Scientists at the University of Southampton have uncovered evidence from ancient rocks that the Earth's climate continued to fluctuate during its most extreme ice age - known as Snowball Earth.
During the Cryogenian Period, between 720 and 635 million years ago, it has long been believed that Earth's climate entirely shut down.
The planet experienced its most severe glaciations, with ice sheets reaching the tropics, and much of the Earth entirely frozen. It's a state known as Snowball Earth, when the planet would have looked like a snowball from space.
These conditions were thought to have shut down much of the interaction between the atmosphere and oceans, muting short-term climate variability for millions of years.
But a new study, published in Earth and Planetary Science Letters , challenges that view. It shows that during at least one interval of Snowball Earth, climate oscillations occurred on annual, decadal, and centennial timescales - cycles strikingly similar to those seen in Earth's climate system today.
The breakthrough comes from analysing exquisitely preserved laminated rocks, known as varves, on the Garvellach Islands off the west coast of Scotland. These sediments were deposited during the Sturtian glaciation, the most severe Snowball Earth event that lasted 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."
Researchers examined 2,600 individual layers within the Port Askaig Formation, each recording a single year of deposition.
Lead researcher 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."
Microscopic analysis showed that the layers likely formed through seasonal freeze-thaw cycles in a calm, deep-water setting beneath ice. When the team used statistics to analyse variations in layer thickness, a surprising signal emerged.
"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."
Brief disturbance in a frozen world
These climate cycles, however, were unlikely to have been the norm for Snowball Earth.
"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."
The research team ran climate simulations for Snowball Earth, which showed that a completely ice-sealed ocean would suppress most climate oscillations. However, if a small fraction, around 15 per cent, of the ocean surface remained ice-free, familiar atmosphere-ocean interactions 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."
This finding supports a scenario in which Snowball Earth was generally frozen solid but punctuated by intervals, sometimes dubbed 'slushball' or more extensive 'waterbelt' states, when small patches of open ocean emerged.
Scotland's unique rock record
The Scottish field site played a crucial role in unlocking this record.
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 behaved during Snowball Earth matters far beyond deep time.
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."
The research was supported by the WoodNext Foundation , a fund of a donor-advised fund program, whose generous support underpins Professor Gernon's research group at the University of Southampton.
