Roger Pielke Jr. has offered an interesting framework for watching the fallout from Hans von Storch's paper in Science last week challenging Michael Mann's "hockey stick" climate graph:

In this highly politicized atmosphere, given how many scientists spoke out in support of or against Mann et al.'s "hockey stick" it will be very interesting to see reactions among scientists to Von Storch's new paper. (It won't be so interesting to see how advocacy groups react, as it will be completely predictable.) Specifically, given the close connection of support or refutation of the earlier paper with explicit political agendas, scientists who were critical of Soon and Baliunas may be very hesitant to comment on Von Storch et al., except in a negative way. Conversely, we can expect howls of support from those scientists who supported Soon/Baliunas.

Mann's hockey stick is the iconic graph, based on proxy reconstructions of past climate, suggesting that the current warming is unprecedented over the past 1,000 years. The new von Storch paper suggests that Mann's technique essentially understates variability - that noise in the climate data recorded in proxies like tree rings effectively masks climate swings, making them appear smaller than they really were.

Andrew Revkin in the New York Times has a good explanation of the science, as does New Scientist. I finally got around to reading the paper last night, and I don't have a lot to add to the explanation of the science itself. It's pretty clear that if von Storch et al. are right, then Mann's hockey stick says something very different than what we thought it did.

From a scientific point of view, this is exciting. Proxy reconstructions are hard, and this is the way science moves forward - thoughtful challenges and aggressive skepticism. Past is prologue, and understanding the way the climate has behaved in the past is critical to understanding what we might expect in the future.

But in the rhetorical climate wars? This is only a huge deal because people on both sides of the global warming debate have misused poor Dr. Mann's hockey stick.

For those in favor of action to reduce greenhouse gas emissions, the hockey stick has been a powerful tool, a perfect PowerPoint slide: throw it up on the screen, point at it with your laser pointer and utter the phrase "unprecedented warming." That's rhetorically powerful. (It also, I think, misses the point. More on that in a moment.)

Those opposed to action to reduce greenhouse gas emissions have made the hockey stick a central focus, acting as though all they need to do is knock Dr. Mann's legs out from under him in order to doom the whole global climate change enterprise. (In the most egregious example, the Calgary Herald, in an editorial last summer, called the hockey stick and the paper in which it first appeared "the sole evidence for the IPCC’s support of the human-induced climate change theory.")

Here, I think, is a useful framework for thinking about where the hockey stick fits in. It's a series of questions that I think are relevant to understanding the issue and role of the hockey stick in it.

1. Is it warming?
2. Are humans causing it?
3. Will it be bad?
4. If so, can the change be prevented, reduce, or mitigated?
5. What are the costs and benefits of the steps necessary to prevent, reduce or mitigate it?

Nobody seriously debates the answer to one any more. It's warming.

The hockey stick might seem relevant to the answer to two, the attribution problem, but it's not, at least not beyond some small margin of understanding. The reason it seems so relevant is the rhetorical power of the hockey stick's image of unprecedented change. If it's unprecedented, doesn't that mean humans must be responsible? But that's just rhetoric, not science. The science of attribution is based on issues far more subtle than the question of whether or not it's unprecedented.

The question of precedence also has rhetorical power on the side of those who oppose action to reduce greenhouse gas emissions. They would argue that the possibility such change has happened before as a result of natural causes weakens the case for doing something about it in this case. But that's clearly fallacious. What happened before is not relevant, except again marginally, to the question of whether it would be bad if it happened now. The fact that the planet warmed a thousand years ago as a result of cause "A" would not make it any less troublesome if it happened again now as a result of cause "B," whatever "A" and "B" might be. (In fact, we know that climate changes in the past, natural ones, have had disastrous effects on human societies.)

My "will it be bad" question is, I realize, a bit of a cheat, a transition point between what science can say and what politics and policy must do. Science can only offer some useful explanation of what the physical changes might be. The question of whether that would be bad is a question of policy, politics and values. Science also can offer data to help inform the final two questions - though I must admit that my reading of the state of the science there is that there is much work to be done in informing us about what effects various carbon reduction strategies might have.

But it's clear that the life or death of the hockey stick does not fundamentally alter what we know or don't know in answer to any of my five questions.