According to the ‘snowball Earth’ hypothesis, our planet was almost entirely covered by ice at least twice in the Neoproterozoic era, between about 710 and 635 million years ago. This deep-freeze climate would have destroyed most of the budding life that existed before the ensuing Cambrian explosion of organisms.

But far from becoming a more hospitable place when the ‘snowball’ finally thawed after the second episode of glaciation, the Earth was swept by winds close to hurricane strength, according to Philip Allen and Paul Hoffman writing elsewhere in this issue (Nature 433, 123–127; 2005). The sustained storms whipped up surface waves in the oceans that moulded the sediments in shallower ocean margins into giant sand ripples, much like their smaller cousins found on beaches today.

The ancient ripples are preserved in sedimentary rocks around the world, and have crest-to-crest distances of several metres. In comparison, the ripples seen in shallow water today typically measure less than 20 cm in wavelength. The picture shows a characteristic giant wave ripple from Namibia that was created during the rise in sea level after the glaciation 635 million years ago. The hammer handle is 33 cm long.

The steepness and height of the ripples are evidence for winds of at least 72 km per hour over long stretches of sea. The structures must therefore have formed along coasts that were exposed to the swell from the open ocean. But what would be considered a one-off extreme storm today must have been characteristic for the climate of the time, otherwise the waves would not have had time to imprint their signature in the sediments.

Allen and Hoffman estimate that the giant ripples formed at depths of 200–400 m, far deeper than the extent of oscillations from surface waves in today's more benign climate. This gives a hint of the profound effect the stormy climate must have had on upper-ocean turbulence and currents.

Such violent mixing could have resulted from the contrast between cold continental ice cover and the warmer, increasingly uncovered tropical oceans as the ice sheets retreated. The difference in temperature and consequently air pressure could have produced the inferred prevalence of storms, the giant ripples bearing witness to the power unleashed by a changing climate.