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Anthropogenic Aerosol Uncertainties
Volume 7, Number 15: 14 April 2004

In a review of our current understanding of the forcing of climate by anthropogenic aerosols and how that highly uncertain knowledge is employed in state-of-the-art climate models, an international group of seven scientists from the United States, Switzerland, France, Sweden and Germany raises some unsettling questions about the objectivity of the science that is being used by various political entities to pressure the nations of the world into ratifying the Kyoto Protocol and/or other like measures designed to reduce their emissions of carbon dioxide.

Anderson et al. (2003) begin their review by noting there are two different ways by which the aerosol forcing of climate may be computed.  The first approach is forward calculation, based on "knowledge of the pertinent aerosol physics and chemistry."  The second approach is inverse calculation, based on "the total forcing required to match climate model simulations with observed temperature changes."  The first approach is a first principles approach, which utilizes known physical and chemical laws and assumes nothing about the outcome of the calculation.  The second approach, in considerable contrast, is a residual approach, where the aerosol forcing is computed from what is required to match the calculated temperature change with the observed temperature change over some period of time.  Consequently; in the words of the authors, "to the extent that climate models rely on the results of inverse calculations, the possibility of circular reasoning arises."

So which approach do the climate models use?  You guessed it.  Anderson et al. report that, "unfortunately, virtually all climate model studies that have included anthropogenic aerosol forcing as a driver of climate change have used only aerosol forcing values that are consistent with the inverse approach."

How significant is this choice of approach?  And why is this specific choice made?

The aerosol scientists indicate that the negative forcing of anthropogenic aerosols derived by forward calculation is "considerably greater" than that derived by inverse calculation, so much so, in fact, that if the unbiased and clearly superior first-principles approach of forward calculation were employed, "the results would differ greatly."  How greatly?  Anderson et al. say that when forward calculation is used, "even the sign of the total forcing is in question," which means that the climate models predict no change in climate in response to the combined effects of anthropogenic-produced greenhouse gases and anthropogenic-produced aerosols, which also implies, in their words, that "natural variability (that is, variability not forced by anthropogenic emissions) is much larger than climate models currently indicate."  Indeed, it would suggest that essentially all climate change we have witnessed over the past century and a half may well have been the result of natural phenomena unrelated to anthropogenic activities, as we have long argued in many of our Editorials.

Such a situation would not sit well with the powers that agitate for the establishment of the Planetary Management Authority they hope to see govern the world via edicts that mandate massive reductions in fossil fuel usage.  And perhaps that is why the inverse approach is employed; the forward approach does not yield the result required by these forces.

To recapitulate, Anderson et al. say "that the magnitude and uncertainty of aerosol forcing may affect the magnitude and uncertainty of total forcing to a degree that has not been adequately considered in climate studies to date," noting that, as a result, current "inferences about the causes of surface warming over the industrial period and about climate sensitivity may therefore be in error."

Is that not reason enough to reject all emotional calls for immediate actions to cut CO2 emissions ? that there may be no problem at all?  At the very least, it would seem to suggest we ought not make an uninformed rush to judgment; for that is precisely what adopting the Kyoto Protocol would be at this stage of the game: the blind leading the blind, before some of the most important facts about the matter of climate change are yet known.

Sherwood, Keith and Craig Idso

Reference
Anderson, T.L., Charlson, R.J., Schwartz, S.E., Knutti, R., Boucher, O., Rodhe, H. and Heintzenberg, J.  2003.  Climate forcing by aerosols - a hazy picture.  Science 300: 1103-1104.