Bradford Plumer has some extremely kind things to say about my earlier post on climate change, followed by some very intelligent criticisms. Everybody should wish for critics this graceful and informed. Let me see if I can address his criticisms one at a time.

Plumer begins with this:

I see a couple problems with this argument. The first is that Manzi is clinging way too tightly to the IPCC report. Yes, the IPCC puts out the best summary of scientific knowledge about our climate system. I rely on it all the time. But the 2007 report is also dated. Climate science is a rapidly moving field, and more recent research has suggested that things may be bleaker than was projected three years ago.

There are recent individual studies that argue damages will be worse than those estimated in the governing Fourth Assessment Report. But the essential step that Plumer makes is to argue that therefore a rational observer should update this forecast and expect more severe damages. Of course, there is a long history of individual studies that project greater future warming or damages than the then-consensus estimates that have not changed subsequent consensus estimates. These go all the way back to the first major attempt to create such a consensus estimate, the NRC Charney Commission of 1979. In spite of these frequent challenges, however, the key scientific estimate for climate sensitivity—the central scaling parameter that projects how much warming will be created by a given amount of emissions—has remained essentially unchanged for decades, from the Charney report through each of the four successive IPCC assessment reports.

Synthesizing the various individual studies that have been released over the past several years to determine if projected damage estimates need to be increased requires integrating knowledge from numerous physical science specialties, software engineering, economics, and various civil and energy engineering disciplines. I’m not competent to do it; but then again, no one person or small group is competent to do this—if it were, why would we bother with the immense time and expense of the IPCC assessment process? We’d just go ask that guy to tell us the answer.

Plumer proceeds to his second criticism:

Second, it's a bit too simplistic to use a single global GDP figure when talking about the effects of climate change. True, a 3 percent drop in global GDP may not sound so bad. We'll all be much richer in 2100, we can take a hit. But that top-line figure can obscure some serious distributional issues.

Plumer is correct that the global poor will be disproportionately affected by climate change damages. But it is also the case that they would be disproportionately affected by any reductions in global economic growth, and there is at least some short-term trade-off between economic growth and mitigation of climate change damages. An empirical question is the relative size of these two effects.

Consider Indur Goklany’s 2008 review of the major research projects that compare the change in climate and wealth under various UN IPCC emissions scenarios. I’ll show the two extreme scenarios to make a point: A1F1 (the IPCC scenario for global development that is most heavily dependent on fossil fuels, and has a projected increase in global temperature of about 4C by the end of the century), and B1 (the scenario that assumes greatest deployment of alternative technologies, and has a projected increase in global temperature of about 2C by the end of the century). Here are the projections for each scenario for the developed then the developing worlds:

Developed Countries Projected GDP / Capita in 2100:

A1F1: $107,300 B1: $72,800

Developing Countries Projected GDP / Capita in 2100:

A1F1: $66,500 B1: $40,200

In other words, at least through the next 100 years, the average person living in the developing world is better off in money terms with more economic development and more climate change damage, on net. A lot better off in fact: $66,500 is more than 65% higher than $40,200.

 Plumer then proceeds to a third criticism.

Harvard economist Marty Weitzman has recently been arguing that there's plenty of uncertainty in climate projections, and the worst-case scenarios could be really freaking bad. Like, civilization-destroying bad. And that prospect, even if it's slim, is a great reason to cut emissions—think of pollution curbs as an insurance policy against total annihilation. In reply, Manzi accuses Weitzman of doing "armchair climate science." But that's unfair.

I agree that it is useful to think of emissions mitigation as a kind of insurance policy against much-worse-than-expected damages (and in fact used precisely this analogy in my post). I also praised Weitzman’s argument at length in my post, but I don’t think that it’s unfair or inaccurate to characterize him as doing “armchair climate science.”

As I indicated earlier, the key scaling parameter in global warming analysis is climate sensitivity, conventionally denoted by the symbol S. Weitzman invents the notation of S1, to represent what he defines as “climate sensitivity narrowly-defined”, and S2, which he defines as “ a ‘generalized climate-sensitivity-like scaling parameter’ that includes heat-induced feedbacks on the forcing from the above-mentioned releases of naturally-sequestered GHGs, increased respiration of soil microbes, climate-stressed forests, and other weakenings of natural carbon sinks.” He takes the probability distributions in the IPCC reports as representing S1. Weitzman then uses a single ice bore study to generate the one number that allows him to lever up S1 into his much larger estimate of S2 as the basis for all of the numerical calculations that follow, which I argued are central to his conclusions. Using this crucial calculation in the relevant passage of the paper, Weitzman says:

Without further ado I am just going to assume for the purposes of this simplistic example that [doubling CO2 has a 1 percent chance of eventually causing a temperature increase of more than 20C], which I will take as my base-case tail estimates in what follows. These are wildly-uncertain unbelievably-crude ballpark estimates—which are most definitely not based on hard science—of small probabilities of what amounts to huge climate impacts occurring at some indefinite time in the remote future. [Sorry for the long bracketed part above, but I was sparing you some nasty mathematical notation.]

I call that armchair climate science. If you think that is pejorative, please feel free to pick the label that you prefer. Whatever you choose to call it, however, I don’t think that this is a reliable basis for forgoing trillions of dollars of cumulative global consumption.

Plumer then proceeds to a fourth criticism:

It's possible I'm misunderstanding the point here, but Manzi seems to be suggesting that, because there's a near-infinite number of potential threats lurking out there, we shouldn't extend ourselves too far to address the big ones staring us in the face—on account of the opportunity costs.

I was arguing that consensus science doesn’t support the idea that climate change is “the big one.” Its expected costs don’t justify aggressive emissions mitigation, its odds-adjusted costs don’t justify this either, and the unquantifiable danger represented by the possibility of damages beyond anything projected in the probability distributions of consensus science can’t be distinguished practically from many other such dimly-understood dangers.

 Plumer finally presents a fifth criticism:

The other big dispute here is over the costs of averting drastic climate change. Is Manzi right that cutting carbon emissions would "cripple our ability to … lead productive and interesting lives"? This seems awfully outlandish.

That would be an outlandish claim for me to make—especially as I argued in the post that the costs of a typical emissions mitigation program would on the order of 6 percent of GDP by 2100. Here is the complete sentence from which this quote is taken:

A healthy society is constantly scanning the horizon for threats and developing contingency plans to meet them, but the loss of economic and technological development that would be required to eliminate all theorized climate change risk (or all risk from genetic technologies or, for that matter, all risk from killer asteroids) would cripple our ability to deal with virtually every other foreseeable and unforeseeable risk, not to mention our ability to lead productive and interesting lives in the meantime. [Italics added]

This was in the context of an extended argument in my post that the Precautionary Principle is not a useful guide to action because it provides no non-arbitrary stopping point for actions against climate change, as we would be chasing an ever-receding horizon of zero danger without regard to costs. It was not a statement about any proposed emissions mitigation plan.