Global Warming Potentials Modified for Water Interference in the Atmosphere

Preliminary work has been carried out on the analysis of the radiative trapping of greenhouse gases in the atmosphere and the downward thermal radiative flux measured at the earth's surface. The radiative fluxes trapped by the atmosphere were measured with the Interferometric Monitor for Greenhouse Gases (IMG) instrument onboard the ADEOS satellite, and the surface flux measurements were obtained with a high-resolution FTIR spectrometer (Bomem model DA8). This work provides an important direct measure of the comparison between the radiative forcing at the earth's surface and the radiative trapping at the tropopause. Currently, the predictions of global climate models are based on the amount of thermal radiation that is absorbed, or trapped, at the tropopause. However, the actual greenhouse effect is due to the downward radiative flux which impinges upon the earth's surface, and it is this quantity that is measured with the ground-based system. A comparison between the measured upward flux and the emitted downward surface flux for methane shows that a significant discrepancy exists between the two quantities. This is due to the non-isothermal atmosphere and the spatial variation of water vapour in the atmosphere. Hence, this comparison suggests that the global warming potentials of greenhouse gases should be modified to account for water interference and the downward radiative forcing. These modifications, which are presented in this paper, will simulate more realistically the true effectiveness of a greenhouse gas in warming the planet's surface.