Researchers have identified a 'tipping point' about 2.7 million years ago when global climate conditions switched from being relatively warm and stable to cold and chaotic, as continental ice sheets expanded in the northern hemisphere.
Following this transition, Earth's climate began swinging back and forth between warm interglacial periods and frigid ice ages, linked to slow, cyclic changes in Earth's orbit. However, glacial periods after this tipping point became far more variable, with large swings in temperature over relatively short timescales of roughly a thousand years.
An international team of researchers, led by the University of Cambridge, analysed the chemical fingerprints in deep-sea cores, from beneath the seafloor off the coast of Portugal. They looked at the chemical makeup of the sediments – in particular the ratios of elements such as calcium, titanium, zirconium and strontium –to reconstruct how fast the climate was changing.
"Things were relatively quiet until 2.7 million years ago, when we began to see the first evidence of severe 'cold snaps'," said Professor David Hodell from Cambridge's Department of Earth Sciences, who led the research. "These events may have been harbingers of things to come because at 2.5 million years ago, we start to see a distinct pattern of multiple rapid swings in the Earth's climate, on thousand-year timescales. From then on, variability was a persistent feature of the glacial climate, which is consistent with what we see in Greenland ice cores during the most recent Ice Age."
Growth of ice sheets and rapid millennial climate change seem to have gone together. This pattern of variability was consistent across different cores taken from different sites, providing strong confirmation that the signals reflect widespread climate events, not just local variation.
"This is the time when the glacial-interglacial cycle really got going," said Hodell. "Glacial periods weren't just cold, they were also highly variable with large swings in temperature over relatively short timescales."
This climate tipping point coincided with the emergence of the genus Homo, the group to which modern humans belong, suggesting that climate swings may have played a role in shaping early human evolution. The results are reported in the journal Science.
The sediment cores were recovered from beneath the seafloor off Portugal during an expedition by the International Ocean Discovery Program (IODP). Sediments in this location accumulate so rapidly and continuously that they record climate history at a resolution usually found only in polar ice cores, but these sediment cores extend millions of years further back in time than the ice cores, allowing the researchers to analyse the past 5.3 million years of climate history.
"I was surprised at the superb quality and resolution of the sediment cores, and the detail of the climate signals recorded within them," said Hodell.
The appearance of rapid climate swings in the sediment cores lined up with another important change in the North Atlantic: the first widespread arrival of 'ice-rafted debris': grains of rock carried out to sea by icebergs.
These debris layers suggest that ice sheets had grown large enough to reach the ocean, calving icebergs that melted as they drifted. These marine terminating ice sheets have the ability to destabilise ocean circulation and cause these abrupt climate flips.
Previous research has found that around 2.5 million years ago, during a period of glacier growth known as Marine Isotope Stage 100, the North Atlantic went through multiple pulses of iceberg calving, matching the timing and number of cold snaps found in the sediment cores.
The cores reveal that these rapid climate swings only occurred when glaciation crossed a particular threshold. The researchers describe this as the climate entering a 'sweet spot' where ice sheets were large enough, seas cold enough, and ocean circulation sensitive enough to tip into a period of instability marked by abrupt changes. Once this threshold was reached, millennial‑scale variability became a built‑in feature of glacial climate during the Quaternary Period (the past 2.6 million years).
"This period of glacial intensification is the most recent of the major climate tipping points of the past 66 million years," said Hodell.
The transition occurred at approximately the time of the emergence of the genus Homo, the group to which modern humans belong. Researchers have previously suggested that rapid changes in the Earth's climate may have had an effect on the way that humans evolved, since they would have been forced to adapt to changes in climate and vegetation in order to survive.
By extending the history of climate change, the research changes scientists' understanding of when and how abrupt climate variability emerged, and shows how the Earth's ice sheets have the ability to affect global climate patterns on multiple timescales.
The research was supported in part by the Natural Environment Research Council (NERC), part of UK Research and Innovation (UKRI). David Hodell is a Fellow of Clare College, Cambridge.
