When civilization stands at the edge of a cliff, a step forward doesn't make much sense.

Listen up, you couch potatoes: each recycled beer can saves enough electricity to run a television for three hours.

These are not exhortations from overwrought extremists, but carefully phrased warnings from some of the world's finest scientists.

The more subtle thing is more speculative. The world is well past its long-term carrying capacity for human beings living a European, much less an American, lifestyle predicated on planned obsolescence. International economic growth is largely a matter of accelerated movement of materials from mines and forests to the dump. Instead of saving and buying decent furniture we can pass on to our children, we charge our credit cards for shaped heaps of sawdust and glue that fall apart in less than three or four years.

There are a few obvious consequences and perhaps one subtle possibility. One obvious thing is that, to stimulate the economy, President Obama has committed to creating millions of green jobs that will leave a legacy - much as Roosevelt's public works did during the new deal.

With time environmental issues got much more complicated. It is pretty easy, if you know what you are doing, to stop a company from pouring poison into a lake where kids swim. It is much harder to address all the myriad greenhouse gases emitted by different sources - from petrochemical refineries to hundreds of millions of peasants cutting down trees for their incredibly inefficient cook stoves.

As a student of conservation biology, I believe that characteristics with survival value will ultimately prevail. There is no survival value in pessimism. If you think failure is inevitable, that view will probably become self-fulfilling.