A mature tree can absorb as much as 21.77 kg (48 lbs) of CO2 a year, which means it would take about 990 such trees to sequester the carbon emissions of one average American. 1 The 216 billion metric tons of CO2 emitted by a hypothetical global population of 10 billion such Americans would require 9.9 trillion trees to sequester its emissions. 2 With a 3.65 m (12 ft) spacing between trees this equates to a 132,250,000 km2 forest — almost the entire 149,000,000 km2 terrestrial area of the planet (minus its current 15,000,000 km2 of ice covered land). Discounting the 39,990,000 km2 of the world's existing forest (including the world's protected areas) we are left with the need to construct a 92,260,000 km2 forest in order to sequester the carbon emissions of the equivalent of 10 billion Americans. 3
If we consider the feasibility of constructing such a forest, we must also subtract from this some 15,300,000 km2 of current crop land and 33,800,000 km2 of current grazing land. (Although this grazing land has some tree cover it is not at the density required to meet the sequestration demand). Furthermore, we must also subtract the 26,780,400 km2 of potentially arable land which is not yet, but surely will be farmed if we are to feed 10 billion by 2100. This leaves a total of only 16,469,600 km2 on which to develop a sequestration forest, all of which is presently desert. This is also assuming no expansion of the world's current grazing land. In short, as this century unfolds there will not be enough land to utilize forestry as the single mechanism for carbon sequestration.
1 NC State University College of Agriculture & Life Sciences, "Tree Facts," http://www.ncsu.edu/project/treesofstrength/treefact.htm (accessed June 1st, 2016). - Sarah Moos, "50,000 Trees," Scenario 04: Building the Urban Forest (2014), http://scenariojournal.com/article/50000-trees/.
2 Of course, considerable CO2 equivalents are already absorbed by the earth through ocean-atmosphere gas exchange (the ocean not only absorbs but also releases carbon), freshwater outgassing, rock weathering, geological activities (volcanic eruptions), respiration and fire, etc. For more detailed information regarding the carbon cycle, please refer to the IPCC Fifth Assessment Report (AR5) http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter06_FINAL.pdf
3 FAO (2015) Global Forest Resources Assessment 2015. Rome. Available at http://www.uncclearn.org/sites/default/files/inventory/a-i4793e.pdf