This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Earth may have been pushed deeper into a global deep freeze by something surprisingly ordinary: salt. Around 700 million years ago, Earth entered periods of extreme glaciation known as Snowball Earth, when ice is thought to have spread across much or potentially all of the planet's surface.
Scientists have long known that expanding ice could have helped drive this transition: As bright ice replaces darker ocean water, more sunlight is reflected back into space, causing the planet to cool further and allowing even more ice to form. Now, Aksel Samuelsberg, from University of Tromsø—The Arctic University of Norway, and colleagues have identified another possible feedback that could have amplified this cooling: salt crystals accumulating on the surface of sea ice. Their modeling suggests that these crystals could have made the frozen surface even more reflective, helping temperatures fall rapidly during the early stages of a Snowball Earth event.
The idea begins with what happens to seawater when temperatures are very low. Although water freezes, the salts dissolved in seawater do not all become part of the ice. Instead, as sea ice forms and becomes colder, salt becomes increasingly concentrated in the remaining liquid trapped within it.
At sufficiently low temperatures, some of this salt can eventually crystallize. If ice at the surface then slowly changes from solid ice into water vapor—a process called sublimation—the salt crystals can be left behind, accumulating in a thin layer on the surface. That matters because salt crystals can be extremely bright.
In previous laboratory experiments, researchers found that a salt crust formed on very cold sea ice could reflect about 93% of incoming sunlight. For comparison, fresh snow reflects around 83%, while melting bare sea ice reflects roughly 67%. The researchers call the resulting process a salt-albedo feedback.
Albedo is simply a measure of how much sunlight a surface reflects, so a high-albedo surface reflects more sunlight and absorbs less energy, making it easier for temperatures to fall. In this case, falling temperatures allow more salt to crystallize, which makes the surface brighter and causes even more cooling. The process works in a similar way to the better-known ice-albedo feedback thought to have played an important role in Snowball Earth.
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