Photobiological uncertainties in the Archaean and post-Archaean world
The notion that ultraviolet (UV) fluxes, and thus biologically weighted irradiances, were higher on Archaean Earth than on present-day Earth has been a pervasive influence on thinking concerning the nature of early Earth. It directly influences calculations concerning protection strategies that may or may not have been required by early life. Our knowledge of the Earth's changing UV radiation climate over time depends upon our knowledge of a diversity of factors, the magnitudes of which are uncertain. Here these uncertainties are explored. During the Archaean Era, calculations of the surface photobiological environment span a three order of magnitude difference in DNA-damage weighted irradiances with consequences for our assumptions concerning the environment for exposed surface life and the role of UV radiation as a mutagen. These differences are primarily caused by uncertainties in the concentrations of trace gases and the partial pressures of carbon dioxide and nitrogen that affect scattering in the atmosphere. To a lesser extent, the luminosity of the Sun in the UV region is also a factor. During the Proterozoic and Phanerozoic, when we know that an ozone column existed, these uncertainties drop to two orders of magnitude and are primarily caused by poor knowledge about the frequency and atmospheric effects of potentially ozone-depleting agents such as volcanism, impact events and supernovae explosions as well as the effects of global temperatures on ozone concentrations and thus surface UV irradiance. Changes in other atmospheric constituents during this time have less of an effect on photobiological consequences, which include a Palaeozoic oxygen pulse. Understanding the cause of photobiological uncertainties and their consequences constitutes a current challenge for atmospheric chemists and photobiologists.