Glaciation: the snowball theory still holds water

Sir

D. M. Williams et al.¹ propose a mechanism² whereby an initial high obliquity for the Earth could have rapidly changed to its present low value of ˜23.5° near the end of the Proterozoic eon. Following G. Williams³, they note that the mean annual insolation would have been lower at the Equator than at the poles if the Proterozoic obliquity exceeded 54°. They infer that low-latitude glaciation observed near the beginning and end of the Proterozoic could be explained without the need for an ice-covered ‘snowball’ Earth⁴, ⁵.

However, neither Williams et al.¹ nor the accompanying News and Views article⁶ discuss the implications of high obliquity for glaciation per se. The basic requirement for glaciation is a net accumulation of winter snow after summer melting. High obliquity enhances seasonality³, ⁷ creating very cold winters with reduced snowfall and very hot summers with maximal melting.

The recognition that glaciation is favoured by cool summers, not cold winters, is the crucial difference between the Milankovitch theory and the much earlier Croll theory of orbital forcing⁸. High obliquity has the greatest effect on seasonality at the poles, but insolation during the warm seasons at the equator is equally high irrespective of obliquity.

Whatever merit it has for the early Earth¹, high obliquity seems insufficient as an explanation for low-latitude glaciation⁷ and glaciating the poles as a means of reducing a high obliquity¹ is highly improbable.