 38 bags of barbecue char. Not very much . But with the above efforts, 2 Gt p.a., it would take more than three hundred years to reach that point.
The global carbon dioxide effluents of today are equivalent of about 7 Gt C. Assume, for a moment, that the people and their leaders around the world will face the imminent danger of a sudden and irreversible climate change and decide to do everything possible to avoid it. They decide to start the above sequestration combined with a sudden braking in carbon emissions, e.g. an 85% reduction in 25 years, leveling out on roughly 1 Gt C per annum, leading to a net sequestration of about 1 Gt C per annum.
Then, given that the reduction is even over time, one could expect that the carbon dioxide cloud could start reversing after about 18 years.
I hope sincerely, that that is not too late.