Salt may have helped turn Earth into a frozen world 700 million years ago
Renaud de RICHTER
Salt may have helped turn Earth into a frozen world 700 million years ago
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.
Brightening a frozen world
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.
Testing salt's cooling effect
The process works in a similar way to the better-known ice-albedo feedback thought to have played an important role in Snowball Earth. As ice spreads, its bright surface reflects more sunlight than the darker ocean it replaces, reducing the amount of energy absorbed by Earth and driving further cooling.
The new study, published in Climate of the Past, suggests salt could have added to this effect once the planet became cold enough. To test the idea, the researchers incorporated salt deposits into a simplified climate model that allowed them to explore how Earth's climate could settle into different stable states.
Their model included a region of bare sea ice around the tropics where evaporation was expected to exceed snowfall. Atmospheric circulation on a Snowball Earth could have transported water vapor away from these regions, leaving the ice exposed. As this ice sublimated, salt crystals could accumulate on its surface.
When the researchers included this process, the model produced two possible stable Snowball Earth states: one with salt deposits and one without. The salt-covered state was substantially colder. Under the model's conditions, a transition from a warm, ice-free climate into a Snowball Earth tended to lead directly to the colder, salt-covered state.
The model shows how Earth's climate could shift between different stable states as the amount of energy (radiative forcing) reaching the planet changes. The salt-covered Snowball Earth state is colder and requires substantially more atmospheric carbon dioxide to trigger melting than a Snowball Earth without salt deposits. Credit: Samuelsberg et al, 2026. Deepening the freeze
The results suggest salt may have been particularly important during the early stages of a Snowball Earth event. As the planet cooled, salt accumulating on exposed sea ice could have increased its reflectivity, allowing less sunlight to be absorbed and pushing temperatures lower still.
The model indicates that this feedback could have started at temperatures warmer than about −36°C (−33°F), the point at which seawater would have frozen almost completely. Different salts crystallize at different temperatures, with some beginning to precipitate at around −8°C (18°F) and others at −23°C (−9°F). This means salt crystals could potentially have started accumulating relatively early in the cooling process.
That could have made the transition to a fully frozen planet harder to stop. In the model, even partial salt deposits were unstable, eventually giving way to salt covering the tropical bare sea ice.
The salt-covered state was also harder to escape from. A much greater increase in atmospheric carbon dioxide was needed to melt the ice than in the version without salt. As carbon dioxide is a greenhouse gas, rising concentrations would gradually warm the planet by trapping more heat in the atmosphere, causing ice loss.
Salt's role remains uncertain
The findings do not mean salt was responsible for freezing the entire planet in the first place. Instead, the researchers propose it as an additional feedback that could have strengthened cooling once widespread glaciation was already developing. What originally pushed Earth into a Snowball state remains an open question.
There are also uncertainties over whether salt deposits could have formed and survived long enough to have a major climatic effect. The model does not include the movement of sea ice, for example. Over time, ice formed from snowfall could have flowed toward the tropics, potentially diluting the salty marine ice from which the crystals formed.
Other processes could also have weakened the effect. Clouds could obscure the bright salt surface, while dust could cover the crystals and reduce their reflectivity. Winds could redistribute salt into areas where snowfall was occurring, preventing large deposits from building up.
For now, the study is best seen as a new piece of the Snowball Earth puzzle rather than a complete explanation for the planet's deep freeze. It suggests that once Earth's oceans began freezing, the salt left behind may have helped make the planet even brighter and potentially even colder.
The authors say future climate models should investigate how strong this feedback would be when factors such as ice movement, clouds and winds are included. If the effect survives those more detailed tests, salt precipitation could prove to have been an overlooked player in one of the most dramatic climate transformations in Earth's history.
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Publication details
Aksel Samuelsberg et al, Amplified cooling of Snowball Earth from a salt-albedo feedback, Climate of the Past (2026). DOI: 10.5194/cp-22-1499-2026
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