First of all, and constituting the centerpiece of the important new paper, plots of δD vs. CO2 derived from the earlier and latter portions of the Dome C ice core (comprising, respectively, marine isotope stages 1-11 and 12-16) and a similar plot from the well-known Vostok ice core are all seen to have essentially the same slope, which suggests, in the words of Siegenthaler et al., "that the coupling of Antarctic temperature and CO2 did not change substantially during the last 650 ky [thousand years]," or as Brook (2005) puts it in his Perspective about the new work, "that climate and greenhouse gas cycles are intimately related."

We agree with both of these assessments.  However, the more important question to be resolved is which parameter is doing the major forcing and which is simply following the other's lead.

In investigating this question, Siegenthaler et al. say they obtained the best correlation between CO2 and temperature "for a lag of CO2 of 1900 years."  Specifically, over the course of glacial terminations V to VII, they indicate that "the highest correlation of CO2 and deuterium, with use of a 20-ky window for each termination, yields a lag of CO2 to deuterium of 800, 1600, and 2800 years, respectively."  In addition, they note that "this value is consistent with estimates based on data from the past four glacial cycles," citing in this regard the work of Fischer et al. (1999), Monnin et al. (2001) and Caillon et al. (2003).  Clearly, therefore, it is temperature that is the robust leader in this tightly-coupled relationship, while CO2 is but the humble follower, providing only a fraction (which could well be miniscule) - of the total glacial-to-interglacial temperature change.

This observation does little to inspire confidence in climate-alarmist claims that the CO2 produced by the burning of fossil fuels will lead to catastrophic temperature increases, which predicted warmings, in some of their scenarios, rival those experienced in glacial-to-interglacial transitions.  Nevertheless, Siegenthaler et al. stubbornly state that the new findings "do not cast doubt ... on the importance of CO2 as a key amplification factor [our italics] of the large observed temperature variations of glacial cycles."

In vivid contrast to this unsupported contention, it is our opinion that when temperature leads CO2 by thousands of years, during both glacial terminations and inceptions (Genthon et al., 1987; Fischer et al., 1999; Petit et al., 1999; Clark and Mix, 2000; Indermuhle et al., 2000; Monnin et al., 2001; Mudelsee, 2001; Caillon et al., 2003), there is plenty of reason to believe that CO2 plays but a minor role in enhancing temperature changes that are clearly induced by something else, which latter italicized point is an undisputed fact that is clearly born out by the new ice core data.

Consequently, whereas Thomas Stocker (the second and corresponding author of the Siegenthaler et al. paper) is quoted by the BBC's Richard Black (BBC News, 24 Nov 2005) as saying of the tight and time-invariant relationship between δD and CO2, without any additional evidence, that it is "a very strong indication of the important role of CO2 in climate regulation," we say it is "a very strong indication of the important role of climate in CO2 regulation."  Why?  Because like Mary's little lamb, and as evidenced by 650,000 years of real-world data, wherever temperature went over this period, CO2 was sure to follow, which by definition is "a very strong indication of the important role of climate in CO2 regulation" and not the opposite.

Sherwood, Keith and Craig Idso

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