> irrigate the Sahara Desert to turn it into a vast grassland that can support a billion cattle.

There are currently about a billion cows[1], and these produce 14.5% of green house gases[2] (in the form of methane, measured as co2-equivalent). Doubling that would not be very good.

[1] https://www.statista.com/statistics/263979/global-cattle-pop...

[2] https://clear.ucdavis.edu/explainers/using-global-emission-s...