A planet locked in ice isn’t necessarily a planet locked in time. New research reveals that even during periods of extreme global glaciation – often referred to as “Snowball Earth” – the climate system continued to exhibit seasonal variations and cyclical patterns. This challenges the long-held assumption that such deep freezes would effectively pause climate activity.
For decades, the prevailing scientific view depicted Snowball Earth as a period of climatic stasis, where continents, oceans, and even tropical regions were encased in ice, seemingly eliminating the possibility of meaningful climate fluctuations. However, a study conducted by researchers at the University of Southampton, published in Earth and Planetary Science Letters, presents compelling evidence to the contrary.
Rocks Reveal Hidden Climate Rhythms
The research team analyzed ancient rocks from the Garvellach Islands off the coast of western Scotland. These rocks, formed during the Sturtian glaciation – a global freeze lasting approximately 57 million years, between and years ago – contain finely layered sediments known as varves. Each varve represents a single year’s worth of sediment deposition, creating a remarkably detailed annual climate record from within a Snowball Earth glaciation.
By meticulously measuring 2,640 of these layers in the Port Askaig Formation, the researchers reconstructed year-by-year environmental conditions. Their analysis revealed clear evidence of continuing climate rhythms, even during the depths of the global freeze. “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,” explained Professor Thomas Gernon.
Sediment Layers Track Seasonal Changes
The key to unlocking this ancient climate record lies in the microscopic structure of the sediment layers. Alternating light and dark layers were observed, with lighter layers composed of coarser sediment deposited during warmer melt seasons and darker layers formed from finer particles settling during colder months. This structure strongly suggests the presence of seasonal freeze-and-thaw cycles, even under complete ice cover.
The researchers explain that even with a surface ocean sealed by ice, water remained calm and deep beneath the thick ice sheet. Partial melting of the ice released sediment, while ice itself carried grains that were dropped into the water as melting commenced. These patterns support the conclusion that yearly sediment formation occurred, rather than being the result of random events.
“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,” said Dr. Chloe Griffin, the study’s lead author. “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.”
Climate Cycles Embedded in Rock
Statistical analysis of the varve thickness revealed repeating climate cycles ranging from just a few years to decades and even centuries. Notably, many of these patterns closely matched known solar cycles, including rhythms driven by sunspot activity. Others resembled ocean-atmosphere oscillations similar to modern El Niño-like systems.
The amount of solar energy reaching Earth fluctuates slightly over time as sunspot cycles alter incoming radiation. Even small variations can influence temperature, ice melting, and sediment movement. The rock record demonstrates strong signals corresponding to both decadal and century-scale solar rhythms, indicating that sunlight continued to exert an influence on Earth’s climate even during the intense global freezing of the Sturtian glaciation.
Ocean-Atmosphere Interactions Persist
To understand how these climate cycles could persist under such extreme conditions, the researchers employed climate models. These models showed that a completely frozen ocean would indeed suppress most climate movement. However, even relatively small areas of open water in the tropics could allow climate oscillations to re-emerge, producing signals similar to those recorded in the rocks.
“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,” said study co-author Dr. Minmin Fu. This suggests that even limited exchange of energy between the air and ocean was sufficient to drive temperature swings and circulation patterns.
Earth’s Deep Freeze Wasn’t Static
It’s important to note that climate movement didn’t dominate during Snowball Earth. The evidence suggests that these active periods were relatively short-lived, lasting only a few thousand years, against a backdrop of an otherwise extremely cold and stable planet. “Our results suggest that this kind of climate variability was the exception, rather than the rule,” said 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.”
Ancient Rocks Inform Modern Climate Research
The rocks from Garvellach Island are considered among the best-preserved Snowball Earth records globally. Their clear layering and minimal disturbance allow scientists to reconstruct the climate history of a frozen planet with unprecedented detail, almost year by year.
“These deposits are some of the best preserved Snowball Earth rocks anywhere in the world,” said Dr. Elias Rugen. “Through them, you’re able to read the climate history of a frozen planet, in this case one year at a time.”
Understanding these extreme ancient climates can help scientists assess the resilience and sensitivity of planetary climate systems. The fact that even near-total global freezing didn’t completely halt climate motion offers valuable lessons for the future. “This work helps us understand how resilient, and how sensitive, the climate system really is,” said Professor Gernon. “It shows that even in the most extreme conditions Earth has ever seen, the system could be kicked into motion.”
Recent research, published in , also suggests that Earth reached a “built-in limit” to climate change at the end of the last global ice age, thawing into a “slushy planet” due to high carbon dioxide levels. This further underscores the complex interplay of factors influencing Earth’s climate history.
