If all humans still led the hunter-gatherer lifestyle of the Mentawai people of Indonesia, we would have reached our carrying capacity long ago.

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What's the Earth's Carrying Capacity?

­Carrying capacity is not a fixed number. Estimates put Earth's carrying capacity at anywhere between 2 billion and 40 billion people [source: McConeghy]. It varies with a wide range of factors, most of them fitting under the umbrella of "lifestyle." If humans were still in the hunter-gatherer mode, Earth would have reached its capacity at about 100 million people [source: ThinkQuest]. With humans producing food and living in high-rise buildings, that number increases significantly [source: ThinkQuest].

As of 2008, there were about 6.7 billion people living on this planet [source: Sachs]. A good way to understand the flexibility of Earth's carrying capacity is to look at the difference between the projected capacities of 2 billion and 40 billion. Essentially, we're working with the same level of resources with both of those numbers. So how can the estimates swing so widely?

Because people in different parts of the world are consuming different amounts of those resources. Basically, if everyone on Earth lived like a middle-class American, consuming roughly 3.3 times the subsistence level of food and about 250 times the subsistence level of clean water, the Earth could only support about 2 billion people [source: McConeghy]. On the other hand, if everyone on the planet consumed only what he or she needed, 40 billion would be a feasible number [source: McConeghy]. As it is, the people living in developed countries are consuming so much that the other approximate 75 percent of the population is left with barely what they need to get by [source: McConeghy].

To the surprise of those scientists who dismissed Malthus' prediction as fatally flawed, this limit on resources appears to stand despite the human ability to develop technologies that alter Malthus' presumed linear growth of the food supply. The issue, then, is why technology isn't saving us from the disaster of naturally mediated population control.

What are we doing wrong?