wilderness areas in them where you can enter and see the planet as it was before humanity began to transformt, then we have a choice. we should not let an irreplaceable resource -- human resource if you want to call it that -- the natural places in the world disappear because once they're gone, they're gone forever. narrator: humanity has reached a point where we affect the earth at the planetary level. the big unknown that influences all earth systems is climate change. linked to human emissions of greenhouse gases like carbon dioxide, global climate change is already changing the earth system. schrag: one often hears environmental scientists talking about tipping points. tipping points are parts of the earth system, when pushed to some threshold, they actually respond in a very abrupt way generally collapsing. we could be talking about fisheries, where we fish up to a certain point, and then all of a sudden as we fish any more, the population collapses. we could be talking about a tropical forest, where we start cutting it down and everything looks okay, until suddenly we cut one tree and a