Michael Gerver cites the planet Venus as a caution about runaway greenhouse warming (Physics Today, September 2017, page 11). Apart from Venus’s being much closer to the Sun and having a very dense carbon dioxide atmosphere, another critical difference between Earth and Venus is highly relevant to our greenhouse effect but rarely mentioned: Earth has seasons; Venus does not.
The importance of seasons struck me when I was puzzling how IR radiation can escape from the tropopause into outer space.
In the troposphere, heat is transported upward by convection. First-year undergraduates are taught how to calculate the lapse rate—the temperature drop with altitude—by considering gas thermodynamics alone. But at the tropopause, the temperature ceases to fall with altitude and begins to rise again. The cause is UV heating from above.
Above the tropopause, convection is no longer a viable mechanism for vertical heat transport, and that is why the stratosphere is stratified. If heat is to escape into space from the tropopause, it is going to be by IR radiation. But there is a problem: greenhouse gases, such as CO2, which provide the IR. The CO2 concentration makes the mean free path quite short for a photon at the center of the molecule’s IR resonance. And with the temperature now rising, the net IR flux at that frequency is actually downward.
Nevertheless, IR radiation can cross the stratosphere at frequencies with a smaller CO2 cross section. Raymond T. Pierrehumbert alluded to that in an excellent feature article he wrote for Physics Today (January 2011, page 33). However, the restriction to off-resonance frequencies severely limits the amount of heat that can be shed into space.
In the case of Venus, the options essentially end there. Any extra heating at the surface will cause thermal runaway until some new mode of heat shedding is activated. For Venus, it would appear that the surface is hot enough for the temperature to fall monotonically with altitude all the way up.
Earth, however, has another savior: its shadow, which in winter shields the stratosphere near the poles and thus prevents UV heating from above. The temperature continues to fall with altitude, and IR radiation, even at the center of the CO2 resonance, can cross the stratosphere.
What is surprising is that the seasonal heat shedding receives so little attention.
