There could be catastrophic consequences for the whole eco-system of the planet if all the deserts are converted into green lands.
Dust from the Sahara Desert is transported through winds across the Atlantic Ocean to the Amazon Rainforest, thereby depositing important minerals like phosphorus, which are essential for the forest. These minerals from the Sahara Desert replenish the minerals lost through rain and flooding in Amazon. Thus, largest desert on Earth is the sources of life for the the largest forest on Earth!
Sahara Desert dust feeds Amazon Rainforest.
If the Sahara Desert is converted to green land, then Amazon Rainforest will lose its source of nutrients and minerals and will most probably wither away and die.
Deserts are like recycling machines of the planet, as underlying soil, rich in minerals, is brought to the surface and then dispersed all over the planet through winds.
So not a good idea to convert all the deserts into green lands, because deserts are as essential a part of the eco-system as forests.