(2 of 3)
First, environmental problems can indeed cause societies to collapse, even societies assaulting their environments with stone tools and far lower population densities than we have today.
Second, some environments are more fragile than others, making some societies more prone to collapse than others. Fragility varies even within the same country: for instance, some parts of the U.S., including Southern California, where I live, are especially at risk from low rainfall and salinization of soil from agriculture that is dependent on irrigation--the same problems that overwhelmed the Anasazi. Some nations occupy more fragile environments than do others. It's no accident that a list of the world's most environmentally devastated and/or overpopulated countries resembles a list of the world's current political tinderboxes. Both lists include Afghanistan, Haiti, Iraq, Nepal, Rwanda and Somalia.
Third, otherwise robust societies can be dragged down by the environmental problems of their trade partners. About 500 years ago, two Polynesian societies, on Henderson Island and Pitcairn Island, vanished because they depended for vital imports on the Polynesian society of Mangareva Island, which collapsed from deforestation. We Americans can well understand that outcome, having seen how vulnerable we are to instability in oil-exporting countries of the Middle East.
Fourth, we wonder, Why didn't those peoples see the problems developing around them and do something to avoid disaster? (Future generations may ask that question about us.) One explanation is the conflicts between the short-term interests of those in power and the long-term interests of everybody: chiefs were becoming rich from processes that ultimately undermined society. That too is an acute issue today, as wealthy Americans do things that enrich themselves in the short run and harm everyone in the long run. As the Anasazi chiefs found, they could get away with those policies for a while, but ultimately they bought themselves the privilege of being merely the last to starve.
Of course, there are differences between our situation and those of past societies. Our problems are more dangerous than those of the Anasazi. Today there are far more humans alive, packing far greater destructive power, than ever before. Unlike the Anasazi, a society today can't collapse without affecting societies far away. Because of globalization, the risk we face today is of a worldwide collapse, not just a local tragedy.
People often ask if I am an optimist or a pessimist about our future. I answer that I'm cautiously optimistic. We face big problems that will do us in if we don't solve them. But we are capable of solving them. The risk we face isn't that of an asteroid collision beyond our ability to avoid. Instead our problems are of our own making, and so we can stop making them. The only thing lacking is the necessary political will.