Snowball Earth

Snowball Earth is the hypothesis that Cryogenian glaciations covered nearly the entire planet in ice, including low-latitude oceans. It proposes an unusually severe state of global glaciation.

Ice–albedo feedback: A climate feedback in which expanding ice reflects more sunlight, cooling the surface and promoting further ice growth. Reflective ice could drive glaciation toward global coverage once cooling crossed a critical threshold.
Sturtian glaciation: A Cryogenian glacial interval lasting roughly 717 to 659 million years ago. Its long duration makes it a central candidate for a Snowball Earth episode.
Brian Harland: A British geologist who argued that Neoproterozoic glacial deposits occurred at low paleolatitudes. His 1960s interpretation helped establish the geological puzzle behind global glaciation.
Hard Snowball Earth: A version of Snowball Earth in which sea ice forms a nearly continuous cover over the oceans. It is the most complete-ice version of the hypothesis.
Cryogenian period: A geological period from about 720 to 635 million years ago, marked by major glaciations. Snowball Earth concerns the period's most extreme proposed climate episodes.
Runaway glaciation: A self-reinforcing expansion of ice that can carry a climate from partial glaciation toward a much colder state. It describes the proposed transition from ordinary cooling to near-global ice cover.
Marinoan glaciation: A Cryogenian glacial interval that ended about 635 million years ago. Its widespread deposits and abrupt termination figure prominently in Snowball Earth arguments.
Joseph Kirschvink: An American geobiologist who introduced the name Snowball Earth in 1992. He framed low-latitude glacial evidence as a hypothesis of near-global ice cover.
Waterbelt Earth: A climate scenario with extensive ice but persistent open-water belts, often near the equator. It preserves liquid-water refuges without requiring a fully ice-covered ocean.
Ocean anoxia: A condition in which ocean water contains little or no dissolved oxygen. Restricted exchange beneath sea ice could have altered oxygen levels in Cryogenian oceans.

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