Lots of accusations are flying around in the climate change debate. People who believe in anthropogenic (human-caused) climate change are accused of practicing religion, not science. People who don’t are called “deniers”, which some of them think is an attempt to draw a moral link with holocaust deniers. Al Gore referred to Sarah Palin as a “climate change denier,” and Palin immediately responded that she believes the climate changes, she just doesn’t think the level of greenhouse gases in the atmosphere has anything to do with it. What’s the right word to use for people like her? And yes, we do need some terminology if we want to be able to discuss the climate change debate!
In a recent blog entry about Climategate I used the term “climate change skeptics” to refer to people who are convinced that the climate effect of trebling or quadrupling the concentration of carbon dioxide in the atmosphere will be small or nonexistent. I used that term instead of “denier” because I’m not trying to pick a fight by using a term that some people feel is morally loaded. But the term “skeptic” isn’t right either, at least for some of these people. A skeptic is someone who isn’t convinced that something is true; that’s not the same as being completely convinced that the thing is not true.
Maybe we should start characterizing people by a single number, as follows. What probability do you assign to the following statement: increasing the atmospheric carbon dioxide concentration above 800 ppm will change the global average surface temperature by more than 2.5 degrees C (4.5 F)? This would imply a climate sensitivity somewhat below the extreme low end of what the Intergovernmental Panel on Climate Change says is credible. (The standard way of looking at climate sensitivity is to quantify the effect of doubling the pre-industrial carbon dioxide concentration, but that number is only relevant when you scale it somehow to much higher concentrations, since we are going to blow way past a mere doubling. I’ve chosen 800ppm, which is a bit less than triple the pre-industrial level. It’s likely that we’re going to zoom right past 800ppm, too).
This single “subjective probability” wouldn’t capture all of the variation in what people believe — two people with the same probability might differ enormously on what they think the response should be (if any); what they think the most likely temperature increase would be at that concentration; whether they think the biggest problems, if that temperature increase occurs, will be related to sea level rise or droughts or storms or floods; and so on. But at least using a number would avoid the pitfalls of having to choose terms — “denier”, “true believer”, and so on — that raise people’s hackles.
I’m not really serious about getting everyone to quote a number, I know it’s not possible: people aren’t going to talk about “0.1 percenters” instead of “denialists.” But I’m not at all convinced that it’s a bad idea, if we could make it happen. Just thinking about the issue, “what’s my number”, really does help me focus on my assessment of the evidence. This phenomenon will be familiar to anyone who has ever said “I’m sure” about such-and-such, and has had his bluff called by someone who says “OK, let’s bet on it, and if you’re really sure you should be wiling to give me 20:1 odds.”
If you believe the IPCC, your number would be…well, I don’t know exactly, perhaps a quantitative probability can be determined from their uncertainty range (I doubt it)… but considering the implied climate variability would be below their lowest credible estimate, someone who accepts the IPCC report completely would have a number somewhere over 95%, I think. Me, I’m a bit more skeptical, and I’d put the probability at 90%. I think there’s a 90% chance that 800ppm will be disastrous.
Everything above is, I hope, non-controversial. I don’t mean climate change is non-controversial, I mean that I think I have fairly characterized the IPCC position, Sarah Palin’s response to Al Gore, my personal assessment of the strength of evidence for anthropogenic climate change, and so on. I probably just ought to leave it at that, but I’m not going to, I’m going to go one step farther and tell you what I think is reasonable.
If someone says their number is 99.99% or higher, that is unreasonable. There is definitely more than 1 chance in 10,000 that the scientists have it wrong and the effect of CO2 will be much less than they think. If you want to say that people with this level of certainty are practicing religion rather than science, you’ll get no argument from me.
If someone says their number is 99.9%…well, would they _really_ offer 1000:1 odds? I don’t think this level of certainty is justified by the science, and in fact I would take this bet: I’d put $1000 on the long side, with a chance to win $1,000,000. This isn’t anything like the crazy level of certainty of a 99.99 percenter — hey, they’re allowing 10 times as big a chance of being wrong! — but it’s pretty far out there. I’d call it unreasonable, but not crazy.
99% is a reasonable number. So is my number of 90%. I think even 10% might be a reasonable number. Once you get below that, you’re in “pretty far out there” territory, the skeptical counterpart of the 99.9 percenter.
And, just as I think that lopping off a factor of 10 in the probability of error by going from 99.9% to 99.99% takes you from “pretty far out there” to “completely unreasonable” on the high side, I think lopping off a factor of 10 by going from 10% to 1% on the low side takes you from “pretty far out there” to “unreasonable.” If you really think there is less than a 1% chance that 800ppm of CO2 will have a major climate effect, I think you’re practicing religion, not science.
If I wouldn’t find a very low climate sensitivity shocking — I’m allowing a 10% chance — then should I be shocked to find that other people are completely convinced the climate sensitivity is very low? The answer is Yes. As with Andrew’s musings about the probability that the world’s greatest boxer will beat the world’s greatest wrestler, there are some subtleties in thinking about this kind of thing. The strength of the evidence is heavily on the side of climate sensitivity not being extremely low. It’s one thing to not allow yourself to be fully convinced by the evidence; it’s another to discount the evidence altogether.
I invite comments about this, of course. But I have one request: along with your comment, state your number. I think it’ll be fun. I also think that you will find that, if you take this exercise seriously, it makes you think hard about what you really think about the strength of the evidence, pro and con, for anthropogenic climate change.