If you’ve read about the “ice ages” of Earth’s recent history before, you probably learned that the cyclical rhythm of these climate changes is controlled by several reliable cycles in Earth’s orbit. That relationship is pretty clear, but there’s also a fascinating and unsolved puzzle here. For about the last 700,000 years, glacial periods were each about 100,000 years long—lining up with a subtle cycle in the shape of Earth’s orbit around the Sun. If you look at the 500,000 years before that, though, you see shorter glacial periods that line up with a 41,000-year cycle in the tilt of Earth’s axis. Satisfying explanations for this change in Earth’s time signature have proven elusive.

There have been ideas, of course. It could be that the ice sheets of North America and Europe reached a sort of critical mass, becoming too big to fail during the weaker 41,000-year warm-up. The culprit could also lie in the ocean, where circulation changes or increases in the wind-blown dust that fertilizes plankton growth could pull greenhouse gas out of the atmosphere, making the ice ages icier.

A new study led by Adam Hasenfratz and Samuel Jaccard at the University of Bern may have found a piece of the puzzle at the bottom of the ocean around Antarctica.

The passage of gas

The core of seafloor sediment collected there—spanning 1.5 million years of history—targets a location where surface water and deep water mix. Carbon dioxide accumulates in the deep ocean as water makes its long journey along the bottom. Where that water mixes up toward the surface in places like this, the gas has an opportunity to vent (perhaps complaining about how dark it is down there as it makes its way into the atmosphere).

This release of carbon dioxide is one of the keys to the ice ages. The changes in Northern Hemisphere summer sunlight caused by Earth’s orbital cycles are weak—enough to melt the continent-spanning ice sheets a bit, but not enough to explain the large temperature swings. For that, you need other parts of the climate system to react and amplify the orbital-driven temperature change. The movement of CO 2 into and out of the ocean is a huge part of this, strengthening or weakening Earth’s greenhouse effect.

This means that anything that influences the exchange of CO 2 between the ocean and atmosphere can have a big impact on the dynamics of ice ages.

The researchers analyzed the shells of tiny critters called forams, some of which live as floating plankton near the surface and some of which live down on the seafloor. By measuring the isotopes of oxygen in their calcium carbonate shells, the researchers hoped to see how much mixing there was between deep and shallow waters—and therefore how much CO 2 venting was going on.

The more mixing takes place in this location, the more similar the isotopic signature of the seafloor and surface shells should be. If this mixing slows because the surface water becomes less dense (and so less able to sink downward), the isotopic signature of the two shell types can remain apart. This is because the oxygen atoms in their shells come in part from the water around them. Mix the water together, and their source material should be the same regardless of the depth. Pull from separate pools, and the chemistry of the shells will differ.

Looking back through the last few hundred thousand years, their data shows the shallow and deep water foram shells holding consistent patterns. But around 700,000 years ago—when the transition from 41,000-year-long glacial periods to 100,000-year-long ones was taking place—there’s a shift in the data. The shallow and deep water shells diverged, implying that deep water had a harder time mixing up toward the surface after that.

If significantly less CO 2 was being vented from the deep ocean to the atmosphere, the slight warming nudge from the 41,000-year cycle might not have been enough to kick off a full-blown global warming. Instead, atmospheric CO 2 would have stayed low until the 100,000-year cycle forced it to rise.

One thing leads to another

How could this have happened? The researchers note that the shallow water is both less salty than the deep water here, which should keep it from sinking, and a little colder, which should cause it to drop. Increased precipitation over the ocean or increased meltwater from Antarctic ice would add more freshwater to the surface, making the shallow water even less salty. Decreasing the salinity would decrease its density, making it less willing to mix downward. And less mixing would also bring less salt up from the deeps—further strengthening the stagnant fresher-on-top layering that stymies mixing in the first place.

If this doesn’t feel like a satisfying explanation for the change in ice age rhythm, it’s because this is still one piece of a broader puzzle. The process envisioned here could solidify the change, but it probably wasn’t the initiator. Another factor—like dust over the ocean fertilizing plankton growth—could have dragged atmospheric CO 2 levels down to a new low, with this shift off the Antarctic coast ensuring they could stay down for the duration of a longer glacial cycle. Each of these things could have helped ice sheets grow past that “critical mass” point, making the glacial climate even more stubbornly persistent.

The reason the shift to longer ice age cycles has been hard to explain is probably that there isn’t one big answer. The change in behavior could be the emergent result of complex interactions between many feedbacks in Earth’s climate system. And that’s the kind of thing we’d like to understand as we look to the future. After all, the ocean is currently soaking up a significant share of our CO 2 emissions, which would otherwise be adding to global warming. Fifty or a hundred years from now, how much will it be soaking up?

Science, 2019. DOI: 10.1126/science.aat7067 (About DOIs).