GFDI Colloquium: Dr. Ming Cai
The Bizarre Antarctic: Why can the greenhouse effect be negative over Antarctic?
Dr. Ming Cai
Florida State University
Abstract: Greenhouse gases warm the climate system by reducing the energy loss to space through a process known as the greenhouse effect. Thus, a common way to measure the strength of the greenhouse effect is by taking the difference between the surface longwave (LW) emission and the outgoing LW radiation. Based on this definition, the existence of a paradoxical negative greenhouse effect is found over the Antarctic Plateau, as greenhouse gases cause an increase in energy loss to space. We find that the strongest negative greenhouse effect is found within water vapor and CO2 bands. A principle-based explanation is provided to show that the sign of the greenhouse effect is fundamentally dictated by the vertical absorptivity and temperature profiles, which are partly determined by non-greenhouse factors. The dependence on temperature and absorptivity (i.e., emissivity) implies the sign of the greenhouse effect will vary seasonally and with wavenumber. The atypical climatological conditions found over the Antarctic Plateau, such as a robust surface-based temperature inversion, severe scarcity of water vapor above the inversion layer, and warmer stratospheric than surface temperatures cause the negative greenhouse effect. We thus conclude that a negative greenhouse effect can occur, but only under very abnormal conditions.