A Climate Tipping Point 2.7 Million Years Ago Reset the Tempo of Earth's Ice Ages

For most of the past 66 million years, Earth's climate changed slowly. Ice ages came and went, paced by cyclic wobbles in the planet's orbit around the Sun, but within any given glacial period the temperature stayed relatively stable. That changed approximately 2.7 million years ago, according to an analysis published in Science by an international team led by Cambridge University. At that point, glacial periods started flickering - lurching between warm and cold on timescales of roughly a thousand years rather than tens of thousands.

The research identifies this as a climate tipping point: a threshold at which the system shifted into a qualitatively different mode of behavior and stayed there. The transition coincided, within geological uncertainty, with the emergence of the genus Homo - the group that includes all modern humans and our closest extinct relatives.

The Evidence: Deep-Sea Cores Off Portugal

The study relied on sediment cores recovered from beneath the seafloor off the coast of Portugal during an International Ocean Discovery Program (IODP) expedition. This location is scientifically unusual: sediments accumulate there rapidly and continuously, allowing climate signals to be reconstructed at resolutions typically found only in polar ice cores, but extending millions of years further back in time than ice cores can reach.

Professor David Hodell of Cambridge's Department of Earth Sciences, who led the research, and his colleagues analyzed the chemical makeup of the sediments - specifically the ratios of elements including calcium, titanium, zirconium, and strontium. These ratios reflect changes in the rate of sediment delivery, which in turn tracks how quickly climate was shifting at the time the sediment was deposited.

"Things were relatively quiet until 2.7 million years ago, when we began to see the first evidence of severe 'cold snaps'," said Hodell. "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."

What Triggered the Flickering

The rapid climate swings appeared in the sediment record only when glaciation crossed a specific threshold. The researchers describe this as the climate entering a "sweet spot" where multiple reinforcing conditions aligned: ice sheets had grown large enough to reach the ocean and calve icebergs; sea temperatures had dropped sufficiently; and ocean circulation had become sensitive enough to tip into instability.

Once ice sheets reach the coastline and begin calving, they gain the ability to disrupt the North Atlantic overturning circulation - the ocean conveyor system that transports heat northward and plays a major role in regulating Northern Hemisphere climate. Icebergs melt as they drift, freshening surface waters and potentially suppressing the density-driven sinking that drives circulation. Multiple pulses of iceberg calving, matching the timing and number of cold snaps found in the sediment cores, have been documented in independent records for the period around 2.5 million years ago.

The consistency of the signal across cores from different sites strengthens the interpretation. "Glacial periods weren't just cold, they were also highly variable with large swings in temperature over relatively short timescales," Hodell said.

A Link to Human Evolution

The timing of the tipping point overlaps with one of the major transitions in hominin evolution. The genus Homo - which encompasses species from early Homo habilis through modern Homo sapiens - appears in the fossil record approximately 2.4 to 2.8 million years ago. Researchers have long proposed that climate instability may have driven key adaptations in early human lineages, including dietary flexibility, increased brain size, and the development of stone tools, by repeatedly disrupting African habitats and forcing adaptation.

The Cambridge study does not establish a causal link between the identified climate pattern and human evolution - correlation in the geological record does not confirm causation, and the spatial distance between the Portuguese sediment cores and African hominin fossil sites means that the climate signal recorded in the cores reflects global patterns rather than local African conditions. What the study does is sharpen the timing of the global climate shift with higher resolution than previous analyses, making the overlap with hominin origins more precise.

Extending the Glacial Record

Until the Portuguese cores were analyzed, the detailed record of millennial-scale climate variability had been largely confined to Greenland ice cores covering the most recent glacial period, roughly the past 120,000 years. The sediment cores extend that style of high-resolution record back 5.3 million years, providing a new window into the dynamics of earlier glacial cycles and demonstrating that the pattern of rapid variability is not unique to the most recent ice age.

The research was supported in part by the Natural Environment Research Council (NERC), part of UK Research and Innovation. David Hodell is a Fellow of Clare College, Cambridge.