At several times throughout its history, the Earth has been almost entirely covered in ice – a phenomenon known as “Snowball Earth” which scientists believe is caused by an outburst of an initially cold climate. But the exact processes that lead to this positive cooling feedback are still unknown. Among several proposed hypotheses (increase in albedo, modification of the carbon cycle, etc.), a team of researchers seems to prefer the hypothesis of the impact of an asteroid, releasing enormous quantities of material into the atmosphere and blocking the solar rays.
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The “Snowball Earth” model describes the Earth as almost entirely covered in ice, with average temperatures well below current temperatures, where ice only remains at the poles. This model was born from the observation of sediments of glacial origin in regions formerly located at low latitudes. Thanks to these sediments, whose formation regions were estimated from paleomagnetic measurements, scientists have identified two major episodes of extreme glaciationglaciation : the first about 2.25 billion years ago, during the Lower Proterozoic, and the second between 720 and 635 million years ago, during a period called the Cryogenian.
Possible multiple origins
During these episodes, the Earth was covered with a layer of ice reaching tropical latitudes – some scientists even believe that the Earth could at times have been completely frozen, including in equatorial regions. If several hypotheses exist to explain these extreme and temporary climatic conditions, all seem to agree on the same point: the phenomenon of “Snowball Earth” is caused by the runaway of an initially cold climate. According to this principle, the decrease in average temperatures leads to the extension of icy surfaces across the globe. However, the presence of ice on a surface increases its albedoalbedothat is to say that the surface reflects more solar rays and therefore retains less heatheat. Then follows a feedback loopfeedback loop positive, where the extension of icy surfaces leads to a decrease in temperatures, which in turn produces an increase in icy surfaces… A grouping of massesmasses continental at the level of the equatorial regions (which receive more solar radiationsolar radiation than the polar regions), as during the CryogenicCryogenicseems to facilitate this retroactive phenomenon, the continents having a higher albedo than the oceans.
Every body reflects part of the solar radiation it receives. The lighter a body is, the more reflective it is: it has a high albedo. © CEA
If scientists seem to agree on the mechanisms of global warmingglobal warming leading to the end of periods of intense glaciations (volcanic episodes leading to significant degassingdegassing of greenhouse gasgreenhouse gasas the carbon dioxidecarbon dioxide or methane), the causes of the initial cooling are, however, still debated. Among the many causes put forward, we find, for example, variations in the solar constantsolar constant (THE SunSun was in fact slightly less bright in the past), variations in theorbitorbit terrestrial, a reduction in atmospheric greenhouse gas concentrations, or the eruption of a supervolcanosupervolcanoemitting aerosolsaerosols in L’atmosphereatmosphere blocking the sun’s rays. But according to a team of scientists, the hypothesis of a winterwinter post-impact seems the most plausible: the impact of a asteroidasteroid could emit so much dust into the atmosphere that the sun’s rays might no longer be able to reach the Earth’s surface. The impact of Chicxulub, which participated in the extinction of dinosaursdinosaurs 66 million years ago, is also associated with a sudden drop in temperatures.
A giant impact like the start of a global winter?
It was based on this example that a team of researchers modeled the effects of an asteroid impact on the Earth’s climate. They present their results in the journal Science Advances. For their models, they applied their estimates of the climate response after the impact of ChicxulubChicxulub to various other initial scenarios: one corresponding to pre-industrial atmospheric carbon dioxide levels (before 1850), another to the last glacial maximum (around 20,000 years ago), a third reconstituting the climatic conditions which prevailed before the impact of Chicxulub on CretaceousCretaceous (with atmospheric carbon dioxide concentrations four times higher than pre-industrial levels), and a final one reconstituting the climatic conditions of 720 million years ago, just before the last episode of “Snowball Earth”.
And according to their simulations, the impact of an object of dimensions similar to racing carracing car de Chicxulub could well have resulted in the “Snowball Earth” effect: this is in fact what they observed in their scenarios modeling the climatic conditions of the last glacial maximum and the Cryogenian (with, in the latter case , atmospheric carbon dioxide concentrations twice as high as pre-industrial levels). However, the Earth was not entirely covered with ice in the scenarios of pre-industrial conditions, the end of the Cretaceous, and a Cryogenian characterized by atmospheric carbon dioxide concentrations four times higher than pre-industrial levels.
Their work thus indicates that the impact of an asteroid can indeed result in a “Snowball Earth”, if it occurs in initially cold conditions. But their conclusion will only be definitively confirmed by the discovery of ancient corresponding craters which, if they existed, are perhaps today already eroded and disappeared.