The partly-Simpsonian model of the water vapour feedback on climate change
William Ingram, Physics Dept., University of Oxford, U.K.

The climate system is even more complicated than our most detailed & physically-based models of it, General Circulation Models (GCMs), which contain of order a million lines of code. To try & make sense of both these we need simpler models, models we know are too simple to be true but hope may be both true enough & simple enough to be useful. A classic example is considering global-mean temperature & a single "climate sensitivity". A more recent one is the "partly-Simpsonian" model of the water vapour feedback. Water vapour is the most important greenhouse gas in our atmosphere, but its concentration is primarily determined by temperature, giving a strong positive feedback. In the partly-Simpsonian model its size is given by assuming that the emission of heat to space from water vapour does not change under global warming, while the emission from everything else increases following Planck's Law. I shall explain the physical basis for this very simple description of an important aspect of the climate system, & show how it provides a useful basis for a higher-order analysis comparing different GCMs.

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