UN plans to reverse desertification to help with climate change

In the aftermath of a UN conference on desertification, 196 countries have signed on to a scheme to reverse desertification with a view to sequestering more carbon in the soil, which UN climate scientists say should provide a 15-20 year buffer in which to look for more permanent solutions to the world's climate crisis.
The scheme, which the scientists estimate will cost around $300 billion (about equivalent to the GDP of Chile, or the world's military spending in just 60 days), is being billed as an affordable short-term solution, which will have the added benefit of increasing the food-producing land of many countries currently struggling to feed it population due to desertification.
Note that this is not making deserts into productive agricultural land - deserts are viable and valuable natural ecosystems in heir own right - merely returning land previously lost to encroaching desertification back to productive status. The UN Food and Agriculture Organization estimates that, of the 2 billion hectares of productive land lost to desertification, through misuse, overgrazing, deforestation and other human activities, 900 million hectares could be recovered and revitalized through a judicial use of fertilizers and irrigation and improved agricultural practices.
It certainly sounds like a win-win situation. What could possibly go wrong?