Even when Earth was locked in a global deep freeze, its climate may have kept moving. Ancient rocks reveal seasonal and decade scale cycles beneath the ice during Snowball Earth.
Researchers at the University of Southampton have discovered signs in ancient rocks that Earth’s climate continued shifting even during its most extreme ice age, known as Snowball Earth.
This dramatic period occurred during the Cryogenian Period, between 720 and 635 million years ago. For decades, scientists believed that the planet’s climate system essentially stalled during this time.
Massive glaciers advanced toward the tropics, and much of the globe was locked in ice. In this state, Earth may have resembled a white sphere from space. Because the oceans were largely sealed off, experts assumed that exchanges between the atmosphere and the oceans nearly stopped, preventing short-term climate swings for millions of years.
However, new research published in Earth and Planetary Science Letters presents a different picture. The study indicates that during at least one stretch of Snowball Earth, the climate continued to oscillate on yearly, decadal, and even century-long scales, patterns that closely mirror cycles observed today.
Scottish Rock Layers Preserve Ancient Climate Cycles
The key evidence comes from finely layered sedimentary rocks called varves found on the Garvellach Islands off Scotland’s west coast. These deposits formed during the Sturtian glaciation, the most severe Snowball Earth episode, which lasted about 57 million years.
Thomas Gernon, Professor of Earth and Planetary Science at Southampton and a co-author of the study, said: “These rocks preserve the full suite of climate rhythms we know from today – annual seasons, solar cycles, and interannual oscillations – all operating during a Snowball Earth. That’s jaw-dropping. It tells us the climate system has an innate tendency to oscillate, even under extreme conditions, if given the slightest opportunity.”
Scientists examined 2,600 distinct layers within the Port Askaig Formation. Each layer represents one year of sediment accumulation, creating a detailed annual record from deep time.
Lead author Dr. Chloe Griffin, Research Fellow in Earth Science at the University of Southampton, said: “These rocks are extraordinary. They act like a natural data logger, recording year-by-year changes in climate during one of the coldest periods in Earth’s history. Until now, we didn’t know whether climate variability at these timescales could exist during Snowball Earth, because no one had found a record like this from within the glaciation itself.”
Under a microscope, researchers determined that the layers likely formed through seasonal freezing and thawing in a quiet, deep water environment beneath ice. When they analyzed variations in layer thickness using statistical methods, consistent repeating patterns appeared.
“We found clear evidence for repeating climate cycles operating every few years to decades,” said Dr. Griffin. “Some of these closely resemble modern climate patterns, such as El Niño-like oscillations and solar cycles.”
A Short-Lived Pulse in a Mostly Frozen World
The team emphasizes that these oscillations probably did not define the entire Snowball Earth era.
“Our results suggest that this kind of climate variability was the exception, rather than the rule,” explained Professor Gernon. “The background state of Snowball Earth was extremely cold and stable. What we’re seeing here is probably a short-lived disturbance, lasting thousands of years, against the backdrop of an otherwise deeply frozen planet.”
To explore how this variability could occur, the researchers ran climate simulations. The models showed that if the oceans were completely sealed beneath ice, most climate oscillations would fade away. But if even a small share of the ocean surface, about 15 percent, remained ice-free, interactions between ocean and atmosphere could resume.
Dr. Minmin Fu, Lecturer in Climate Science at the University of Southampton, who led the modelling work, said: “Our models showed that you don’t need vast open oceans. Even limited areas of open water in the tropics can allow climate modes similar to those we see today to operate, producing the kinds of signals recorded in the rocks.”
These findings support the idea that while Snowball Earth was largely frozen, it may have experienced intervals sometimes called ‘slushball’ or more extensive ‘waterbelt’ states, when patches of open ocean temporarily appeared.
Why the Garvellach Islands Matter
The Scottish field site proved essential to reconstructing this ancient climate history.
Dr. Elias Rugen, Research Fellow at Southampton, who has worked on the Garvellach Islands for the past five years, said: “These deposits are some of the best-preserved Snowball Earth rocks anywhere in the world. Through them, you’re able to read the climate history of a frozen planet, in this case, one year at a time.”
Understanding how Earth functioned during Snowball Earth offers insights that extend beyond ancient history.
Professor Gernon said: “This work helps us understand how resilient, and how sensitive, the climate system really is. It shows that even in the most extreme conditions Earth has ever seen, the system could be kicked into motion. That has profound implications for how planets respond to major disturbances, including our own in the future.”
Reference: “Interannual to multidecadal climate oscillations occurred during Cryogenian glaciation” by Chloe Griffin, Thomas M. Gernon, Minmin Fu, Elias J. Rugen, Anthony M. Spencer, Geoffrey Warrington and Thea K. Hincks, 4 February 2026, Earth and Planetary Science Letters.
DOI: 10.1016/j.epsl.2026.119891
The study was supported by the WoodNext Foundation, a fund of a donor-advised fund program, whose generous support sustains Professor Gernon’s research group at the University of Southampton.
Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.