In early August, at the Goldschmidt Conference on geochemistry, a team of scientists from Columbia University presented evidence from seafloor cores that suggest that a million years ago ice sheets in the Northern Hemisphere began sticking to their bedrock. The team proposes that as the glaciers grew thicker, it led to a global cooling that disrupted both the Atlantic Meridional Overturning Circulation (AMOC) and the ice age cycle. But how exactly might glaciers have been involved in this perplexing shift in paleoclimate ice age patterns?
As skeptics of anthropogenic climate change often note, Earth’s climate changes and has changed before. Aside from humans’ unabashed consumption of greenhouse gases, a wide variety of natural factors cause shifts in this complex system. For instance, scientists have long acknowledged how tiny changes in the Earth’s orbit around the sun, collectively known as the Milankovitch Cycles, drive the coming and going of ice ages. As the Milankovitch Cycles interact, the planet’s movements displace the incoming solar radiation across the globe, dramatically affecting the Earth’s climate system and the advancement and retreat of glaciers.
For a while, ice ages were known to occur steadily every 40,000 years. However, a million years ago, that metronome inexplicably got off course. Instead of periods of intense glaciation occurring every 40,000 years, it shifted to every 100,000 years. But the likely culprit, the Milankovitch Cycles, hadn’t changed a million years ago. It didn’t add up.
And that’s not all. Around the same time, the massive AMOC— the conveyor belt that brings warm, shallow water to the North Atlantic, where it cools and sinks to the sea floor before returning south— nearly collapsed. Were these events related? If so, how and what was behind them?
These questions have perplexed scientists for years, as was apparent even at last month’s conference. But through an analysis of the chemical composition of basin-wide ocean sediment cores over several years, geochemist Steve Goldstein from Columbia University, who led the study presented at Goldschmidt, found unique shifts in isotopic signals that reflect a slower turn of the AMOC 950,000 years ago.
For the present study, the team examined five more ocean cores, in addition to two analyzed earlier in the decade, that also demonstrated signs of a weak AMOC. The group believes two of the cores from the North Atlantic indicate possible triggers for the AMOC crisis. They suggest that such a slowdown could have rapidly cooled the North Atlantic region, in turn lengthening the ice age rhythm.
Peter Clark, a glaciologist at Oregon State University in Corvallis, has advanced this hypothesis as the only plausible explanation for many years, wrote Paul Voosen in Science last month. Three million years ago, a sustained warming period allowed for the build-up of thick soil in the Northern Hemisphere. Ice sheets would often collapse as the soil acted as an oiled buffer. But repeated glaciations wore down the warm protective layer and enabled glaciers to dig deeper into older rock that stabilized them and helped them thicken and advance.
But as exciting as the findings may be, not everyone is sold on the hypothesis. Climate scientist Amy Clement from the University of Miami told GlacierHub it sounded like an interesting concept, but she has problems with how the AMOC idea is applied in the modern climate. Clement explains how some argue that variations in the AMOC strength control the North Atlantic surface temperature on these multi-decadal timescales.
“The problems are (1) timescale and (2) magnitude. On these short timescales, the AMOC doesn’t seem to be the driver,” she noted to GlacierHub. “Instead we think the North Atlantic surface temperatures are controlled by external forcing (some natural, such as the sun and volcanoes) and some anthropogenic (such as greenhouse gases and aerosols).”
Others including Henrieka Detlef, a paleoclimatologist at Cardiff University in the U.K., told Science that while she accepts something important happened in the North Atlantic to lead to AMOC crisis, she has yet to see conclusive evidence that northern ice sheets were increasing in thickness prior to the AMOC slowdown.
Still, most agree that ice age rhythm shifts were likely caused by more than one trigger. The Columbia team is confident that thickening ice sheets in addition to other factors played a role in the perplexing transition. “The interactions between the different components of the Earth’s climate are elusive, but understanding them is crucial for reconstructing past changes,” Maayan Yehudai, part of the research group and a graduate student at Columbia, told GlacierHub. “We still have a long way to go as scientists before we can characterize them perfectly, but I think this is another important step forward on this account.”