But there isn�t any more of what we now consider arable � that is, farmable land. In fact, the amount of arable land hasn�t changed appreciably for more than half a century.  This looks like a big increase, but it�s only 10%.  Meanwhile, the population has doubled, which means we�ve gone from 3 acres of arable land per person half a century ago to less than a third of that today.  And despite pockets here and there, the overall amount of arable land is not likely to increase much in the future because we continue to lose it to urbanization, salinization, and desertification as fast as we�re adding it.  So doubling the food supply by doubling the amount of land farmed is not an option.