Misquotes on Global Warming

I’m doing some research to try to get a handle on Paul Krugman’s accusations against my former colleague Pat Michaels in the Ukia Daily Journal. (It was also in the New York Times, but it costs money there–methinks the paywall people at the Times need to talk to the syndication people) Michaels responds to the accusation on Cato’s blog.

I haven’t figured out what I think about the dispute yet, but I wanted to flag an inaccuracy I found in the course of my research. RealClimate.org is a prominent blog about “climate science by climate scientists.” They’ve done a number of blog posts criticizing Michaels for alleged inaccuracies, including the one featured in Krugman’s column. One of the unfortunate things about the climate debate is that it’s extremely vitriolic: the pro-Kyoto folks consider the “skeptics” to be pseudo scientists in the pay of private industry, while the “skeptics” accuse their opponents of trumping up the results to get more research funding. As a result, they often accuse each other of lying and misleading the public, and it’s often hard to tell for sure who’s telling the truth.

So here’s one example where Michaels’s critics wrongly accused him of misrepresenting his opponents. Gavin writes that Michaels misquoted climatologist James Hansen by removing the qualification “Given these constraints on climate forcing trends” from the sentence “we predict additional warming in the next 50 years of 3/4 +/- 1/4°C.”

“What are these constraints that Hansen mentions?” he asks, “Precisely the control of CO2, methane and black carbon emissions that Michaels insists are unnecessary!”

But that’s not quite right, as I’ll explain below the fold.

If you look at the paper, you can see that he uses a variety of different assumptions about the growth of greenhouse gas emissions (the most important of which is CO2) to project different possible temperature paths:

If CO2 continues to increase 1.5 ppm year, which would require that global emissions be kept about the same as today, the added forcing in 50 years will be 1.08 W/m^2 (5). Achievement of flat CO2 emissions will require major efforts in energy efficiency, fuel switching, and renewable energies. If, rather than being constant, CO2 emissions increase exponentially at 1.5% year, the added forcing in 50 years is 1.54 W/m^2. This growth rate is perhaps the largest plausible one, exceeding that of the past 25 years. If 1.5% year growth occurs in developing countries and emissions in developed countries are constant at the 2000 level, the added forcing is 1.28 W/m^2 (Table 1).

With that background, they then project warming trends based on those assumptions:

We contend that a forcing much smaller than 0.85 W/m^2 is unlikely, because fossil fuels are expected to be the primary energy source for at least several decades. Rapid introduction of nonfossil energies or CO2 sequestration might reduce the forcing by a few tenths of 1 W/m^2. However, much of the warming in the next 50 years will be from presently ”unrealized warming” caused by the existing planetary radiative imbalance of at least 0.5 W/m^2 (8, 37). Slowing CO2 emissions in the second quartile of the century, although crucial for stabilizing atmospheric composition later in the century, would have only a small effect on the warming in 2050. These considerations suggest a minimum warming of 0.5°C by 2050.

At the other extreme, CO2 growth exceeding exponential at 1.5% year would be inconsistent with historical trends and with the negative feedback caused by human concern about climate change. Thus the maximum CO2 forcing is 1.28–1.54 W/m^2 (Table 1). BC and O3 are unlikely to be much greater in 2050 than today. Indeed, China has already begun to reduce its air pollution (38) and other developing countries are probably near their limits. Continued global warming would produce at least moderate public concern, thus limiting added forcing to about 1.5 W m2 and realized warming to about 1°C.

Given these constraints on climate forcing trends, we predict additional warming in the next 50 years of 3/4 +/- 1/4°C, a warming rate of 0.15 +/- 0.05°C per decade. A slower warming rate will occur in the second half of the century, assuming that the climate forcing growth rate begins to trend downward before 2050.

Now, my reading of this is that their “high” estimate of 1.5 percent growth in CO2 emissions would be consistent with not adopting the Kyoto treaty. (Kyoto requires that “industrialized countries will reduce their collective emissions of greenhouse gases by 5.2% compared to the year 1990”) And the low end of the estimate is the value we’d expect if we were more aggressive than Kyoto requires (by including developing nations as well as industrialized ones).

In other words, according to Hansen, under any reasonable assumptions–either with Kyoto or without it–the Earth will warm between 1/2 a degree and 1 degree over the next half-century. The difference between Kyoto and no Kyoto is a couple tenths of a degree over 50 years.

It doesn’t look to me like Michaels misquoted Hansen in this instance. Realclimate is also one of the most prominent people claiming that Michaels misrepresented Hansen in his 1998 testimony, so this doesn’t bolster my confidence in that claim, either.

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