The Snowball Earth is an idea about Earth's history that suggests during one or more very cold periods, Earth's surface was almost completely frozen, with very little or no liquid water visible. This theory is most often linked to the Cryogenian Period, a time that included two major ice events: the Sturtian (about 717 to 660 million years ago) and the Marinoan (about 650 to 635 million years ago).

Supporters of the theory say it best explains certain rock layers that scientists think were formed by glaciers in areas that were once near the equator, as well as other mysterious features in Earth's geological record. People who disagree with the theory question the evidence for worldwide freezing and whether it would be physically possible for Earth's oceans to be completely covered in ice or slush. They also point out the difficulty of Earth escaping a fully frozen state. Some questions remain unanswered, such as whether Earth was completely frozen ("snowball") or had a narrow area near the equator with open water ("slushball"). The Snowball Earth events are thought to have happened before the sudden growth of complex life forms known as the Avalon and Cambrian explosions. The most recent Snowball Earth event may have helped multicellular life begin to develop.

History

Long before the idea of a global ice age was first suggested, scientists made several discoveries that showed evidence of ancient ice ages from the Precambrian time. The first discovery was made in 1871 by J. Thomson, who found ancient material shaped by glaciers (called tillite) on the island of Islay in Scotland. Similar findings were later reported in Australia (1884) and India (1887). A fourth important discovery, known as "Reusch's Moraine," was made by Hans Reusch in northern Norway in 1891. Many more discoveries followed, but scientists at the time struggled to understand them because they did not accept the idea that continents move over Earth's surface.

Douglas Mawson, an Australian geologist and explorer, studied rock layers from the Neoproterozoic period in South Australia. He found thick layers of rock formed by glaciers. Later in his career, he suggested that a global ice age might have happened.

Mawson’s ideas were based on the incorrect belief that continents like Australia had always stayed in the same place. Later, when scientists accepted the theory of continental drift and plate tectonics, they realized that the glacial sediments could be explained by the fact that continents were once located at higher latitudes.

In 1964, W. Brian Harland published a paper showing that glacial sediments in Svalbard and Greenland were deposited in tropical regions. He used this evidence, along with other geological findings, to argue that an extremely cold ice age occurred, with glaciers forming even in tropical areas.

In the 1960s, Mikhail Budyko, a Soviet climatologist, created a simple model to study how ice affects Earth’s climate. His model showed that if ice sheets spread far from the poles, a cycle could begin where ice reflects more sunlight, causing more cooling and more ice. This could lead to Earth being completely covered in ice. However, Budyko concluded that such a situation had never happened because his model did not show a way to escape it.

In 1971, Aron Faegre, an American physicist, used a similar model to suggest that Earth could have three stable climate states, one of which was a completely frozen Earth, or "snowball Earth." His work introduced the idea that Earth could suddenly shift from one climate to another, including into a snowball Earth state.

The term "snowball Earth" was first used by Joseph Kirschvink in 1992. His research showed that banded iron formations could be linked to a global ice age. He also proposed that Earth could escape a frozen state if volcanic activity released enough carbon dioxide, creating a strong greenhouse effect.

Franklyn Van Houten discovered a repeating pattern in lake levels, now called the "Van Houten cycle." His studies of phosphorus deposits and banded iron formations led him to support the snowball Earth idea, suggesting Earth’s surface froze more than 650 million years ago.

Interest in the snowball Earth theory grew in 1998 when Paul F. Hoffman and others used Kirschvink’s ideas to study Neoproterozoic rocks in Namibia. They added evidence, such as the presence of cap carbonates, to support the hypothesis.

In 2010, Francis A. Macdonald and others reported that the supercontinent Rodinia was near the equator during the Cryogenian period. They found evidence that glacial ice covered areas at or below sea level, and that the Sturtian glaciation was a global event.

Evidence

The Snowball Earth hypothesis was created to explain geological clues that suggest glaciers once existed near the equator. Models show that if glaciers spread within 25° to 30° of the equator, an ice–albedo feedback could cause ice to quickly reach the equator. Finding glacial deposits in tropical regions supports the idea of global ice cover.

To assess the theory, scientists must evaluate the reliability of the evidence showing ice reached the tropics. This evidence must confirm three key points:

Proving the third point is very difficult. Before the Ediacaran period, markers used to match rocks from different areas are missing, so scientists cannot be certain rocks were deposited at the same time. Instead, they estimate rock ages using radiometric dating, which is rarely precise to better than a million years.

The first two points often cause debate. Some glacial features can form without glaciers, and estimating ancient landmass positions, even as recently as 200 million years ago, is challenging.

The Snowball Earth hypothesis was first proposed to explain what were then thought to be glacial deposits near the equator. Determining the positions of tectonic plates in Earth’s history is difficult because they move slowly. Scientists use palaeomagnetism to estimate the latitude where sedimentary rocks formed. Magnetic minerals in these rocks align with Earth’s magnetic field, allowing scientists to estimate latitude (but not longitude). Palaeomagnetic data suggest some glacial sediments from the Neoproterozoic were deposited within 10 degrees of the equator, though this accuracy is debated. These findings imply glaciers may have extended to tropical latitudes, but it is unclear whether this means global glaciation or localized glacial areas. Some argue the data do not confirm glacial deposits were within 25° of the equator.

Skeptics question whether ancient magnetic fields were different from today’s. If Earth’s core cooled more slowly during the Proterozoic, the magnetic field may have had multiple poles instead of just two. This could affect how palaeomagnetic data are interpreted. Additionally, determining whether a rock’s magnetic signal is original or altered by later events is difficult. For example, mountain-building processes can reset magnetic signatures in rocks far from their original location. This complicates the study of rocks older than a few million years.

Only one deposit, the Elatina deposit in Australia, is confirmed to have formed at low latitudes. Its age is well-determined, and its magnetic signal is original.

Sedimentary rocks formed by glaciers have unique features that help identify them. Before the Snowball Earth hypothesis, some Neoproterozoic sediments were thought to have glacial origins, including those possibly near the equator. However, some features linked to glaciers can also form through other processes. As of 2007, only one reliable piece of evidence confirmed tropical tillites, making claims about equatorial ice cover uncertain. Evidence of tropical sea-level glaciation during the Sturtian glaciation is growing. Features that suggest glacial origins include:

Some deposits from the Snowball Earth period could only form if an active hydrological cycle existed. Thick layers of glacial deposits, separated by thin non-glacial layers, indicate glaciers melted and reformed repeatedly over millions of years. This would not be possible if Earth were completely frozen. Ice streams, similar to those in Antarctica today, may have caused these patterns. Sedimentary features that require open water, such as wave-formed ripples and signs of photosynthetic activity, are found in Snowball Earth sediments. These may represent meltwater "oases" on a frozen Earth. However, computer models suggest large ocean areas likely remained ice-free, making a fully frozen Earth unlikely.

Two stable carbon isotopes exist in seawater: carbon-12 (C) and the rarer carbon-13 (C), which makes up about 1.109% of carbon atoms. Photosynthesis and other biochemical processes favor carbon-12, making ocean-dwelling photosynthesizers slightly depleted in C compared to volcanic carbon sources. This leads to lower C/C ratios in organic remains and higher ratios in ocean water. Organic material in lithified sediments remains slightly depleted in C.

Silicate weathering, an inorganic process that removes carbon dioxide from the atmosphere and stores it in rocks, also affects carbon isotopes. The formation of large igneous provinces before the Cryogenian period, combined with the weathering of continental flood basalts exposed by the breakup of Rodinia, likely caused significant changes in carbon isotopic ratios. This may have contributed to the start of the Sturtian glaciation.

During the proposed Snowball Earth period, there were rapid and extreme drops in the C/C ratio. Detailed analysis of these changes shows…

Mechanisms

A snowball Earth event begins when Earth cools, causing more snow and ice to cover its surface. This increase in ice and snow raises Earth's albedo, which is the amount of sunlight reflected back into space. Higher albedo leads to more cooling, creating a cycle that can cause runaway cooling. This process is helped by continents located near the equator, where sunlight is strongest, allowing ice to build up more easily.

Many factors could start a snowball Earth, such as lower levels of greenhouse gases like methane and carbon dioxide, eruptions from supervolcanoes, changes in the Sun's energy output, or shifts in Earth's orbit. The Cryogenian period began around the same time as a rapid increase in atmospheric oxygen, known as the Neoproterozoic Oxygenation Event. This rise in oxygen reduced methane, a strong greenhouse gas. No matter the cause, cooling leads to more ice and snow, which reflect more sunlight, causing further cooling and more ice. This cycle can eventually freeze the equator as cold as modern Antarctica.

Global warming from large amounts of carbon dioxide in the atmosphere, mainly from volcanic activity, may have melted a snowball Earth. Because of the feedback loop, melting the ice covering most of Earth could take as little as a thousand years.

Tropical continents, despite seeming counterintuitive, are important for starting a snowball Earth. These continents reflect more sunlight than oceans, absorbing less heat. They also receive more rainfall, leading to more river discharge and erosion. When silicate rocks on land weather, they remove carbon dioxide from the air. An example is the weathering of wollastonite, a type of rock. Calcium from this reaction combines with bicarbonate in the ocean to form calcium carbonate, a sedimentary rock. This process moves carbon dioxide from the atmosphere into the geosphere, balancing the carbon dioxide released by volcanoes over long periods.

In 2003, scientists struggled to determine the exact position of continents during the Neoproterozoic because there were not enough suitable sediments to study. Some reconstructions suggest polar continents, which are common in other major glaciations and provide surfaces for ice to form. Changes in ocean circulation may have triggered the snowball Earth event.

Other factors that may have started the Neoproterozoic snowball Earth include the rise of atmospheric oxygen, which reacted with methane to form carbon dioxide, a weaker greenhouse gas. The Sun was also weaker during this time, emitting 6% less radiation than today.

Normally, when Earth cools, weathering reactions slow, reducing the removal of carbon dioxide from the atmosphere. This allows carbon dioxide to build up, warming Earth. However, during the Cryogenian, all continents were in tropical regions, where weathering remained high even as Earth cooled. This led to ice spreading beyond the poles. Once ice reached within 30° of the equator, its high albedo caused further cooling and more ice, eventually covering the entire planet.

Polar continents are too dry to remove much carbon dioxide from the atmosphere because of low evaporation. Sediments before global glaciation show a slow decrease in carbon dioxide levels, but the start of snowball Earth is marked by a sharp drop in carbon isotope ratios, possibly due to reduced biological activity from cold, ice-covered oceans.

In 2016, scientists proposed the "shallow-ridge hypothesis," linking the breakup of the Rodinia supercontinent to the formation of hyaloclastites along shallow ridges. These rocks increased ocean alkalinity, explaining the thick cap carbonates that formed after snowball Earth events.

Dating of the Sturtian glaciation shows it occurred at the same time as a large igneous province in the tropics. Weathering of this area removed enough carbon dioxide to allow major glaciation.

Global temperatures dropped so low that the equator was as cold as modern Antarctica. Ice reflected most sunlight, and the lack of clouds (due to frozen water vapor) worsened the cooling. Carbon dioxide levels were unusually low during the Cryogenian, helping glaciers persist.

Scientists estimate that carbon dioxide levels needed to thaw Earth would be 350 times today's levels, about 13% of the atmosphere. With Earth almost fully ice-covered, carbon dioxide couldn't be removed from the air by weathering siliceous rocks. Over 4 to 30 million years, volcanic activity and microbes under the ice produced enough carbon dioxide and methane to melt ice in the tropics. This created ice-free areas that absorbed more sunlight, speeding up melting.

The first ice-free areas may have been in mid-latitudes, not the tropics, because rapid water cycles prevented melting at lower latitudes. Dust from these areas settled on ice sheets, lowering their albedo and accelerating melting. Methane trapped in permafrost may also have released, helping to warm Earth.

Methanogens, a type of microbe, played a key role in ending the Marinoan Snowball Earth. Increased microbial activity in surface waters led to the production of methyl sulfides, which may have contributed to the end of the snowball Earth event.

Scientific dispute

The argument against the snowball Earth hypothesis includes evidence of changes in ice cover and melting found in "snowball Earth" rock layers. Signs of melting come from glacial dropstones, chemical clues showing repeating climate patterns, and layers of glacial and shallow ocean sediments. A long record from Oman, located at 13°N, covers the time from 712 to 545 million years ago, which includes the Sturtian and Marinoan glaciations. This record shows both glacial and ice-free conditions. The snowball Earth idea does not explain the changes between glacial and ice-free periods or the shifting edges of glaciers.

Scientists have faced challenges in using climate models to recreate a snowball Earth. Simple models with oceans that mix quickly can freeze even at the equator, but more advanced models with dynamic oceans failed to produce equatorial sea ice. Additionally, calculations suggest that extremely high levels of carbon dioxide—130,000 ppm—would be needed to melt global ice, a number some scientists find too high.

Strontium isotope data do not match predictions from snowball Earth models about weathering stopping during ice ages and resuming quickly afterward. Instead, scientists suggest that methane released from frozen soil during rising sea levels could explain the large carbon changes observed after glaciation.

Nick Eyles argues that the Neoproterozoic Snowball Earth was not unique compared to other ice ages in Earth's history and that finding a single cause for it is unlikely. The "zipper rift" hypothesis suggests two major events in Earth's history—first, the breakup of the Rodinia supercontinent, forming the early Pacific Ocean, and later, the separation of the Baltica and Laurentia continents, forming the early Atlantic Ocean. These events happened during ice ages and may have created high plateaus, similar to how the East African Rift forms high ground, which could support glaciers.

Banded iron formations are sometimes seen as proof of global ice cover because they form in oxygen-free water with dissolved iron. However, the limited size of Neoproterozoic banded iron deposits suggests they may have formed in inland seas rather than frozen oceans. These seas can have varying chemistry, with high evaporation concentrating iron and limited water movement creating oxygen-free conditions. Continental rifting, which causes land to sink, often creates such inland seas. This rifting could allow rapid sediment buildup, reducing the need for massive melting to raise sea levels.

Another idea to explain equatorial ice is that Earth's tilt was much higher, around 60°, placing land in high latitudes. However, evidence for this is limited. A simpler possibility is that Earth's magnetic pole moved to this tilt, as magnetic data depends on the magnetic and rotational poles being similar. In either case, ice would have been limited to small areas, not requiring extreme climate changes.

Evidence of glacial deposits in low latitudes during snowball Earth has been reinterpreted using the concept of inertial interchange true polar wander. This idea, developed to explain ancient magnetic data, suggests Earth's orientation shifted relative to its rotation axis during the snowball Earth period. This could explain the distribution of glacial deposits without requiring them to form at equatorial latitudes. However, removing one incorrect data point from the original study made this explanation less valid.

Survival of life through frozen periods

A huge ice age would reduce plant life that uses sunlight, leading to less oxygen in the air. This would allow rocks rich in iron that have not reacted with oxygen to form. Some scientists argue that such an ice age would have caused all life to die out. However, tiny fossils like stromatolites and oncolites show that in shallow ocean areas, life did not face major problems. Instead, life developed more complex food webs and survived the cold period without harm. Others suggest that life may have survived in these ways:

However, based on the fossil record, organisms and ecosystems do not seem to have changed greatly, as would be expected during a mass extinction. More accurate dating has shown that a phytoplankton extinction event, once linked to the "snowball Earth" ice age, actually happened 16 million years before the ice age began. Even if life survived in the places described, a global ice age would likely change the variety and types of life on Earth. This change has not been found in the fossil record—in fact, the organisms most likely to be affected by climate changes survived the "snowball Earth" period. One explanation for this is that in many areas where scientists debate whether a mass extinction happened during the ice age, the Cryogenian fossil record is limited. Sponges, an early group of animals, survived the ice age after appearing in the Tonian period. Later, during the Ediacaran period, the diversity of animals increased greatly.

Implications

A snowball Earth had important effects on the history of life on Earth. Many places where life could survive during ice-covered times have been suggested, but global ice would have severely harmed ecosystems that needed sunlight. Evidence from rocks near low-latitude glaciers shows that ocean life likely dropped sharply during these icy periods. Large drops in ice coverage helped macroalgae survive.

About half of the ocean’s water would have frozen, leaving the remaining water twice as salty as today. This high salt level would lower the freezing point of the water. When ice melted under a hot atmosphere with high carbon dioxide levels, it would have created a thick layer of warm (50°C) freshwater over the oceans, up to 2 kilometers deep. Only after this warm water mixed with colder, saltier deep water did the ocean return to a warmer, less salty state. The melting of ice may have created new chances for life to diversify and may have caused the fast evolution seen at the end of the Cryogenian period. If global ice cover existed, it might have, along with heat from Earth’s interior, created an ocean with strong mixing and movement of water from top to bottom.

The Neoproterozoic era was a time of major growth in multicellular organisms, including animals. After snowball Earth events ended, organism size and complexity increased greatly. This rapid development of multicellular life may have been caused by repeated changes between cold and warm periods, which could have pushed evolution forward, similar to how ice ages in the Pleistocene increased diversity in Antarctica. Another possibility is that changes in copper levels and rising oxygen played a role. Many Sturtian diamictites (rock layers formed by glaciers) sit above copper-rich layers in Greenland, North America, Australia, and Africa. During the Sturtian glaciation, glaciers broke apart and eroded rocks rich in copper, and chemical weathering of the Franklin Large Igneous Province increased copper in the ocean. Copper is important for many biological processes, such as handling oxygen, making energy, and creating proteins like elastin and collagen. This increase in copper likely helped multicellular life evolve rapidly during the Neoproterozoic. High copper levels continued into the Cambrian explosion, which marked the start of the Phanerozoic era and may have influenced this event as well.

One idea gaining attention is that early snowball Earth events may have resulted from life’s evolution rather than caused it. These two ideas are not necessarily opposite. The theory suggests that life affects Earth’s carbon cycle, so major evolutionary changes could alter how carbon is stored in Earth’s systems, temporarily reducing carbon in the atmosphere until new balance is reached. For example, the cool period during the Huronian glaciation may have been linked to lower greenhouse gas levels during the Great Oxygenation Event. Similarly, the possible snowball Earth during the Cryogenian period (580 to 850 million years ago) may have been connected to the rise of more complex multicellular life and the spread of life onto land. However, a 2022 study suggested that Cryogenian glaciations may have driven the evolution of land plants, explaining why their close relatives, the Zygnematophyceae, became single-celled, cold-adapted, and developed sexual reproduction.

Occurrence and timing

The Snowball Earth idea is used to explain glacial deposits found in the Huronian Supergroup of Canada. However, some evidence about ice sheets in low latitude areas is debated, and rock layer evidence shows only three separate layers of glacial material (the Ramsay, Bruce, and Gowganda Formations) with long periods of no glacial activity in between. Glacial sediments in the Makganyene Formation of South Africa are slightly younger than those in Canada (about 2.25 billion years old) and may have formed near the equator. Scientists suggest that the rise of free oxygen during the Great Oxygenation Event removed methane from the atmosphere by reacting with oxygen. At that time, Earth’s weaker sunlight may have made methane, a strong greenhouse gas, essential for keeping temperatures above freezing. Without methane, temperatures could have dropped, leading to global ice coverage between 2.5 and 2.2 billion years ago during the Siderian and Rhyacian periods of the Paleoproterozoic era.

There were three to four major ice ages during the late Neoproterozoic era. The Marinoan was the most significant, and the Sturtian glaciations were widespread. Even the main supporter of the Snowball Earth idea, Hoffman, says the 350,000-year Gaskiers glaciation did not cause global ice coverage, though it may have been as intense as the late Ordovician glaciation. The status of the Kaigas "glaciation" or "cooling event" is unclear. Some scientists do not consider it a glacial period, others think it may be misdated Sturtian rock layers, and others believe it could be a third ice age. It was less significant than the Sturtian or Marinoan glaciations and likely not global in scale. New evidence suggests Earth experienced several glaciations during the Neoproterozoic, which challenges the Snowball Earth hypothesis.