The idea rests on a curious property of seawater. When ocean water freezes, most of the salt is expelled from the ice crystals, concentrating in pockets of brine. But as temperatures drop far below the normal freezing point of seawater, the brine itself becomes saturated and the salt begins to precipitate out as solid crystals. For the entire seawater system, this eutectic point sits at about minus 36 degrees Celsius. Crucially, laboratory experiments and field measurements by other research groups have shown that these salt crystals, particularly the mineral hydrohalite, are extraordinarily reflective, with an albedo of about 0.93, meaning they bounce back nearly all of the sunlight that strikes them. Fresh snow is one of the few natural surfaces that can compete.

For such crystals to matter climatically, they must sit at the surface where sunlight can reach them. This is where the peculiar atmospheric circulation of Snowball Earth comes in. Previous modeling work by Raymond Pierrehumbert, Dorian Abbot, and colleagues indicates that on a fully glaciated planet, a net ablation zone would have existed at lower latitudes, a band where sublimation of ice exceeds precipitation. There, the frozen ocean surface would have been slowly eaten away by the dry air, exposing bare sea ice for thousands of years. As that ice sublimated, any salt crystals precipitating within it would have been left behind, accumulating into what scientists call a lag deposit, a reflective crust of salt sitting directly on the tropical sea ice.

Samuelsberg and his colleagues asked a simple question: what happens if you add this salt-albedo feedback to a climate model? To find out, they used a diffusive one-dimensional energy balance model, a classic and well-studied framework that describes the zonal mean surface temperature of the planet. The model balances incoming solar radiation against outgoing longwave radiation, redistributes heat from the equator toward the poles through diffusion, and includes the Planck response along with the familiar nonlinear ice-albedo feedback. The team modified the model’s albedo function so that latitudes colder than a critical isotherm carry snow or ice albedo, bare sea ice in the net ablation zone carries a lower albedo, and any region where temperatures fall below the eutectic temperature takes on the extremely high albedo of the salt deposit.

Solving the model analytically using boundary integral methods, the researchers traced out bifurcation diagrams showing all the stable and unstable climate states the system can occupy as radiative forcing changes. The results were striking. In addition to the familiar hothouse, high-latitude ice line, and Snowball Earth states, and the Waterbelt states with a tropical water belt produced by the so-called Jormungand mechanism, the salt feedback gave rise to two new, stable, coexisting Snowball Earth states: one with a salt deposit covering the bare sea ice of the deep tropics, and one without. The salt-deposit state is significantly colder than its salt-free counterpart, because its surface reflects so much more sunlight. Several unstable states with smaller or multiple salt deposits also appeared in the analysis.

Which of these states would the real Snowball Earth have occupied? To answer this, the team applied a test previously used to evaluate Waterbelt hypotheses: a candidate climate state must be accessible from a warm climate, and it must persist over a large range of atmospheric carbon dioxide concentrations, consistent with the well-established hysteresis loop in which the planet plunges into glaciation and later escapes as carbon dioxide builds up. The geological record adds a constraint here, because the roughly 1.5 billion years before the Neoproterozoic glaciations, known as the Proterozoic glacial gap, show no evidence of ice sheets, meaning the climate system must have rested in a warm, largely ice-free state before the plunge.

The verdict was unambiguous. For plausible model parameters, the Snowball Earth state without a salt deposit is essentially inaccessible from a warm climate. Instead, once global glaciation begins and temperatures in the tropical ablation zone fall below the eutectic point, the salt-albedo feedback activates and drives the system into the colder, salt-covered state. The analysis showed that this outcome is robust: higher rates of meridional heat diffusion and larger ice-albedo contrasts favor the direct transition, and no matter at what temperature the salt precipitation begins, the system ultimately collapses into the fully salt-covered state. Only very large bare sea ice regions combined with extremely low eutectic temperatures would allow the planet to avoid it. In other words, it may have been very difficult for Snowball Earth to cool down without salt crystals amplifying the chill.

The findings carry implications for one of the thorniest puzzles surrounding Snowball Earth: how the planet ever deglaciated. In the model, escaping the salt-deposit state requires a substantially higher atmospheric carbon dioxide concentration than escaping the classical salt-free state, because the reflective salt crust suppresses the warming that volcanic carbon dioxide would otherwise deliver. Several earlier modeling efforts have tried to lower the carbon dioxide deglaciation threshold to match geochemical proxy data, and the new results suggest that if a salt deposit persisted for a significant time, it would have raised that threshold even further, making the escape from global glaciation harder to explain. The authors also note that the full seawater system contains numerous salt species that begin precipitating well above the eutectic temperature, with mirabilite crystallizing around minus 8 degrees and hydrohalite around minus 23 degrees, meaning some salt precipitation could be expected even before the deepest cold set in, potentially helping to trigger the glaciation in the first place.

Important caveats remain, and the authors are careful to frame the work as an initial investigation. The simplified model includes no ice dynamics, yet it is widely recognized that thick sea glaciers must have formed and flowed equatorward on Snowball Earth, gradually freshening the ice in the ablation zone as meteoric ice arrived from higher latitudes. Previous work by Jacob Goodman found that only a narrow band of marine ice remained near the equator over million-year timescales, so the salt deposit probably became less climatically important in the later stages of the glaciations, and the study cannot determine how long such a deposit lasted. The strength of the feedback is also uncertain: clouds can mask the albedo of both bare sea ice and salt crusts, surface dust may gradually dim the crystals, and winds could blow salt into regions of net precipitation, keeping larger areas salt-free. Yet even with a lower assumed crystal albedo, the qualitative structure of the model’s solutions holds. What the study establishes is that salt precipitation, long absent from Snowball Earth simulations, is a physical process powerful enough to reshape the planet’s climate trajectory, and one that future models of Earth’s deepest freeze can no longer afford to ignore.

Subject of Research: A salt-albedo feedback from precipitated salt crystals on tropical sea ice amplifying cooling during Snowball Earth

Article Title: Amplified cooling of Snowball Earth from a salt-albedo feedback

Article References: Amplified cooling of Snowball Earth from a salt-albedo feedback. (n.d.). https://doi.org/10.5194/cp-22-1499-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1499-2026

Keywords: Snowball Earth, salt-albedo feedback, sea ice, hydrohalite, eutectic temperature, energy balance model, Neoproterozoic, albedo, climate modeling, degaciation, hysteresis, Climate of the Past