Climate varies continuously, but with occasional spikes and excursions and long periods of slow change, and at different rates; any condition it may happen to be in now is simply described relative to some condition it may have been in before. It probably makes no sense to even talk about a "normal" or "typical" climate. Even the idea of an "average" climate would be misleading, since at any one time there are so many different conditions simultaneously occuring throughout the world.

From the point of view of living things, including us, an undesirable climate change would be one in which changes in our environment occured faster than we could adapt to them, biologically or technologically. No doubt, whatever happens to the climate will generate both winners and losers, but if organisms and ecosystems adapt slower than the climate is changing, the losers will by far outnumber the winners. As humans, we are extremely adaptable, but if our climate ever changes faster than we can, we are in big trouble.

What amazes me is that although the climate appears to be as fickle and changeable as the weather, over the last few billion years it has somehow managed to stay within the limits that life can tolerate. It does not evolve, it is in a state of dynamic equilibrium. In other words, there have been no runaway excursions leaving a lifeless wasteland like Venus or Mars. This is really remarkable, and suggests that there are some very effective natural feedback mechanisms at work which help keep this planet habitable. Using the constraint of the temperature range at which water remains liquid, at the longest time scales, climate appears to be extremely stable. I can think of very few natural systems, even in astronomy, which remain within such narrow limits for such long periods of time.

The question arises, is this kind of stability common in the cosmos? Are climatically stable bio-friendly earthlike planets the norm, or are we very, very lucky?