The figure they measured, in terms of global acreage, works out to an increase in forestry across 8% of the Earth's land area. That's completely viable, in fact some multiple of that is as well. The overwhelming majority of land on this planet is doing pretty much nothing owing to our habit of packing ourselves into tiny densely packed cities which are supported by a fairly small amount of agriculture - 39% of all land there to be exact. [1]
So this is completely viable, and then some! Even more so if one accounts for the fact that increases in CO2 are creating a substantially greener planet making this even easier.
