Making the deserts bloom
The first step of the process - heating limestone up until it breaks down and produces lime also generates carbon dioxide. This can be used to help grow algae - either for food or for biofuels. It also allows the growth of biomass in very dry environments. How so? Think of it this way:
Say you wanted to grow a crop in an arid environment - you haven’t got much water, so you seal your water and the crop that you are growing inside a greenhouse. Sealing it up stops the water evaporating away, but as the plants grow they use up all the carbon dioxide in the greenhouse. So you decide to put some fresh air into the greenhouse, but when you do this you have to remove the old air, and when the old air leaves you lose all the moisture … back to square one - it doesn’t work.
If, however, you have a sealed tank of water and into it you introduce pure carbon dioxide, you are in a much better position. The tank has a transparent lid, so you have sunlight; the water in the tank contains algae - you have everything you need for photosynthesis. Because the tank is sealed you will not lose water by evaporation. (You will need to vent off a small amount of oxygen generated by the photosynthesis, but you will only lose a small amount of moisture because of this).
Some fairly simple chemistry shows that for every kg of sugars that you produce, you need to use 600g of water. Undoubtably, you will lose some more water in the practical application of the process, but the water usage will be less than 10kgs of water for every kg of sugar produced. This sounds like a lot, until you realise that growing crops in really dry places like Egypt requires 1000 kgs of water for every kg of crop produced.
Or to put it another way, it is possible to grow crops with 1% of current water usage. So, paradoxically, the way to make the deserts bloom is to heat the right kind of rocks up.