You may have seen them before—the graphs from Antarctic ice cores showing the heartbeat of “ice ages” (or glaciations). If so, you probably noted a cyclical pattern, with each glaciation lasting about 100,000 years before being abruptly interrupted by a relatively brief warm period—the interglacial. Soon, the slow freeze inexorably gripped the planet again. There's a reason for this rhythmic pattern—cycles in Earth’s orbit that subtly alter the sunlight reaching the Earth.

But the graphs have long contained a couple head-scratching mysteries to climate scientists, though. First, why is the 100,000 year cycle dominant? There are several orbital cycles—some around 20,000 years long, another about 41,000 years long, and then the 100,000 year cycle. By itself, the 100,000 year cycle changes things the least, yet it drives the glacial heartbeat.

There are some good answers to that question, but then there’s the other mystery: once you look back about a million years into the past, the heartbeat changes. Instead of glacial cycles 100,000 years long, a more rapid pulse of 41,000 years becomes the norm. Something happened to change that. Here, too, there are some hypotheses, but the data to test them has been scarce.

Enter a new study from researches at the University of Cambridge. The study presents 1.5 million years of climate history recorded in ocean sediments off the eastern shore of New Zealand. As with most ocean cores, the team measured isotopes of oxygen in the calcium carbonate shells of single-celled foraminifera. (The same isotopes are used to extract climate records from ice cores.)

While those cores are astonishing libraries of climate history, they are complicated by the fact that the oxygen isotopes in those shells are tracking more than one variable. The amount of water that ends up trapped in ice sheets on the continents (lowering sea level) alters the isotopic ratio in the ocean. This means that changes in the record indicate changes in the volume of ice present on the planet. At the same time, the temperature of the ocean water affects the chemistry of the foraminifera’s shell growth, and this affects the isotopic signature as well.

One way to account for this confusion is to find a separate proxy that only records temperature. The ratio of magnesium to calcium (magnesium can take the place of calcium in carbonate shells) does just that. When you subtract the effect of temperature change from the oxygen isotope signal, you’re left with only one thing: ice volume.

This isn’t a new technique, but its application to the transition from 41,000 year glacial cycles to 100,000 year ones makes this record very valuable. Researchers studying the transition have mostly had to do so through smudged lenses—using climate records that couldn’t differentiate between temperature change and ice sheet behavior.

Existing records generally indicate that the transition was gradual, phasing in over a period of 500,000 years or so as glaciations grew colder. In contrast, this new record shows no trend in temperature and a sudden transition in ice volume, which hit a new maximum 900,000 years ago. Glaciations flipped into a different mode from then on.

While the details of this study may be different, it's not the first to provide evidence that ice sheets grew larger during this time. This growth is the best explanation available for the change to 100,000 year glacial cycles. Beyond a certain size, ice sheets become more stable in some ways—the high altitude region of the ice sheet sticks its head into cooler air at higher elevation, for example. Larger ice sheets could withstand the orbital warming nudge that previously ended glaciations after 41,000 years.

The most likely candidates for that increase in ice sheet volume have been the Northern Hemisphere ice sheets, but this new study proposes that the critical change may have occurred in Antarctica. The record reflects ocean conditions near Antarctica, whereas the coverage of other ocean sediment cores covering this timeframe have been biased toward the North Atlantic. Since the new record differs from the others, it suggests the Antarctic ice sheet were not marching in step with the Northern Hemisphere ice sheets.

And there’s good reason to think that the Antarctic ice sheet could have “gone rogue.” The rise in incoming solar radiation at the end of the 41,000 year period just prior to the increase in ice volume was very weak, which could have allowed the Antarctic ice to skip the usual melt, and then grow to a new maximum size. (Because it’s summer solar radiation that matters, orbital changes for the two hemispheres are not synchronized— only the Southern Hemisphere experienced these unusual conditions.) After skipping that beat, the researchers think this larger Antarctic ice sheet could have guided the climate into the 100,000 year groove.

In a perspective published in the same issue of Science, Oregon State’s Peter Clark writes, “Confirmation of these hypotheses will require generation of similar-quality… data sets, which should help to better understand the range of regional variability in deep-ocean temperature and [oxygen isotope signature].” Are there other explanations for the pattern seen in this climate record? Answering that question will require more ice volume records from more locations, each of which will dispel a bit more of the mystery surrounding this pivotal transition in Earth’s climate system.

Science, 2012. DOI: 10.1126/science.1221294 (About DOIs).