Monday, August 23, 2010

Visual impact

OJ Simpson's defense attorneys knew something that the prosecution didn't - that wet leather shrinks when it dries. You'd think that this would make the prosecution's case even stronger: the blood-soaked gloves shouldn't fit, because they had been blood soaked.But they used this knowledge to make a courtroom display that burned through the masses of data, and lodged squarely in the jury's mind. Johnny Cochran captured this is a phrase that advertising people must admire for its memorability: It the glove does not fit, you must acquit.

This visually striking moment was when the trial was basically over. People remembered the image, not the data.

Here's another image that captured the public's imagination just a few years after OJ's trial.

It's the "Hockey Stick" temperature graph from the cover of the UN's Intergovernmental Panel on Climate Change (IPCC) - specifically, the Third Assessment Report (AR3) in 2001. The graph itself was from Michael Mann et al, in a 1998 paper. There's background on this paper here, but the point is that this graph had a similar impact to OJ's glove: it captured the imagination of the public. It is reasonable to argue that this graph led directly to the Kyoto Protocols, and ultimately to today's proposals like Cap And Trade.

Just like OJ's glove, it gets interesting when you look into the data itself. We don't have temperature records going back a thousand years, so Mann et al used proxies - things that seem to work sort of like thermometers, but which aren't. They used tree rings, because trees grow faster in warmer years and slower in colder ones. No problem so far - this is pretty standard in the field.

What Mann did that has been the source of controversy is use a pretty unusual statistical analysis method. It's been shown that his method generates hockey stick shaped graphs when even random data is used as the input - say, the list of telephone numbers in your town. So far, the debate about this has had more than a bit of the playground did too/did not to it. This means that the hockey stick graph has won, because it is so visually striking.

That may be ending. There's a new paper out, written by professional statisticians, that pretty well breaks Mann's approach (including his revised 2008 method). You can find overviews here and here, but in my mind the most interesting commentary comes from William Briggs (himself a statistician), concerning another striking graph:

The jaggy red line is their prediction, over which they lay bands of uncertainty due to various factors. Just look at that envelope of possible temperatures!—the dull gray window. The straight yellow line is mine: notice how it slides right through the envelope, never poking out through it at any point. This suggests that a flat-line, i.e. unchanging, temperature fits just as well as the boys’ sophisticated model. At least, the unchanging “null” model cannot be rejected with any great certainty.

This is just as beautiful as a shorthanded goal. It means we cannot tell—using these data and this model—with any reasonable certainty if temperatures have changed plus or minus 1oC over the last thousand years.

What this is trying to visually represent is that the uncertainties are huge - plus or minus 5° or more, at least until we get to the modern age with the thermometer record. Mann's "there was no Medieval Warm Period" reconstruction is no more valid - mathematically speaking - than "there has not been a climate change in 1000 years".

Scientifically, this is enormous - certainly the biggest news in climate science since ClimateGate last autumn. One of the harshest criticisms leveled against scientists postulating rapid warming is that they underestimate/under report the uncertainties in their calculations. This paper quantifies that.

Overall, does it matter? The hockey stick is still a powerful visual image, and because of that it's not likely to go away. But it doesn't seem to be working - the public doesn't seem to be buying the scare (and note that this was before the ClimateGate scandal broke). Ultimately, this isn't a scientific issue, it's is a political one. The public doesn't believe that there's a need to act, just like the public believes that OJ killed his wife.

Hockey stick or not, the public has the image of corrupt scientists and politicians lodged in its mind. That image isn't going away.


bradley13 said...

This may be huge, if you are a statistician. If you are the average Joe on the street, the paper is unreadable. It doesn't even provide any nice sound bites that can be taken out and used to explain it. Worst of all, the hallmark graph (which you show) still looks like a hockey stick.

Really, on the popular front, this paper is a failure.

B said...

The paper may be a failure, but the factis that espite the hype, (and the climategate hype as well, few people are believing thtthre is any validity to any viewpoint besides their own.....Either you believe tha the climate is changing for the worste, or you don't. Until someone can accurately predict the weather more than 2 days out, few of us proles will believe anything other than what we want to believe:

Either Mother Gaia is angry, or that there is nothing more than seasonal variations.

The "hockey stick" only convinced those who wanted to believe.