Wenyi Zhong and
Joanna D. Haigh
Department of Physics and Grantham Institute for Climate Change, Imperial College London
Introduction
It is well known that carbon dioxide plays an important role in the natural greenhouse warming of the Earth’s atmosphere but the extent to which increases in its concentra- tion might enhance the warming has, over the years, been controversial. The idea of climate warming related to CO2increases, as propounded by Arrhenius among others in the late nineteenth century, was chal- lenged by various scientists in the early twentieth century, including Ångström who argued that the overlap of the CO2spectral bands with those of water vapour, com- bined with the saturation of absorption near the centre of the 15μm band, would leave little scope for additional effects. In the 1930s and 1940s Guy Stewart Callendar at Imperial College (London) revived the warming theory and by the 1970s it was generally accepted that global surface tem- peratures would increase as CO2concentra- tions increased. The band-filling and overlap effects meant the increase would not, how- ever, be in direct proportion to CO2but would rather vary with the logarithm of its concentration. (For good reviews of the his- tory of this discussion, see Mudge (1997) and, in much more detail, Weart (2008 and website update 2011)).
More recently the saturation issue has been resurrected in attempts to deny the existence of anthropogenic climate change. Very clear explanations (e.g. by Archer, 2007; Pierrehumbert, 2011) have been given of the basic physics as to why these arguments are flawed. Here we show in detail how, although the very centre of the 15μm band does become saturated, greenhouse trap- ping by CO2at other wavelengths is far from saturation and that, as its concentration exceeds approximately 800ppmv1, its effect
11ppmv indicates one molecule of the gas for100every million molecules of air.
