Climate Science – PollyAnnas, Pessimists & the IPCC

Posted on May 30, 2011

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Ah, here’s Simple Sam.  Hi, Sam.  Come on over here.  I want you to look at a graph, or four graphs, really, that have been charted together.  Do you know what the diagram stands for?

“I’m sure you’re going to tell me.”

Yes, I am.  This diagram puts together four different estimates of climate sensitivity.  Each of the different kind of lines represents a different study.

“What do you mean by climate sensitivity?” asks Sam.

Well, in this case, the chart is attempting to set how much warming we can expect from a doubling of CO2 in the atmosphere.  Each of the lines sets out the probability of a particular rise in temperature occurring.  Where a line rises highest, that is the the most likely temperature increase according to that set of calculations.  As you can see, all the highest probability results cluster around a 2.5o to 3.0o C. rise in global average temperature with a doubling of CO2.

“So what are the pretty colours for?”

The pink represents the predicted range of climate sensitivities as set out by the IPCC, from 2 to 4.5o C.  The white areas on either side of the pink represent the possibilities that exist outside of this range.

“What do you mean exactly?”

Well, ideally all these graphs should be read separately.  Roughly, though, the area below any of the four lines adds up to 100% of the probabilities.  As you can see, the IPCC range represents the fat part of the probability curve.  The IPCC range has the highest probability of happening, according the four studies charted.  However that range doesn’t exhaust the probabilities.

The white area to the left of the IPCC range—under the yellow—represents the possibility that the climate is less sensitive to CO2 increases than the IPCC is assuming.

The white area to the right of the IPCC range—under the blue—represents the possibility that the climate is more sensitive to CO2 increases than the IPCC is assuming.

“So what’s your point?”

My point, Sam, is that the media has set up a false dichotomy in the climate change discussion.

“No, they haven’t.  The way the liberal media…”

Yes, they have, Sam.  If you think of it, the discussion in the media is almost always set out as an opposition between the IPCC point of view and those few PollyAnna scientists (about 3% of climate scientists according to studies) who assert that things aren’t as bad–or maybe aren’t as bad–as the vast majority of scientists say.

But mathematically, the possibility that things will be worse than what the IPCC says is much greater than the possibility that it will be better.  And, reflecting this, there are a fair number of scientists out there who say that the IPCC is lowballing climate change, and that we ought to be more worried than we are.  This group of scientists—let’s call them the Climate Pessimists—are almost unheard of in the discussion as we hear it in the media.  This is despite the fact, as this chart shows, it is much more likely that the Pessimists are right than the PollyAnnas.  And they most likely outnumber them as well, although I don’t think anybody has actually counted the Pessimists the same way they have counted the PollyAnnas.

The IPCC still has the strongest case.  But if I was going to pay attention to an alternative viewpoint, the Climate Pessimists are the most legitimate dissenters.

Anyway, when it comes to protecting people, PollyAnnas can be dangerous.  It was some PollyAnna, after all, who didn’t buy enough lifeboats for the Titanic.  That boat will never sink, he said.  We can skimp on the lifeboats, he said.

We all know how that worked out.