The world we live in is greatly shaped by an event called the Mid-Pleistocene Transition (MPT) during which the dominant climate cycle lengthened and intensified. Instead of breaks in ice ages occurring every 41,000 years, they started to take place every 100,000 years and the long cold spells got even colder. New evidence pins the blame on a surprising suspect: erosion of North Atlantic continental shelves.
Until humans started messing with the planet's thermal balance, the shift between glacial and interglacial periods was dominated by Milankovich cycles, slow changes to the Earth's orbit and polar orientation. The combination of three cycles with different periods produces the 100,000-year climate cycles we experience today.
When paleoclimatologists discovered the glacial cycles used to be much longer, they suspected Milankovich cycles were different as well. That may have been true when dinosaurs ruled, but the evidence is against anything different on this score in the early Pleistocene. Instead, a paper in Proceedings of the National Academy of Sciences argues the shift took place in the oceans when the Atlantic Meridional Overturning Circulation (AMOC) lost much of its power.
AMOC's most familiar component is the Gulf Stream, which brings warm water up the American coast before turning east. It's the reason Northern Europe has a climate so much more tolerable than anywhere else on the planet at similar latitudes. The water then cools and sinks as a result of its high density, taking immense quantities of carbon with it, before flowing south far below the surface. AMOC is also part of the much larger phenomenon known as the Global Thermohaline Circulation.
Geologists have been aware AMOC's strength plummeted in the Mid-Pleistocene, but whether this was connected to the MPT has been debated. Dr Maayan Yehudai studied deep-sea sediments from the north and south Atlantic to investigate AMOC's strength over time for her PhD at Columbia University.
"What we found is the North Atlantic, right before this crash, was acting very differently than the rest of the basin," Yehudai said in a statement.
The ultimate cause of this event, according to Yehudai and co-authors, is something that might seem an improbable driver of global change: the removal of soils deposited on Europe and North America's continental shelves. According to a long-standing hypothesis, these soils made the continental outskirts slippery, so glaciers slid over them quickly.
Over time, the idea goes, successive ice ages wore away the soils until the glaciers were sitting on bedrock, making them far more stable. The stabilized glaciers grew thicker during cold periods and were less likely to melt entirely during brief warm spells. They continued to reflect light, cooling the planet so shorter interglacials never got properly warm. Consequently, it took a more powerful combination of orbital cycles to break the ice's grip, and the coldest periods became even colder.
The team's findings have put support behind what was once a somewhat speculative explanation. Although it's hard to prove causality between more stable ice sheets, a weaker AMOC, and a longer glacial cycle, the timing they reveal is certainly suggestive. A period of intense erosion around the North Atlantic occurred from 980-950 million years ago and was closely followed by the drastic fall in AMOC's strength, and the first 100,000-year-long glacial cycle.
The results also strengthen the case for a swift MPT, rather than one that took place over 600,000 years as some have proposed.
"It was one of the most substantial climate transitions and we don't fully understand it,” Yehudai said. Addressing that is important for our capacity to predict future climate change where a further weakening of AMOC is thought to be significant again.