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When the Earth was a “snowball”

An international collaboration has published a synopsis of two climatic episodes during which the Earth was covered in ice and snow. This report takes stock of more than 35 years of scientific questioning regarding the functioning of a "snowball" Earth.
Published on 13 November 2017

The Earth experienced two periods during which it was totally covered in ice and snow (Snowball Earth), between 717 and 659 million years (myr), and then again between 649 and 635 myr before the present era.

Since the 1960s, these episodes have been described using geological and glaciological data. However, no known process has been able to explain how the planet withdrew from this glaciation so quickly. Indeed, under the sole effect of sunlight, the thaw would have lasted several billion years.

At the end of the 1980s, scientists proposed a process lasting only several tens of millions of years. In this model, CO2 emitted by volcanoes is no longer absorbed by carbon sinks such as the ocean and biosphere, due to the ice barrier. Instead, it accumulates in the atmosphere to very high levels, provoking a powerful greenhouse effect, followed by the thawing of the ice.

Climatologists from the LSCE, in collaboration with researchers from the Géosciences Environnement laboratory in Toulouse, have made various contributions on this theme. They show that the shift to a snowball Earth could have been favored by a unique distribution of the continents in the tropics. They refined the initial model by assuming that part of the ocean surface remained ice-free, allowing CO2 exchange between the atmosphere and the ocean. According to the researchers, exiting this Snowball state would have happened gradually, throughout a succession of glacier-interglacial oscillations, and finally leading up to the warming necessary for the ice thaw.

Thus, well after the appearance of life 3.7 billion years ago, the Earth's climate experienced major deregulation for millions of years. These episodes led to a functioning of the Earth that is very different from what we know to be, ever since the much later Cambrian explosion (540 myr).

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