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Quantifying climate change – not so certain?

Olive Heffernan

In the latest issue of Nature Reports Climate Change, there’s an interesting Commentary by a group of atmospheric scientists who argue that, in assessing the skill of climate models by their ability to reproduce warming over the 20th Century, the latest IPCC Working Group I report may give a false sense of the climate models’ predictive capability.

In ‘Quantifying Climate Change – Too Rosy a Picture?’, Stephen Schwartz of the Brookhaven National Laboratory in Upton, New York; Robert Charlson of the University of Washington, Seattle and Henning Rodhe of Stockholm University, Sweden say that, as it stands, the narrow range of modelled warming misrepresents the certainty with which past temperature changes can be reproduced.

The temperature changes over the 20th Century summarized in the IPCC report represent results from 58 runs with 14 climate models. Although the models fared well at simulating past temperature changes (see Figure 2 for the comparison between simulated and observed warming), Schwarz and co-authors point out the uncertainty range is only half what it would be if uncertainties in the factors driving simulated climate change were accounted for – e.g, cooling by aerosols.

Richard Kerr has an interesting news piece on this Commentary in the July 6 issue of Science entitled ‘Another global warming icon comes under attack’. Kerr’s take on it is that, unlike climate skeptics taking cheap potshots at their choice picks of climate science, the Commentary by Schwarz and co-authors represents a group of mainstream atmospheric scientists challenging an emerging icon of global warming, with climate scientists giving some ground.

Co-author Charlson points out clearly in the Science news piece that “it is not a question of whether the Earth is warming or will continue to warm” under human influence. Rather, Schwarz and co-authors maintain that it is important to give an accurate picture of the range of sensitivities of current models, so that we have a better gauge of what the future might hold.

Kerr reports that IPCC authors say the group has a point, but that their latest WGI report, if read thoroughly, does reflect the uncertainties highlighted in the NRCC Commentary. We will have a response from IPCC authors on the Commentary soon, and will update you when we do….

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