Since first reading about it in the 1970s, I've been intrigued by the Tunguska Event. It's one of those fascinating blends of fact and speculation that science has never been able to clear up completely. This much is known: in 1908 something exploded over a thinly-populated part of Siberia, flattening trees and throwing up huge amounts of dust. Explanations for the phenomenon have included an impact by an asteroid, a comet, a miniature black hole, and even an odd variation on an earthquake. Whatever caused it, it was a remarkable occurrence and unique in recorded history. Now, a Russian scientist has analyzed temperature data before and after Tunguska and concluded that it may have triggered the climate change we are now observing, by changing the distribution of water and ice crystals in the upper atmosphere. Could there be anything to this?
As Chaos Manor, the blog where I ran across this, points out, the response from climate scientists has been quick and dismissive, though their arguments seem heavy on handwaving and light on actual analysis. That doesn't mean they are wrong, however. It would be highly coincidental and awfully convenient if the whole problem of climate change could be blamed on a single event a century ago--especially such a mysterious one--letting all of us off the hook for our emissions of greenhouse gases. Things are rarely that simple.
At the same time, there's no question about the relative importance of water vapor as the primary greenhouse gas in the atmosphere, and anything that changed its proportions or distribution could indeed upset the climate, at least for a while. I'm skeptical, though, that Tunguska was large enough to do this, when volcanic eruptions and numerous atmospheric H-bomb tests in the 1950s and early 1960s, including a 50 megaton Russian test, presumably weren't.
So why even mention this item? It reminds us that we still don't know as much as we'd like about the complex interactions of the earth and its atmosphere. That shouldn't deter us from taking steps to manage the human contribution to the greenhouse effect, because that is still the explanation that best fits the observable evidence. But it does justify working concurrently on adaptation to climate change, in case it turns out that our emissions aren't the main show, or that we can't do enough to mitigate them.