Les Chevaliers de l’Ordre de la Terre Plate, Part II: Courtillot’s Geomagnetic Excursion

In a paper entitled "Are there connections between the Earth’s magnetic field and climate?" published recently in Earth and Planetary Science Letters, Courtillot and co-authors attempt to cast doubt on carbon dioxide as a primary driver of recent (and presumably future) climate change; he argues instead that fluctuations in the Earth’s magnetic field (partly driven by solar variability) have an important and neglected role. Like most work of this genre, it is carried out in an intellectual void — as if everything we know currently about physics of climate had to be set aside in order to make way for one new (or in fact not-so-new) idea. But the problems don’t end there. With the help of a Comment published by Bard and Delaygue (available here at EPSL or here as pdf) , we’ll expose a pattern of suspicious errors and omissions that pervades Courtillot’s paper. Sloppiness and ignorance is by far the most charitable interpretation that can be placed on this pattern.

Let’s set the stage by noting that, as a significant competitor to anthropogenic greenhouse forcing of recent climate change, the direct radiative forcing by solar irradiance variations is dead on arrival. The solar output has been monitored by accurate satellite instruments since 1978. Measured peak to trough over the 11 year solar cycle, averaging over the Earth’s surface and allowing for albedo, the radiative forcing amplitude is under 0.2 W/m2. The trend left after averaging over the solar cycle is even smaller. This pales by comparison with over 2 W/m2 of radiative forcing arising from long-lived greenhouse gases that have accumulated in the atmosphere since 1750; it pales yet more by comparison with the forcing to come in the future if action is not taken to control emissions. There is nothing in climate physics to suggest that the sensitivity of climate to solar irradiance variation differs substantially from the sensitivity to infrared radiative forcing arising from greenhouse gas changes. As far as the climate cares, a Watt is (for the most part) a Watt, regardless of whether it comes from changes in the incoming solar energy or greenhouse-induced changes in the infrared radiation loss.

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