The Green Hills of Urth

I was contemplating the technology used to make and keep Lune green and forested and wondered, why couldn't the same effect be used to renew Urth, instead of involving the White Fountain and flooding and earth quakes and such? (of course, it would be a much shorter book that way :) Has anyone found mention of how Lune was terraformed? Some giant, lunar-sized Bracelet of Air? It does seem odd that Lune remains green while the vegetation on Urth is dying.