The greenhouse effect and carbon dioxide

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 concentration might enhance the warming has, over the years, been controversial. The idea of climate warming related to CO 2 increases, as propounded by Arrhenius among others in the late nineteenth century, was challenged by various scientists in the early twentieth century, including Ångström who argued that the overlap of the CO 2 spectral bands with those of water vapour, combined 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 temperatures would increase as CO 2 concentrations increased. The band-filling and overlap effects meant the increase would not, however, be in direct proportion to CO 2 but would rather vary with the logarithm of its concentration. (For good reviews of the history 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 trapping by CO 2 at other wavelengths is far from saturation and that, as its concentration exceeds approximately 800ppmv, its effect actually increases at a rate faster than logarithmic.