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What is the “Snowball Earth” hypothesis?

Credits: CNRS

The Snowball Earth theory has long contested that the Earth was entirely covered in ice over 700 million years ago. A climatologist has been investigating this question for years, and the scientific response now seems certain. 

Over the course of its history, the Earth has known several glaciation periods, when the climate became cold enough for the surface of the Earth to be entirely covered in ice. It appears that these “snowball” episodes occurred at least three times since the planet came into existence. Since the year 2000, the researcher and climatologist Gilles Ramstein has been investigating this phenomenon.

Interviewed in a long publication by the French national centre for scientific research (CNRS), the expert explains that first of all, a planet’s climate depends on its radiation variations, or the difference between the energy received from the sun and that which is sent back out into space by the continents, oceans and the atmosphere. He explains that during the formation of the Earth 4.56 billion years ago, temperatures were hotter than they are nowadays, while there was 30% less energy taken from the sun.

In this case, the atmosphere, referred to by the expert as “a heat blanket”, plays a very important role in the regulation of temperatures on the Earth’s surface. Two gases with very powerful greenhouse effects, such as carbon dioxide (CO2) and methane (CH4), were present in highly variable amounts. Furthermore, climate variation is strongly linked to the sources of these greenhouse gases, for example volcanic activity, which is the main natural source of CO2 emissions.

“Its main source is based on erosion of silicate continental rocks under the effect of water runoff. By reacting with the rock, the CO2 is removed from the atmophere and fixed in the form of carbonates that accumulate as underwater sediments. Over 3 billion years ago, volcano activity was already active, but the continents’ surfaces were still very small and erosion had only a negligible effect: the concentration of atmospheric CO2 was thus very elevated”, explains Gilles Ramstein.

Equally, methanogenic archaea appeared 3.5 billion years ago. These are unicellular microorganisms which generate significant quantities of methane, and whose greenhouse effects are thirty times greater than those of CO2.

If CO2 and methane in a way ensured that the Earth would be hot during the first few years, something happened to upset this state: the Great Oxygenation Event (GOE). 2.4 billion years ago, the concentration of oxygen increased with the appearance of cyanobacteria, microorganisms capable of photosynthesis, producing oxygen as a waste product. These cyanobacteria multiplied, contributing to the disappearance of the methanogenic archaea in the ocean and stopping the massive production of methane, while the methane present in the atmosphere changed into CO2 when it came into contact with oxygen.

Thus, the Earth cooled down, which also led to an increase in albedo, or the capacity of the surface of the earth to reflect sunlight. The Earth thus became covered in ice, during a period known as Huronian glaciation, which lasted 300 million years. The Earth later underwent two similar episodes: Sturtian glaciation and Marinoan glaciation.

Source: CNRS