A model of solar-cycle effects on palaeoclimate and its implications for the Elatina Formation

Abstract

Recently published analysis of periodicities observed in the characteristics of the annual deposits of a Precambrian (680 Myr ago) lake in South Australia^1,2 suggest a physical link between ancient solar variability, surface temperature and cyclicity in varve thickness. Solar-cycle effects on the present climate have remained elusive³. The smaller abundance of molecular oxygen and ozone in the Precambrian atmosphere made it possible for solar ultraviolet radiation to interact directly with the troposphere. In those conditions, global climatic perturbations may have been associated with the 11-year solar cycle. We use a radiative–comvective–photo-chemical model to investigate the sensitivity of the global surface temperature to changes in the ultraviolet solar irradiance. We show that the response maximizes for an O₂ atmospheric content of 3 × 10⁻³ the present atmosphere level. Two alternative hypotheses about the ultraviolet variability are examined. In the first, the solar-cycle variation is restricted to wavelengths below 200 nm. The induced surface temperature fluctuation, δT_s, is very small (<0.03 K), except that the amplitude of the ultraviolet modulation is enhanced with respect to average values for solar cycle 21. The second hypothesis assumes that the ultraviolet variability extends up to 300 nm. In this case, a δT_s perturbation of several tenths of kelvin is predicted. It is also shown that other local effects such as the amount of water vapour in the troposphere may restrict the conditions in which solar-cycle variations have affected the palaeoclimate.