Joe Romm has helpfully posted up his full reply to Nature on PWG (PDF), and we are happy to link to it as promised. And after reading Joe’s original letter and his comments, the source of his complaint — and confusion — is now clear. This post explains that Joe has confused the differences between different IPCC SRES scenarios with spontaneous decarbonization within each individual scenario.


Figure 1.png

The Figure above is from our Nature paper. It shows for the six SRES families (A1B, A1FI, A1T, A2, B1, B2) cumulative emissions to 2100. For now lets ignore the light blue part of each bar (which represents the spontaneous or automatic decarbonization that we discuss in the paper, and which I return to below).

Joe Romm points out in his critique:

The Special Report on Emission Scenarios (SRES), which the Commentary cites, makes clear that while the SRES scenarios don’t technically have climate policies, they can and do have energy efficiency and decarbonization policies, which are the same thing. That’s clear from examining the B1 scenario, which includes aggressive policies that help limit total global warming to about 2°C

He is correct in this assertion. The effect of these policies in the B1 sceanrio can be seen in the difference between the height of the green plus red (G+R) parts of the B1 bar and the same G+R portion of the bars for the other scenarios. Clearly, the B1 G+R is closer to the dotted line than any of the others (though A1T is also close). The “energy and decarbonization policies” that Joe Romm refers to are those that account for the difference in height between the G+R parts of the bars in our graph across scenarios — which is completely different than the assumptions of automatic decarbonization within each scenario which are reflected in the light blue parts of the bars.

Automatic decarbonization occurs in the IPCC scenarios not because of specific policies that the report discusses, but because of assumptions that it uses within individual scenarios (specifically, assumptions of decreasing carbon and energy intensities). Whatever policies are associated with these assumptions are not discussed by the IPCC. The decarbonization of the global economy reflected by the light blue portions of the bars in the figure above are indeed accurately characterized as being “automatic” or “spontaneous.”

In its editorial discussing our paper, Nature clearly understood this. Joe Romm apparently does not. He has confused the differences between aggregate emissions across scenarios with assumptions of automatic decarbonization within scenarios.

Now that Joe has released his original letter to Nature, it is clear why they asked him to correct his error of interpretation. It is also clear why his claims that we have made an error in our analysis is incorrect.