The article “The changing width of Earth’s tropical belt” by Thomas Birner, Sean Davis, and Dian Seidel (Physics Today, December 2014, page 38) was an interesting read. However, I was surprised to find no mention of the stratospheric cooling effect that must result from increases in carbon dioxide concentrations.
Stratospheric temperatures are regulated almost entirely by the balance between heating by sunlight-induced photodissociation of ozone and cooling due to 14-μm band emission by CO2. As is well known, that band is radiatively thick—almost opaque—in the troposphere, so the escape of 14-μm radiation from the ground surface or from the troposphere itself is severely limited; thus it inhibits tropospheric cooling. By contrast, the same band is radiatively thin—almost transparent—in most of the stratosphere, with the result that the 14-μm radiation emitted locally can escape freely to space. The net result is that higher atmospheric CO2 concentrations lead to increased warming in the lower atmosphere but accelerated cooling of the stratosphere.
Warming of the troposphere combined with cooling of the stratosphere leads to rising tropopause altitudes and to reduced convective stability of the troposphere and the resultant increases in turbulent mixing rates, vertical water-vapor transport, and precipitation. Those effects should show up in sufficiently comprehensive global circulation models.
