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Michael Mann vs. Nate Silver on Global Warming

Ask Nate Silver about global warming skeptics, and he will say something like:

One type of skepticism flows from self-interest. In 2011 alone, the fossil fuel industry spent about $300 million on lobbying activities.... What they say should not be mistaken for an attempt to make accurate predictions.... A second type of skepticism falls into the category of contrarianism. In any contentious debate, some people will find it advantageous to align themselves with the crowd, while a smaller number will come to see themselves as persecuted outsiders.... “If you look at climate, if you look at ozone, if you look at cigarette smoking, there is always a community of people who are skeptical of the science-driven results,” Rood told me.

Most importantly, there is scientific skepticism.... Scott Armstrong, a professor at the Wharton School at the University of Pennsylvania, is such a skeptic.... [His] critique of global warming forecasts is that they are unrealistically complex, the alternative would be a simpler forecast.... Suppose... one... looked solely at CO2 levels and temperatures, and extrapolated a prediction from these variables alone.... How accurate would such a prediction have been? In fact, it would have been very accurate—quite a bit better, actually, than the IPCC’s forecast. If you had placed the temperature record from 1850 through 1989 into a simple linear regression equation, along with the level of CO2 as measured in Antarctic ice cores and at the Mauna Loa Observatory in Hawaii, it would have predicted a global temperature increase at the rate of 1.5°C per century from 1990 through today, exactly in line with the actual figure....

The Armstrong and Green critique of model complexity thus looks pretty good... [but] Armstrong’s critique may have won the battle but not the war.... [Since] the simple methods correctly predicted a temperature increase in line with the rise in CO2, they are also evidence in favor of the greenhouse-effect hypothesis. Armstrong’s no-change forecast, by contrast, leaves some of the most basic scientific questions unanswered. [Armstrong's no-change] forecast used 2007 temperatures as its baseline, a year... warmer than all but one year in the twentieth century. Is there a plausible hypothesis that explains why 2007 was warmer than 1987 or 1947 or 1907—other than through changes in atmospheric composition?... Armstrong told me he made the no-change forecast because he did not think there were good Bayesian priors for any alternative assumption.... [A]s Armstrong told a congressional panel in 2011,98 “I actually try not to learn a lot about climate change. I am the forecasting guy.”

This book advises you to be wary of forecasters who say that the science is not very important to their jobs, or scientists who say that forecasting is not very important to their jobs! These activities are essentially and intimately related. A forecaster who says he doesn’t care about the science is like the cook who says he doesn’t care about food. What distinguishes science, and what makes a forecast scientific, is that it is concerned with the objective world. What makes forecasts fail is when our concern only extends as far as the method, maxim, or model.

But if Armstrong’s critique is so off the mark, what should we make of [the fact that]... global temperatures did not increase at all in the decade between 2001 and 2011.... As the growth rate of atmospheric CO2 increases... periods of flat or cooling temperatures should become less frequent. Nevertheless, they are not impossible, nor are the odds anything like 100-to-1 against them. Instead, if you assume that CO2 levels will increase at the current pace of about 2 ppm per year, the chance that there would be no net warming over the course of a given decade would be about 15 percent according to this method....

In practice, [Michael] Mann’s street fight is between “consensus” Web sites like and “skeptical” ones like Watts Up With That, and revolves around day-to-day scuffles about the latest journal article or weather pattern or political controversy. Both sides almost invariably stick up for others in their circle and refuse to yield ground. When you’re a Jet, you’re a Jet all the way. I do not mean to suggest that the territory occupied by the two sides is symmetrical. In the scientific argument over global warming, the truth seems to be mostly on one side: the greenhouse effect almost certainly exists and will be exacerbated by manmade CO2 emissions. This is very likely to make the planet warmer. The impacts of this are uncertain, but are weighted toward unfavorable outcomes.

The street-fighter mentality, nevertheless, seems to be predicated on the notion that we are just on the verge of resolving our political problems, if only a few more people could be persuaded about the science. In fact, we are probably many years away.... What I do know is that there is a fundamental difference between science and politics. In fact, I’ve come to view them more and more as opposites. In science, progress is possible...

This makes Michael Mann unhappy:

Nate devotes far too much space to the highly questionable claims of a University of Pennsylvania marketing Professor named J. Scott Armstrong... [who] made a name for himself in denialist circles back in 2007 by denouncing climate models as having no predictive value... Armstrong's arguments were... belied by a large body of primary scientific literature.... As discussed in detail by my co-founder, NASA scientist Gavin Schmidt, Armstrong simply didn't understand the science well enough to properly interpret, let alone, assess, the predictive skill of climate model predictions.

That Nate would parrot Armstrong's flawed arguments is a major disappointment, especially because there are some obvious red flags.... Armstrong... [has] close ties to fossil fuel industry front groups like the Heartland Institute, which earlier this year campaigned to compare people who accept the reality of climate change to the Unabomber.... I suspect Nate's failing here arises from a sort of cultural bias. There is a whole community of pundits with origins in economics and marketing who seem more than happy to dismiss the laws of physics when they conflict with their philosophy of an unregulated market. Nate may not share that philosophy, but he was educated by those who do....

Most disappointing to me of all was the false equivalence that Nate draws between the scientific community's efforts to fight back against intentional distortions and attacks by an industry-funded attack machine, and the efforts of that attack machine itself. He characterizes this simply as a battle between "consensus" scientists and "skeptical" individuals.... This framing is flawed on multiple levels, not the least of which is that those he calls "skeptics" are in fact typically no such thing... denialism posing as "skepticism" for the sake of obscuring, rather than clarifying, what is known.... Nate makes it sound like the "street fight" was of the scientists choosing, completely turning on its head what Nature was actually talking about: scientists finding a better way to defend science from cynical attacks whose sole aim is to confuse the public....

[T]his book was a lost opportunity when it comes to the topic of climate change. Nate could have applied his considerable acumen and insight to shed light on this important topic. But the result was instead a very mixed bag of otherwise useful commentary marred by needless misconceptions and inappropriately laundered denialist memes.

My reaction is that Michael Mann needs some more practice in street fighting.

Had I been Mann, I would have said something like:

Nate Silver's useful chapter underscores how solid and effective at forecasting even the simplest greenhouse-gas based climate models are, and rightly warns against paying attention to climate forecasters like Armstrong who claim that knowing the climate physics is not very important. My major complaint is that Silver puts Armstrong in his category III of critics--scientists with serious questions"--when long experience has led me to believe that he belongs in category I--energy industry spokesmen exercising their constitutional right to speak in order to mislead and confuse.

I agree think that anybody who holds himself out as an expert on climate change while expressly claiming that he "try not to learn a lot about climate change. I am the forecasting guy" is either (a) an idiot, or (b) an industry shill. But that Armstrong is such does not mean that the more general "overfitting" arguments do not need to be considered thoughtfully...