More than 7 billion people currently inhabit the planet, compared to only 3 billion in 1967. Every year about 135 million people are born and 55 million people die, adding 80 million to our global population.

Amid this unstable growth scenario, more than 1 billion human beings do not have enough food and safe drinking water; global warming is disrupting our ecosystems and threatening billions of people with dislocation and conflict; energy resources are becoming scarce, dangerous and expensive; and sprawl destroys 2.2 million acres of forest habitat and farmland every year.

While there are many numbers and arguments for the Earth’s maximum sustainable human population, they all hinge on the question: In what manner shall the Earth’s species live?