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Uncertainty About CO2 as a Climate Driver:
Is It Increasing or Decreasing?

Volume 5, Number 40: 2 October 2002

In a brief review of what we know and do not know about earth's climate and our ability to model it, Kump (2002) correctly notes that the direct contributions of CO2 and other greenhouse gases to global warming "are relatively small."  When fed into modern-day climate models, however, these inducements to warming are strongly amplified, so much so, in fact, that the ultimate increase in temperature predicted by the models is "well in excess" of the direct effects produced by the greenhouse gases themselves.  This result seems highly suspicious; and as we shall see from further study of Kump's review, there are many good reasons to question it.

Kump begins by stating - and, again, rightly so - that earth's climate system is complex, "with numerous feedback mechanisms (interdependent cause-effect relationships) that create indirect effects that are more difficult to predict" than direct effects.  Indeed, the preeminence of this complexity permeates his entire commentary.  In his very next sentence, for example, Kump again rightly states that any climatic transition to new states "may involve counterintuitive transients."  A little further into his essay, he also correctly notes "it is difficult to predict the extent of amplification or damping of the external forcings by internal feedbacks."

Other uncertainties, in Kump's words, result "from the coarseness of the gridded representation of spatially continuous physical processes that numerical models must adopt, especially as it affects our ability to predict cloud cover and rainfall."  He also notes "the possibility that the models are missing key climate feedbacks, with biotic processes perhaps being neglected most."  As one specific example, he again correctly notes that "most models neglect the potentially critical role that marine algae play in the formation and reflectivity of clouds over the remote ocean."

In spite of these many acknowledged shortcomings, Kump still tries to make a case for believing in model predictions.  He says, for example, that "the 400,000-year Vostok ice-core record of atmospheric pCO2 and temperature would seem to provide the ideal demonstration [our italics] that CO2 drives climate."  Yet he goes on to admit, with respect to this record, that "cause and effect is difficult to assign."  Based on other palaeoclimatic data sets from the more distant geologic past, Kump also says "there are good reasons [our italics] to suspect that atmospheric pCO2 has been a primary climate driver."  But he admits in the very same sentence that "the evidence is not conclusive."  Are these the things one would imagine would be said about "the ideal demonstration" that CO2 drives climate, or about "good reasons" for believing in this presumption?  We think not.

In summing up the gist of the palaeoclimatic evidence near the end of his essay, Kump claims, yet again, that there is "general support for the notion than an increase in atmospheric pCO2 will cause global warming."  But he admits, yet again, that this relationship "is neither linear nor in phase on all timescales."  And he further admits that "proxy indicators of global warmth do not always coincide with proxy indications of elevated pCO2, and when they do, as in the Late Pleistocene, there is no lead-lag relationship from which one might hope to assign cause and effect."  Hence, we ask, yet again: Is this the stuff of what convincing arguments are made?  We think not.

Nevertheless, Kump seems to think so, claiming that these observations give us "an elevated confidence in the models."  For us, however, they do just the opposite.  But he has an amazing answer for this problem.  "Fortunately," he says, "improved models ... are on the horizon, and should be adequate to support policy decisions concerning the reduction of fossil-fuel CO2 emissions."

Well, we surely hope improved models are on the horizon.  But on what basis should we believe they will "support policy decisions concerning the reduction of fossil-fuel CO2 emissions"?  As deficient as Kump has convinced us current models are, why should we not at least consider the possibility that new and improved models might suggest a more neutral position, i.e., that there is no need to reduce anthropogenic CO2 emissions?  Or the possibility that future models might suggest the earth would not suffer from even greater CO2 emissions?  Or that the earth might even benefit from them?

From what Kump has revealed about climate model deficiencies and inadequacies, none of these possibilities can be ruled out.  Nevertheless, Kump - and many others - have clearly eliminated them from their thinking.  Hence, we can only presume they do so on a basis other than science and logic.

Sherwood, Keith and Craig Idso

Kump, L.R.  2002.  Reducing uncertainty about carbon dioxide as a climate driver.  Nature 419: 188-190.