Ancient Scottish rocks reveal Earth's climate was never fully frozen

Seven hundred million years ago, Earth was supposed to be completely locked in ice. The "snowball Earth" theory suggested our planet had frozen solid—no seasons, no thaw, just a dead globe wrapped in white. Scientists have believed this for decades.

But rocks from a remote Scottish island are telling a different story.

Thomas Gernon and Chloe Griffin from the University of Southampton examined 2,600 exquisitely preserved layers in ancient rocks from the Garvellachs islands, off Scotland's west coast. These layers formed during the snowball Earth period, and under a microscope, they revealed something unexpected: climate cycles. The thickness of each layer captured year-by-year changes in climate—patterns that looked remarkably like the solar cycles and El Niño oscillations we see today.

Another closeup view of varves. By analysing thousands of these layers, the researchers identified climate cycles operating during Earth's deep-freeze period. Photograph: Prof Thomas Gernon/University of Southampton

The rocks revealed a slushy interlude—a few thousand years when a small fraction of ocean thawed and the climate briefly woke up. It wasn't a full escape from the ice age, but it was something: proof that even during Earth's deepest freeze, the climate system never completely shut down.

This matters because it changes how we understand Earth's climate sensitivity. The discovery, published in Earth and Planetary Science Letters, suggests that climate fluctuations during snowball Earth were rarer than previously thought, but they happened. And that tells us something crucial about how responsive our climate system is to disturbances—both ancient and modern. Understanding how Earth's climate bounced back then offers clues about how it might respond to the major disruptions we're causing now.