Approximately what percentage of the Earth's deserts would need to be converted to photosynthesis producing plants to reverse global warming?
This is a very interesting question. The most effective thing we can do to reduce global warming would be to insure we have as many "green" plants growing as possible. There is a certain amount of the population that does not realize the beneficial effects plants have in removing carbon dioxide from the atmosphere. Carbon dioxide is one of the causes of global warming. It is classified as a "greenhouse gas", meaning it tends to hold heat close to the Earth, much the same way a greenhouse traps incoming solar radiation.
There is a movement called "geoengineering" that is a specific coordinated effort to address this very idea. One of their approaches is to maximize the number of green plants on Earth. But there are realistic promblemary "glitches" that have to be dealt with, such as, deserts. There is a reason the deserts on Earth exist as barren, plantless areas; low annual rainfall, soil conditions tend to predominate, for starters. How do we overcome the natural tendencies of the desert biome? These are the things the geoengineering movement has to deal with to make the idea of "greening the deserts" feasible.