well realistically you only need to get the governments of the UN to agree which is a little less daunting. I can easily imaging taking the Sahara, Siberia, the interior of Australia, and the steppe would get us a good chunk of the way there. Adding on the amazon might be possible with effort.
Of course, these areas have substantial mineral/oil deposits that make the current political situtation difficult. But it's not outside of the realms of possibility that this may eventually stop being the case, such as how Britain's one ubiquitous coal mines are all closed now.
