It pains me to see this Neo-Malthusianism still abundant in modern thought when it was debunked over a century ago by Henry George. The fact is that 9 billion people is well within the carrying capacity of earth. If you go to any third world country, you'll find that there is plenty of food available, but that many people cannot afford to buy it. It is not the stinginess of nature but the policies and practices of people which prevents some from enjoying a basic sustenance. The best land for farming, housing, and business is monopolized by speculators and big industry. This leaves the poor either with extremely fallow land or none at all. The solution is not population control, but land reform.
I HIGHLY suggest reading Progress and Poverty by Henry George and World Hunger: Twelve Myths by Frances Moore Lappe.