What we need to be doing is replanting the planet. Given time, nature will rearrange itself and plants will move where climate is optimal for them, that is if they aren't eaten by humans or animals first. Given that we and our livestock eat a lot of those plants, we need to get ahead of our consumption and plant areas that are now defoliated.
Somalia is a good example. Overgrazing has reduced the carrying capacity of that country to the point where the population is starving. However, if local native species were given a head start (by a government capable of planning and carrying out projects on a national scale) it could be much more productive. If you want to see what long-term planning can do for desertified areas, check out this video:
http://www.youtube.com/watch?v=K9dhtu15DXY (the link is to part2, but all 6 are worth watching).