Posted on 08/11/2007 12:40:50 AM PDT by neverdem
Climate scientists are used to skeptics taking potshots at their favorite line of evidence for global warming. It comes with the territory. But now a group of mainstream atmospheric scientists is disputing a rising icon of global warming, and researchers are giving some ground.
The challenge to one part of the latest climate assessment by the Intergovernmental Panel on Climate Change (IPCC) "is not a question of whether the Earth is warming or whether it will continue to warm" under human influence, says atmospheric scientist Robert Charlson of the University of Washington, Seattle, one of three authors of a commentary published online last week in Nature Reports: Climate Change.
Instead, he and his co-authors argue that the simulation by 14 different climate models of the warming in the 20th century is not the reassuring success IPCC claims it to be. Future warming could be much worse than that modeling suggests, they say, or even more moderate. IPCC authors concede the group has a point, but they say their report--if you look in the right places--reflects the uncertainty the critics are pointing out.
Twentieth-century simulations would seem like a straightforward test of climate models. In the run-up to the IPCC climate science report released last February (Science, 9 February, p. 754), 14 groups ran their models under 20th-century conditions of rising greenhouse gases. As a group, the models did rather well (see figure). A narrow range of simulated warmings (purple band) falls right on the actual warming (black line) and distinctly above simulations run under conditions free of human influence (blue band).
But the group of three atmospheric scientists--Charlson; Stephen Schwartz of the Brookhaven National Laboratory in Upton, New York; and Henning Rodhe of Stockholm University, Sweden--says the close match between models and the actual warming is deceptive. The match "conveys a lot more confidence [in the models] than can be supported in actuality," says Schwartz.
To prove their point, the commentary authors note the range of the simulated warmings, that is, the width of the purple band. The range is only half as large as they would expect it to be, they say, considering the large range of uncertainty in the factors driving climate change in the simulations. Greenhouse-gas changes are well known, they note, but not so the counteracting cooling of pollutant hazes, called aerosols. Aerosols cool the planet by reflecting away sunlight and increasing the reflectivity of clouds. Somehow, the three researchers say, modelers failed to draw on all the uncertainty inherent in aerosols so that the 20th-century simulations look more certain than they should.
Modeler Jeffrey Kiehl of the National Center for Atmospheric Research in Boulder, Colorado, reached the same conclusion by a different route. In an unpublished but widely circulated analysis, he plotted the combined effect of greenhouse gases and aerosols used in each of 11 models versus how responsive each model was to a given amount of greenhouse gases. The latter factor, called climate sensitivity, varies from model to model. He found that the more sensitive a model was, the stronger the aerosol cooling that drove the model. The net result of having greater sensitivity compensated by a greater aerosol effect was to narrow the apparent range of uncertainty, as Schwartz and his colleagues note.
"I don't want certain interests to claim that modelers are dishonest," says Kiehl. "That's not what's going on. Given the range of uncertainty, they are trying to get the best fit [to observations] with their model." That's simply a useful step toward using a model for predicting future warming.
IPCC modelers say they never meant to suggest they have a better handle on uncertainty than they do. They don't agree on how aerosols came to narrow the apparent range of uncertainty, but they do agree that 20th-century simulations are not IPCC's best measure of uncertainty. "I'm quite pleased with how we're treating the uncertainties," says Gabriele Hegerl of Duke University in Durham, North Carolina, one of two coordinating lead authors on the relevant IPCC chapter, "but it's difficult to communicate" how they arrived at their best uncertainty estimates.
Hegerl points out that numerical and graphical error ranges in the IPCC report that are attached to the warming predicted for 2100 are more on the order expected by Schwartz and his colleagues. Those error bars are based on "a much more complete analysis of uncertainty" than the success of 20th-century simulations, she notes. It would seem, as noted previously (Science, 8 June, p. 1412), IPCC could improve its communication of climate science.
You have chosen to believe stupid things, and as a result you are living ‘terrified’.
You’re free to live that way, and I don’t care. Just don’t impose your foolishness on the rest of us who know better. You’re not mamas favorite anymore.
Leave us alone.
“Unworthy of reasoned discourse.”
This line appears to be insurance against having to debate me further—a safe strategic move on your part in the interest of preserving your perception of your own intellectual integrity. Having debated many leftists, I am too familiar with the tactic of running away from debate while hiding behind the false slogan “I won’t honor it with a response.”
“and that your religious beliefs and imperatives can be sustained in the face of all contrary evidence.”
There is no evidence contrary to Christian doctrine.
I am happy to debate the topic with you, and I mean this sincerely: you will have the opportunity to emerge from the discussion wiser than when you started. I ask you to think very carefully about your statements—there are some sloppy arguments going around, advocated by atheists such as Richard Dawkins or Christopher Hitchens. But please use such ideas if you would like an education regarding their errors.
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