What is the best description of the greenhouse effect?

Posted by K R on

What exactly is the greenhouse effect? And what does it look like if we view it from a new angle? Of course, we know the answer, and Raymond Pierrehumbert has written an excellent paper about it (Infrared radiation and planetary temperature). Computer code used in climate models contain all the details. But is it possible to provide a simple description that is physically meaningful and more sophisticated than the ‘blanket around earth’ concept? I wanted a description that could be grasped by physicists. Without the clutter of too much details – just the essentials. A ‘back-of-the-envelope’ type derivation of the greenhouse effect. The starting point was to look at the bulk – the average – heat radiation and the total energy flow. I searched the publications back in time, and found a paper on the greenhouse effect from 1931 by the American physicist Edward Olson Hulburt (1890-1982) that provided a nice description. The greenhouse effect involves more than just radiation. Convection also plays a crucial role. How does the understanding from 1931 stand up in the modern times? I evaluated the old model with modern state-of-the-art data: reanalyses and satellite observations. With an increased greenhouse effect, the optical depth increases. Hence, one would expect that earth’s heat loss (also known as the outgoing longwave radiation, OLR) becomes more diffuse and less similar to the temperature pattern at the surface. An analysis of spatial correlation between heat radiation estimated for the surface temperatures and that at the top of the atmosphere suggests that the OLR has become more diffuse over time. The depth in the atmosphere from which the earth’s heat loss to space takes place is often referred to as the emission height. For simplicity, we can assume that the emission height is where the temperature is 254K in order for the associated black body radiation to match the incoming flow of energy from the sun. Read More: RealClimate

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