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Climate Forcing vs. Climate Sensitivity: Is it a Valid Issue?
Hansen, J.E., Sato, M., Lacis, A., Ruedy, R., Tegan, I. and Matthews, E.  1998.  Climate forcings in the industrial era.  Proceedings of the National Academy of Sciences, U.S.A. 95: 12,753-12,758.

What was done
In "an update of the Intergovernmental Panel on Climate Change," the authors attempted to quantitatively evaluate changes in climate forcing factors that have occurred over the last 150 years of the industrial era.  The factors they considered were: well-mixed greenhouse gases (CO2, CH4, N2O and CFCs), tropospheric ozone, stratospheric ozone, tropospheric aerosols, forced cloud changes, vegetation and other planetary surface alterations, solar variability, and volcanic aerosols.

What was learned
There were so many uncertainties in some of the anthropogenic forcings that the authors ended up concluding that "the forcings that drive long-term chimate change are not known with an accuracy sufficient to define future climate change."

What it means
The authors used this conclusion to infer another: "uncertainties in climate forcings have supplanted global climate sensitivity as the predominant issue" in the climate change debate.  This second conclusion, however, is by no means a logical extension of the first.  Just because one aspect of the climate problem, i.e., the question of climate forcings, is so uncertain does not mean that another, i.e., the question of earth's climatic sensitivity, is known any better.  In fact, this attempt to shift the emphasis of the debate appears to us to be a simple way to claim victory on a crucial point that cannot otherwise be rigorously supported.  We and others, for example, would argue vigorously that earth's climatic sensitivity to radiative forcings is not nearly as great as the authors of this paper claim it is.  Consequently, the fact that they conclude -- on the basis of forcing uncertainties alone -- that we cannot "define future climate change" doubles our confidence in the validity of this same conclusion.  And it allows us to continue to hold to it, if at some future date the uncertainties in the forcings are significantly reduced.  This is now the second time in but a matter of months that Hansen has attempted to create a convenient straw man to draw attention away from a crucial aspect of the ongoing climate change debate (see our Vol. 1, Number 1 Editorial Commentary).

Reviewed 15 December 1998