Bradford Plumer quotes at length a post called “Debunking Jim Manzi in 5 Easy Steps.” He lost me at hello.

Here is what the author of the original post says in Step 1 of his debunking:

1.       "There is only one SRES scenario that reasonably tracks real world emissions growth per observations and infrastructure legacy: A1FI."

There’s a lot of jargon there for the uninitiated. The background is that the IPCC considers a range of scenarios (“SRES scenarios”) for global development over the next century. Each scenario is an integrated picture of population growth, economic development, technological change, and so forth. They describe six of these as “marker scenarios” that are meant to represent a reasonable range of possible future paths of development. The IPCC says that all individual marker scenarios “should be considered equally sound.” The IPCC also generates projections for a range of possible warming levels for each scenario. The scenario labeled A1F1, as indicated in my original post, is the one of these six with the highest projected emissions over the course of twenty-first century, and therefore the one with the greatest projected warming.

In plain English, the author is claiming in Step 1 that emissions over the past several years have tracked scenario A1F1 more closely than others, and that because of this, plus our “infrastructure legacy,” we should use scenario A1F1 alone when estimating the possible range of warming the world will face in the future. He then proceeds to use this as the starting point for an alternative cost / benefit comparison that “debunks” my argument in Steps 2 through 5.

But presumably, once the premise is clearly stated, the problems with the purported debunking become evident.

First, as I made clear in my initial post, I used the IPCC governing assessment report as the technical basis for my reasoning. Obviously, the IPCC doesn't believe that there is no point in considering alternative scenarios, or they wouldn’t include any. They would simply call A1F1 “the baseline” and get on with it. If the author wants to argue that they’re wrong, fair enough. But then he should have titled his post “Debunking the IPCC in 5 Easy Steps.”

Second, the IPCC was wise to consider a wide range of scenarios. We can no more precisely predict human progress or folly over the next century than an observer in 1910 could have predicted the course of the twentieth century. If emissions have more closely tracked those estimated for scenario A1F1 for the past ten years, that does not mean that we know they will continue to do so for the next 90. What course the world polity and economy follow over the rest of the century, and the resulting quantity of human emissions between now and 2100, is not dependent on some physical laws, but on how history unfolds.