Why the ancient Earth may have frozen almost entirely over

A new modelling study offers a simple explanation for why the planet could have cooled so much. It wasn't just the composition of the atmosphere. A fainter Sun, the ancient supercontinent Rodinia at the equator, and a nearly bare landmass with no plants could have played an important role.

The idea is that light-coloured surfaces reflect more sunlight back into space. If the land has no forests, grasses, or dark vegetation, but mostly light-coloured rocks and bare rock, such a surface is less likely to heat up. Under certain conditions, this could help set off a chain reaction: it gets colder, more ice appears, the ice reflects even more light - and the planet cools further.

Important: this is not "proof" of a single cause of ancient glaciation, but the result of climate modelling. The authors showed under what conditions the Earth could have transitioned to a state of near-global ice.

Details

The researchers studied not modern climate, but the ancient Earth of the late Proterozoic - the period before the dramatic increase in the complexity of animal life. At this time, the supercontinent Rodinia existed. According to reconstructions, much of the landmass was at low latitudes, closer to the equator, where sunlight is usually particularly strong.

But about 700 million years ago, the Sun shone more faintly than it does today. In the model, the authors used a value of about 95 per cent of today's solar luminosity. That doesn't seem like a big difference, but it could be significant for the planet's climate.

The second factor is the land surface. Much of the continents are now covered with vegetation. Plants are usually darker than bare rock, so they absorb more light and heat. And in that era, the land was almost devoid of terrestrial vegetation. The authors of the paper point out that bare granite surfaces may have had a higher reflectivity than forests.

This reflectivity is called albedo. In simple words, it is the fraction of light that a surface reflects rather than absorbs. Ice and snow have high albedo, so they reflect sunlight well. Dark oceans and forests have a lower albedo, so they heat up better.

In the model, scientists tested different options: how much light the Earth received from the Sun, where the continents were located, how much carbon dioxide was in the atmosphere, and how light the land was. They compared the ancient configuration with Rodinia and the more modern arrangement of the continents.

The result was revealing. With sunlight at 95% of today's levels and bare continents with reflectivity similar to granite, Rodinia at equatorial latitudes could have pushed the Earth to a snowball state even with CO₂ concentrations as low as 1000 ppm. If the continents were positioned as they are today, such a scenario would only be triggered at lower CO₂ levels of up to 400 ppm.

Why was Rodinia near the equator so important? Firstly, the light-coloured land in an area of strong sunlight reflected a lot of energy back into space. Second, in the warm and humid low latitudes, silicate rocks broke down faster. This process gradually took some CO₂ out of the atmosphere, weakening the greenhouse effect.

As the planet began to cool, ice came into play. Ice is also light and reflects a lot of sunlight. So the more ice there was, the less heat remained at the surface. This is called ice-albedo feedback: cold creates ice, and ice enhances cold.

A separate conclusion of the paper concerns plants. According to the authors' calculations, if the continents were covered with vegetation and reflected less light, the Earth would have a harder time transitioning to a snowball state. In the model, reducing the albedo of the land from about 0.35 to 0.15 could prevent such a state from being triggered when CO₂ is about 400 ppm and solar luminosity is 95% of today's.

Why it matters

The study helps us understand why Earth's ancient climate may have changed so dramatically. It shows that a near-global glaciation doesn't have to be caused by a single cause. A combination of factors could have worked: a weaker Sun, less greenhouse heat, continents near the equator and a light-coloured landmass without vegetation.

This is also important for understanding the role of life in climate. Plants not only produce oxygen and participate in the carbon cycle. They change the appearance of the planet: they make the land darker, help it absorb more sunlight and thus influence the temperature balance.

The work also explains why a new "Snowball Earth" is highly unlikely today. The sun is brighter, the continents are arranged differently, the landmass is covered with vegetation, and greenhouse gas levels now do not resemble the conditions under which the model triggered near-glaciation. The authors write that at today's solar luminosity, the "snowball" state only occurred in their calculations at very low CO₂ - about 100 ppm or lower and with very light bare land.

Background

The "Snowball Earth" hypothesis arose to explain traces of ancient glaciers at low, almost tropical latitudes. If glacial deposits did form close to the equator, it means that the Earth may have been much colder in the past than during normal glacial epochs.

The best known such events date back to the Cryogenian period, roughly between 720 and 635 million years ago. Scientists debate whether the planet was then completely encased in ice or retained "belts" of open water. But even the milder version means an extremely harsh climate compared to modern Earth.

The new study doesn't close this debate, but it does clarify the mechanism. It shows that ancient Rodinia may not have been just a backdrop, but an active participant in the climate tipping point: its position and bare, light-coloured surface may have helped the planet transition to an icy state.

Source

Research: Erica Bisesi, Giuseppe Murante, Antonello Provenzale, Jost von Hardenberg, Michele Maris, Laura Silva, "Interaction between vegetation and Snowball phases in the late Proterozoic Earth", International Journal of Astrobiology, 2026.