Stratospheric cooling and the tropical belt

Birner, Davis, and Seidel reply: We appreciate John Zinn’s bringing up the stratospheric cooling effect due to increases in carbon dioxide. Although the influence of that stratospheric cooling on tropospheric circulations is certainly interesting, how it will manifest in changes to the tropical belt is presently unclear. A rise in tropopause altitude does not necessarily translate into reduced convective stability of the troposphere. Furthermore, CO₂ is not a dominant factor in driving temperature changes in the lowermost part of the stratosphere—the part that arguably matters most for tropospheric circulations. The complex interplay of changes to concentrations of radiatively active gases, such as water vapor and ozone, and to the stratospheric circulation determines the temperature response to climate change in the lowermost stratosphere. A detailed understanding of that response with all its consequences remains a significant research goal.