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Time to give up the fantasy

10 Jun 2020 | Science Notes

Speaking of climate sensitivity, a correspondent alerted us to an amusing exchange a few years back in a climate journal on the seemingly obscure topic of aerosol forcing. Far from being esoteric, it like ECS is central to the climate debate. Although there’s not a lot of warming over the 20th century to explain, you can use a climate model with a high sensitivity to greenhouse gases that generates alarming future projections as long as you incorporate something that causes strong offsetting cooling during the 20th century. Typically the modelers use aerosol pollution, i.e. sulphur dioxide and soot, whose cooling impact supposedly canceled out the supposedly large warming from CO2. But it’s no good if sulphur and soot have limited cooling impact because then CO2 only had a small warming effect. In 2015 a scientific paper appeared that concluded that sulphur at least only cools a little. To which another paper appeared arguing, yeah but my climate model says sulphur cools a lot. To which the original author replied that’s because you told it to and your model is useless.

OK, scientists rarely talk that way at least in print. The actual retort was “We are averse to the idea that climate models, which have gross and well-documented deficiencies in their representation of aerosol–cloud interactions... provide a meaningful quantification of forcing uncertainty. Surely after decades of satellite measurements, countless field experiments, and numerous finescale modeling studies that have repeatedly highlighted basic deficiencies in the ability of comprehensive climate models to represent processes contributing to atmospheric aerosol forcing, it is time to give up on the fantasy that somehow their output can be accepted at face value. If progress is to be made in narrowing the bounds on aerosol forcing, it will arise not through a reduction of model spread—which fails to penalize the most erroneous models—but rather by developing and testing specific physical hypotheses.” But the implication is clear and commendably blunt.

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