In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 10 February 2009, Susan Solomon and three coauthors describe a climate modeling exercise that claims to show that "climate change that takes place due to increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop." In the virtual world of computer-run climate models, such may well be so; but that need not be the case in the real world, where researchers still struggle to even understand -- much less model -- the suite of complex interactions that occur among the many physical, chemical and biological phenomena that combine to determine the planet's multi-faceted climatic trajectory in response to a rise in the air's CO2 content. Hence, there is a great need to carefully evaluate Solomon et al.'s claim.

The four scientists begin their analysis by setting forth three important criteria they feel should be met by the modeled climatic parameters they study: "(i) observed changes are already occurring and there is evidence for anthropogenic contributions to these changes, (ii) the phenomen[a] [are] based upon physical principles thought to be well understood, and (iii) projections are available and are broadly robust across models." Consequently, we analyze these criteria to see how well they pertain to the first -- and most basic -- of the climatic phenomena Solomon et al. study, namely, atmospheric warming; for if there is a problem in this regard with respect to this phenomenon, there will be even greater problems with ancillary climatic phenomena that are driven by atmospheric warming.

Starting with the last of their three criteria, we note that the atmospheric warming studied by Solomon et al. is in absolute harmony with it, since that criterion precisely describes what the studied warming consists of: projections that "are available and are broadly robust across models." Criterion number two, however, presents a bit of a problem; for just because projections are based upon physical principles that are thought to be well understood (and then only by some and not by others) does not mean that they really are well understood. And when one realizes there are non-modeled chemical and biological principles that may be equally as important as the physical principles employed in the models, one begins to realize just how tenuous the models' projections might be. And if the model projections are extremely tenuous, any conclusions that might be drawn about the irreversibility of their projections will be extremely tenuous as well. And if true, this problem alone would render Solomon et al.'s conclusions of very little worth.

That this situation is likely to be the case, i.e., that the climate model projections analyzed by Solomon et al. are indeed extremely tenuous, is suggested by the results of the several "natural experiments" employed by Idso (1998) to quantitatively evaluate the response of earth's near-surface air temperature to both short- and long-wave radiative perturbations caused by a number of well-defined, naturally-occurring, real-world phenomena. The set of results obtained from this exercise, as described by Idso, indicates that "a 300 to 600 ppm doubling of the atmosphere's CO2 concentration could raise the planet's mean surface air temperature by only about 0.4°C." And he goes on to note that "even this modicum of warming may never be realized," stating that "it could be negated by a number of planetary cooling forces that are intensified by warmer temperatures and by the strengthening of biological processes that are enhanced by the same rise in atmospheric CO2 concentration that drives the warming."

Lastly, with respect to the first of Solomon et al.'s three criteria -- their assumption that the modeled atmospheric warming is already occurring, and that there is evidence for anthropogenic (i.e., CO2-induced) contributions to it -- the situation is much the same: real-world data provide little to no support for this contention. We note, for example, that the global warming of the past few decades was actually part of a much longer warming, which began in many places throughout the world a little over three centuries ago (about 1680) with the dramatic "beginning of the end" of the Little Ice Age (LIA, see figure below), well before there was any significant increase in the air's CO2 content. And this observation suggests that a continuation of whatever phenomenon -- or combination of phenomena -- may have caused the greater initial warming could well have caused the lesser final warming, the total effect of which was to transport the earth from the chilly depths of the Little Ice Age into the relative balm of the Current Warm Period.
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The mean relative temperature history of the earth (blue, cool; red, warm) over the past two millennia - adapted from Loehle and McCulloch (2008) - highlighting the Medieval Warm Period (MWP) and Little Ice Age (LIA), together with a concomitant history of the atmosphere's CO2 concentration (green).

We also note that earth's current temperature is no higher now (and maybe just a tad less, in fact) than it was during the peak warmth of the Medieval Warm Period (MWP), when (just as at the "beginning of the end" of the LIA) there was over 100 ppm less CO2 in the air than there is today. Consequently, since the great MWP-to-LIA cooling occurred without any significant change in the atmosphere's CO2 concentration, just the opposite could occur just as easily, and the planet could warm, and by an equal amount -- just as it actually did over the past three centuries -- all without any help from an increase in the atmosphere's CO2 content, which remained essentially constant for the first 1850 years of the 2000-year record depicted in the figure above, and which did not begin to really take off until just the last few decades of the 20th century, which brief period of correlation is simply too short to use as justification for claiming that the late 20th-century CO2 increase was responsible for the late 20th-century warming of the globe, and especially since that warming actually ceased at the end of the 20th century, even though the atmosphere's CO2 content has subsequently continued to climb at an unprecedented rate and to ever greater heights.

In light of these several observations, it should be clear to all that Solomon et al.'s analyses of the irreversibility of climate-model-based atmospheric warming projections due to projected increases in the air's CO2 content are utterly meaningless, because the projections themselves are meaningless, due to (1) their being based on overly-inflated values of the strength of the CO2 greenhouse effect, and (2) the long history of the non-correlation between historical climate change (both warming and cooling) and the concomitant history of the air's CO2 content over the past two millennia.
Sherwood, Keith and Craig Idso

Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.

Loehle, C. and McCulloch, J.H. 2008. Correction to: A 2000-year global temperature reconstruction based on non-tree ring proxies. Energy & Environment 19: 93-100.

Solomon, S., Plattner, G.-K., Knutti, R. and Friedlingstein, P. 2009. Proceedings of the National Academy of Sciences USA 106: 1704-1709.