I don't think climate change will turn the Earth into another Venus--or a Mars, for that matter. I believe it is irresponsible to suggest that AGW will destroy the planet. We know there have been violent swings in climate in the past, caused by things like continental drift, or orbital variations, or solar evolution. But they have been gradual. Sudden changes result from things like asteroid strikes or violent, continent-wide volcanic episodes. No doubt there are other causes too, things we haven't even thought of.

But we also know the Earth, and Life, have an enormous capacity to absorb stress, survive damage, and evolve. Earth Abides, as the Good Book tells us. We may get an occasional Ice Age or lose a dominant life form like the Terrible Lizards, but life has never been wiped out. In the long run, it has endured and even flourished. It can adapt itself to just about any disaster, sudden or slow, and the atmosphere and oceans are filled with feedback loops and self-modifying systems which can adjust to change, perhaps at some new level of stability, but stability nevertheless. A 4 billion year history proves this without a doubt.

What is much more likely to happen is that the Earth's atmosphere and oceans will adjust until a new plateau of stability is achieved, as it has done innumerable times in the past. This process goes on all the time, naturally, even without the provocation of extraordinary events. Global climate changes over geologic time scales just like local weather changes over the course of a year, dramatically, but within limits. The question is can our industrial civilization adapt to keep up with that change within a generation or two?

We can now alter the planet by our presence, our agricultural practices, resource extraction and industrial activities. We can't be sure what we are witnessing now is evidence of this, but neither should we dismiss it as impossible simply because it hasn't happened during our recorded history, or because it might be inconvenient for our investments. Most of our stay on Earth has been low-impact. The Industrial Revolution and the population explosion are only a couple of centuries old, a heartbeat in Earth's history; and we often forget how dependent contemporary civilization is on relatively minor things, like abundant rainfall in the Midwest or sufficient snowfall in the Rockies, or relatively mild winters in Northern Europe. The changes we are seeing now are not going to take centuries and we won't have time to adjust. Summer Arctic sea ice has dropped forty percent over the last 30 years, this has never happened before, excpt in times of global catastrophe. Perhaps we are that catastrophe.

We can't count on the rapid changes we are seeing in the Arctic being reversed by natural adjustments, they may continue to affect some of the environmental conditions we have come to depend on. Neolithic cultures could survive in the shadow of the glaciers in Ice Age France. Modern man cannot.