At several times throughout its history, the Earth has been almost completely covered in ice—a phenomenon known as “Snowball Earth” that scientists believe was caused by the explosion of an initially cold atmosphere. But the exact processes that lead to this positive cooling response are still unknown. Among the many proposed hypotheses (increase in albedo, changes in the carbon cycle, etc.), one team of researchers prefers the hypothesis of an asteroid impact, which releases enormous amounts of material into the atmosphere and blocks solar rays.
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The “Snowball Earth” model describes the Earth as almost completely covered in ice, with average temperatures much lower than present-day temperatures, where ice remains only at the poles. This model was born from earlier observations of sediments of glacial origin in regions located at low latitudes. Thanks to these sediments, whose regions of formation were estimated from paleomagnetic measurements, scientists have identified two major episodes of extremes. GlacierGlacier : The first about 2.25 billion years ago, during the Lower Proterozoic, and the second from 720 to 635 million years ago, during a period called the Cryogenian.
Multiple origins possible
During this episode, the Earth was covered by a layer of ice that reached tropical latitudes – some scientists even believe that the Earth, including the equatorial regions, may have been completely frozen at times. If many hypotheses exist to explain these extreme and temporary climatic conditions, all seem to agree on one point: the phenomenon of “Snowball Earth” is initially caused by the escape of a cold atmosphere. According to this theory, the decrease in average temperature leads to the expansion of icy surfaces around the world. However, the presence of snow on the surface increases it AlbedoAlbedoThat means the surface reflects more solar rays and therefore retains less the heatthe heat. Then it follows Responsive loopResponsive loop Positive, where the expansion of icy surfaces causes a decrease in temperature, which in turn increases the icy surfaces… A group groupgroup At the level of the equatorial regions continental (which receives more solar radiationsolar radiation than the polar regions), such as during CryogenicCryogenicThis seems to facilitate the preceding phenomenon, with continents having higher albedos than oceans.
Every body reflects part of the solar radiation it receives. The lighter the body, the more reflective it is: it has a higher albedo. © CEA
If scientists seem to agree on the mechanisms of global warmingglobal warming leading to the end of periods of intense glaciation (volcanic episodes leading to significant degassingdegassing No Greenhouse gasGreenhouse gasas carbon dioxidecarbon dioxide or methane), however, the causes of the initial cooling are still debated. Among the many reasons put forward, we find, for example, variation in solar stillsolar still (The the sunthe sun In fact was slightly less bright in the past), variationorbitorbit Decline or explosion of terrestrial, atmospheric greenhouse gas concentrations SupervolcanoSupervolcanoemissions AerosolsAerosols In L’atmosphereatmosphere Blocks the sun’s rays. But according to a team of scientists, A winterwinter A post-impact seems most plausible: the impact of a asteroidasteroid can throw so much dust into the atmosphere that the sun’s rays can no longer reach the earth’s surface. The effect of Chicxulub, which participated in the extinction DinosaurDinosaur 66 million years ago, also associated with a sudden drop in temperature.
A massive impact like the onset of global winter?
It was based on this example that a team of researchers modeled the effects of an asteroid impact on Earth’s climate. They present their results in a journal Science Advances. For their models, they applied their estimates of the climate response after the impact of ChicxulubChicxulub For various other initial scenarios: one corresponding to pre-industrial atmospheric carbon dioxide levels (before 1850), another to the Last Glacial Maximum (about 20,000 years ago), a third reconstructing the climatic conditions that prevailed before the Chicxulub impact. CretaceousCretaceous (Atmospheric carbon dioxide concentrations are four times higher than pre-industrial levels), and before the last episode of “Snowball Earth”, the final re-establishment of climate conditions 720 million years ago.
And according to their simulations, the effect of objects of similar dimensions Racing carsRacing cars de Chicxulub may well result in a “snowball Earth” effect: in fact they have seen in their scenarios modeling the climatic conditions of the last glacial maximum and the Cryogenian (with, in the latter case, a double concentration of carbon dioxide in the atmosphere. As high as the pre-industrial level). However, the Earth was not fully covered by ice in the Cryogenian scenarios, with pre-industrial conditions, the end of the Cretaceous, and atmospheric carbon dioxide concentrations four times higher than pre-industrial levels.
Their work thus suggests that an asteroid impact can indeed result in a “snowball Earth”, if it occurs in cold conditions to begin with. But their conclusion will be confirmed only by the discovery of ancient corresponding pits which, if they existed, have probably already been destroyed and disappeared today.
