The world's surface may be conveniently divided into thirds. Two thirds are covered by the oceans, and if the planet was ground flat by a giant scraper, it would be covered by seawater, a mile deep. Thus while we are short of fresh water, we have an abundance of seawater.

Of the land's surface, roughly one third is occupied by mankind in various states of development, one third is forest (and shrinking) and the remaining third is desert (and growing). Many, if not all of the world's deserts formerly supported vegetation, and were it not for the lack of fresh water, they could do so now.