Scientists are now speaking of what they call the "coupled human-environment system." It's a new way of thinking about the relationship between nature and human beings.

Kai Lee said, "I think one of the great dilemmas we face is acknowledging this interdependence in a way which also enables us to live our lives. We don't want to be encumbered all the time with thinking about all the indirect consequences of what we're doing in the world. And I think that's a dilemma that we don't even recognize is a dilemma yet ... "

Urbanites soon to outnumber country dwellers

B.L. Turner is a professor at Arizona State University. He talks about the history of the biophysical system and the social system as one of a coupled system.

Consider three requirements for a sustainable world. Everyone on the planet would have their human needs met. Hunger and poverty would be substantially reduced. And, the natural systems that support life on the planet would be preserved.

Nobel Laureate Kenneth Arrow, a prominent economic theorist of the 20th century, talks about how to measure the impact of the anthropogenic causes of change.

There is no consensus on desirable levels of consumption, or even what "sustainable consumption" looks like. Read more with Alex de Sherbinin.