Direct observational evidence from space of the effect of CO₂ increase on longwave spectral radiances: the unique role of high-spectral-resolution measurements

Abstract

We present a direct measurement of the impact of increased atmospheric CO2 on the spectra of the Earth's longwave radiation obtained from space. The goal of this study is to experimentally confirm that the direct effects of CO2 increase on the Earth's outgoing longwave spectra follow theoretical estimates, by developing a methodology that allows for a direct and more precise comparison between theory and observations. In this methodology, a search is performed to find selected ensembles of observed atmospheric vertical profiles of temperature and water vapor that are as close as possible to each other in terms of their values. By analyzing the spectral radiances measured from space by the Atmospheric Infrared Sounder (AIRS), corresponding to the selected ensembles of profiles, the effects of increased CO2 on the spectra can be isolated from the temperature and water vapor effects. The results illustrate the impact of the increase in CO2 on the longwave spectra and compare well with theoretical estimates. As far as the authors are aware, this is the first time that the spectral signature of the increase in CO2 (isolated from temperature and water vapor changes) has been directly observed from space.