Reforestation can offset 15bn ton of CO2 per year from 3bn global acres. But is there potential to afforest any of the world’s 11bn acres of arid and semi-arid lands, by desalinating and distributing seawater? Our 18-page note answers this question. While the energy economics do not work in the most extreme deserts (e.g., the Sahara), $60-120/ton CO2 prices may be sufficient in semi-arid climates, while the best economics of all use waste water from oil and gas, such as in the Permian basin or potentially the Middle East.