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4.1.3 The Greenhouse Effect and Basic Atmospheric Physics

So how does the greenhouse effect work? To begin, we should note that as some skeptics[1] of anthropogenic climate change have pointed out, the term “greenhouse effect” is somewhat misleading: the mechanics of the effect bear only a passing resemblance to the mechanics of man-made greenhouses. Artificial greenhouses are kept warmer than the ambient environment primarily through a suppression of convection: that is, the glass in the greenhouse prevents warm air—which is less dense than cold air, and so will tend to rise above it—from rising away from ground level, and thus keeps conditions warmer than they would be otherwise. A similar mechanism is at work when you leave your car parked in the sun on a warm day: the interior heats up, but because the cabin is air-tight (at least on the timescales of interest to you during your trip to the shopping mall or grocery store), the warmer air inside the car and the cooler air outside the car cannot circulate, so the temperature increase can build up over time. The planetary greenhouse effect operates very differently. The layers of the Earth’s atmosphere are not closed systems in this sense, and while convection impediment can play a role in increasing radiative forcing felt on the ground—the fact that cloudy nights are generally warmer than clear nights is partially explained by this effect—it is not the driving factor in keeping the surface of the Earth warm.

Rather than blocking the motion of air itself—convection—the greenhouse effect operates primarily by altering the balance of radiation that is emitted by the planet (conveniently, this is

  1. Gerlich and Tscheuschner (2009). This paper should be taken with a very large grain of salt (a full shaker would perhaps be even better), as the arguments Gerlich and Tscheuschner make about the “falsification” of the greenhouse effect are highly suspect. Halpern et. al. (2010) argue convincingly that Gerlich and Tscheuschner fundamentally misunderstand much of the involved physics. Still, they are (at least) correct on this point: the atmospheric greenhouse effect is very different from the effect involved in glass greenhouses.