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Original Commentary dated 27 June 2007

“The latest report from the Intergovernmental Panel on Climate Change assesses the skill of climate models by their ability to reproduce warming over the twentieth century, but in doing so may give a false sense of their predictive capability.

…

Cooling, or negative forcing, occurs primarily through direct effects such as scattering of light in cloud-free air and indirect effects such as enhanced reflection of light by clouds. These aerosol forcings are much less certain than the greenhouse-gas forcings, and hence the total forcing is likewise quite uncertain. The new report estimates total anthropogenic forcing to be 0.6 to 2.4 W m^-2 (5–95% confidence range). This factor of four range greatly limits the ability to evaluate the skill of climate models in reproducing past temperature changes and to infer climate sensitivity from observed change because a given temperature increase might result from a large forcing and low climate sensitivity or alternatively from a small forcing and high climate sensitivity.”

wrote Stephen E. Schwartz¹, Robert J. Charlson² & Henning Rodhe³ in Quantifying climate change — too rosy a picture?

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To the Editor (of Nature, posted today)

“…However, they have misinterpreted the use of AR4 Fig. SPM-4 (shown here as Fig. 1) and, as such, their criticisms are misplaced. In the following, we summarize key aspects of the AR4 uncertainty analyses that were neglected by Schwartz et al.

…

We conclude that the skill of climate models in reproducing warming over the twentieth century shown in AR4 does not imply that the IPCC has under-estimated uncertainty in future warming. Uncertainty ranges are not based on the ability of the multi-model ensemble to reproduce twentieth century global mean warming. Instead, the AR4 assessment of future warming provides important new probabilistic information that incorporates multiple sources of uncertainty, including observational data.”

wrote Piers Forster¹, Gabriele Hegerl², Reto Knutti³, V. Ramaswamy⁴, Susan Solomon⁵, Thomas F. Stocker⁶, Peter Stott⁷ & Francis Zwiers⁸ in Assessing uncertainty in climate simulations

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Authors’ response (posted today)

“In their reply to our Commentary¹, Forster et al.² mainly criticize us for points which we did not make regarding climate sensitivity and future global warming and fail to come to grips with our central point, namely that in assessing the skill of climate models by their ability to reproduce warming over the twentieth century, the latest report from the IPCC³ may give a false sense of their predictive capability.”

wrote S. E. Schwartz¹, R. J. Charlson² & H. Rodhe³ in Assessing uncertainty in climate simulations – authors’ response

That original Commentary and its reinforcing Authors’ response echoes points a certain professor made in London, which is what drew my attention to this exchange of letters.

(Here’s the Nature correspondence in PDF format: one is the original ‘Commentary’ and the other is ‘To the Editor’ combined with ‘Authors’ response’.)

If there is more uncertainty in climate change than current research indicates, we should be even more aggressive in tackling emissions—immediately! Strangely, the “uncertainty is too great” camp often prefers to argue that we should do nothing to jeopardize economic growth, favoring further study instead. In so doing, they put zero value on our natural world. (Could it be they just don’t care?)

Still, I am not convinced that the IPCC reports present “too rosy a picture” in terms of scientists’ ability to quantify climate change. (That is just another ‘talking point’, i.e. what the doubters want us to think.) If anything, recent indicators in the natural world tend to show that the IPCC reports under-estimate the rate and extent of change, and that is what we should concentrate efforts on, rather than the uncertainties.

It’s time to look at solutions (that is, what we can do now—and do as much as we can) instead of dwelling on unknowns (that is, what we do not know for sure—hurdles ensuring we do as little as possible).