Low-latitude glaciation and rapid changes in the Earth's obliquity explained by obliquity-oblateness feedback

Palaeomagnetic data suggest that the Earth was glaciated at low latitudes during the Palaeoproterozoic(1,2) (about 2.4-2.2 Gyr ago) and Neoproterozoic(3-8) (about 820-550 Myr ago) eras, although some of the Neoproterozoic data are disputed(9,10). If the Earth's magnetic field was aligned more or less with its spin axis, as it is today, then either the polar ice caps must have extended well down into the tropics-the 'snowball Earth' hypothesis(8)-or the present zonation of climate with respect to latitude must have been reversed. Williams(11) has suggested that the Earth's obliquity may have been greater than 54 degrees during most of its history, which would have made the Equator the coldest part of the planet(12). But this would require a mechanism to bring the obliquity down to its present value of 23.5 degrees. Here we propose that obliquity-oblateness feedback(13) could have reduced the Earth's obliquity by tens of degrees in less than 100 Myr if the continents were situated so as to promote the formation of large polar ice sheets. A high obliquity for the early Earth may also provide a natural explanation for the present inclination of the lunar orbit with respect to the ecliptic (5 degrees), which is otherwise difficult to explain.