Salt could have made ‘Snowball Earth’ even colder

Ordinary sea salt may have exacerbated one of the most severe cooling events in the planet’s history. Climate modelling has shown that salt crystals on the surface of sea ice made the ancient Earth more reflective and helped it lose even more heat.

The proposed mechanism does not explain why global glaciation began. It may have kicked in after the ice had already spread and accelerated the transition to a colder state around 700 million years ago.

Details

During the Cryogenian period, approximately 715–635 million years ago, the Earth experienced several extreme glaciations. According to the ‘Snowball Earth’ hypothesis, ice then spread as far as the tropical latitudes and covered most or nearly all of the ocean’s surface.

One of the known mechanisms behind such cooling is the feedback between ice and albedo. Light-coloured ice reflects more sunlight than dark ocean water. Therefore, the expansion of the ice cover reduces surface heating and creates conditions for the formation of new ice.

Aksel Samuelsberg of the Arctic University of Norway and his colleagues have proposed an additional mechanism. When seawater freezes, salts do not generally become incorporated into the ice structure but instead concentrate in the remaining liquid. At sufficiently low temperatures, they crystallise.

In the tropical regions of the ancient frozen Earth, the evaporation of ice directly into water vapour may have exceeded snowfall. Following such sublimation, salt crystals remained on the surface and gradually formed a light-coloured crust.

Laboratory data show that a surface covered with hydrogalite crystals can reflect around 93 per cent of incident light. By comparison, fresh snow cover reflects approximately 83 per cent, whilst melting bare sea ice reflects around 67 per cent. The lighter the surface became, the less energy it absorbed and the more it cooled.

The researchers incorporated this salt-albedo feedback into a simplified one-dimensional climate energy balance model. It took into account the distribution of solar radiation across latitudes, heat transfer, energy radiation into space, and the varying reflectivity of water, snow, ice and salt deposits.

The model revealed two stable states for a completely frozen planet: one with salt deposits and one without. The version with salt proved to be significantly colder. During the transition from a warm climate to global glaciation, the system in the calculations tended to immediately shift to the colder state with a salt crust.

For the entire mixture of seawater salts, complete crystallisation is expected at approximately −36 °C. However, individual salts begin to precipitate as early as approximately −8 and −23 °C. Therefore, the effect could have occurred before the ocean ice reached its lowest temperatures.

Why this is important

The salt crust may not only have accelerated the cooling, but also made it more difficult to emerge from it. In the model, significantly more carbon dioxide was required to melt the salty ‘snowball Earth’ than in the version without salt. The greenhouse gas would have had to accumulate in the atmosphere to a level capable of overcoming the surface’s increased reflectivity.

This helps to explain why ancient global glaciations may have been so severe and persistent. Previously, the salt-albedo feedback had not been included in ‘Snowball Earth’ climate models, although the formation of bright salt deposits on very cold sea ice had been observed both in the laboratory and in the natural world.

However, the result is currently based on a simplified model. It does not take into account the movement of sea ice, which could have transported ice from snow into the tropics and diluted the salt layers. Clouds, dust and wind are also capable of obscuring or transporting the crystals, thereby weakening the effect.

The authors emphasise that salt did not necessarily trigger global freezing. To assess its actual contribution, the mechanism needs to be tested in more complex climate models incorporating ice dynamics, atmospheric circulation and clouds.

Source

Aksel Samuelsberg, Per Kristen Jakobsen and Martin Rypdal, ‘Amplified cooling of Snowball Earth from a salt-albedo feedback’, Climate of the Past, 2026. DOI: 10.5194/cp-22-1499-2026. The article is published under a CC BY 4.0 licence.

The original material was published on Phys.org. Image: Song et al., Nature Communications, 2023, CC BY 4.0.