2005 has been an extraordinary year for natural disasters and unusual weather, and there are still two months remaining. People of many persuasions are looking at these events for signs, whether of millenarian portents or signposts of the accelerating impact of human activities on the earth's climate. We've already set a new record for the number of named Atlantic basin cyclonic storms, following a year that was no slouch, either. Are all these hurricanes telling us something about the climate, or are they just bad weather?
Several months ago, I cited a scientific paper correlating warmer ocean temperatures with more intense hurricanes, suggesting that further climate change will deliver more hurricane seasons like the one that's now nearing its official end. Nevertheless, the author of that paper is adamant that Katrina and Rita should not be claimed as evidence in support of climate change. If you find that distinction confusing, you aren't alone.
Part of the difficulty in filtering out the noise from any signal here lies in the distinction between climate and weather. Weather is what we experience day to day and year to year, while climate is a long-term picture at the local, regional, or global level, over time spans that are long relative to human perception. "Climate change" isn't just a bureaucratic euphemism for global warming; it's an accurate description of the territory that scientists must examine when they look for evidence of global warming. A single year's weather, however bizarre, may or may not represent a statistically significant departure from the previous norm.
The other problem is a human one. Because we're only alive for "threescore and ten", give or take, there's a natural tendency to assign extra significance to events that occur during our lifetimes. Hurricane Katrina affected people we know, or at least could see on TV, while the comparably destructive hurricane of 1900 is now just a historical footnote, even though it permanently altered the course of development of east Texas. (Without it, "Galveston", not "Houston" might have been the first word spoken from the Moon.)
I have to agree with Dr. Emanuel that it's probably premature to assign extra significance to the hurricanes of the last two years. At the same time, though, these storms--along with recent pictures of a summertime ice-free path around the north pole--provide worrying glimpses of a future in which climate change could alter the planet in ways that we may not find conducive to "life, liberty, and the pursuit of happiness." So while they may not serve as evidence, these hurricanes are still important signposts.