OK, but remember that the experts said the same thing about oil and natural gas, and were wrong in that case. Here the estimate of ~10 megatonnes assumes that seawater extraction will never be economically viable, one possible source of error.

If you're interested in the history of "peak resources" here is a good book about Paul Ehrlich's famous bet with Julian L. Simon:

http://www.amazon.com/The-Bet-Ehrlich-Julian-Earth%C2%92s/dp...