The glacial cycles that have dominated the Earth's recent past have been driven by a combination of orbital cycles and greenhouse forcing. But the relatively cool, rhythmic cycles appear to be an anomaly in the planet's history, as long periods of warmer and colder conditions seem to have dominated. Is there something unique about the current climate? A new study that looks at part of the Eocene climate record more carefully finds that some of the same cycles may have been in operation about 50 million years ago, creating events that have picked up the name "hyperthermals."

The modern glacial cycles are triggered by periodic shifts in the Earth's axis and orbital eccentricity, which alter the amount of sunlight and its distribution across the planet. On their own, these changes are relatively minor; however, they do trigger some significant feedbacks. The retreat of ice sheets, for example, limits the surface area of the planet that's covered by highly reflective ice, and has a warming effect. It also enables exchange between the atmosphere and deep ocean near Antarctica, which releases dissolved carbon compounds; those enhance warming through the greenhouse effect. The net result is that a small change in the Earth's orbit and/or tilt can trigger a large change in its climate dynamics.

These orbital changes, however, have been going on for far longer than recent glacial cycles. Was the Earth's climate previously indifferent to them? Apparently not, according to a paper released by Nature this week.

The researchers looked at the Eocene, a time when the climate in general was much warmer than it is now; the period was also dominated by the Paleocene-Eocene Thermal Maximum, when the planet experienced a relatively sudden warming of 5-7°C. It wasn't the only event of its kind, though. A variety of studies have identified a number of smaller events, called "hyperthermals," that were typically 2-4°C warmer than the surrounding period.

The new paper focuses on a sediment core from the Atlantic that covers 2.4 million years of the Eocene, starting about 50 million years ago; where possible, the data was confirmed using a sample from the Pacific, which indicated whether the trends in the Atlantic were global. In this record, the authors identify over a dozen hyperthermal events, indicating they were much more common that had previously been suspected. Each warming event was accompanied by changes to the planet's carbon cycle. (Both temperature and carbon cycle status are derived from isotope ratios within the sample.)

But the other thing that was notable about them is their regularity. Each hyperthermal lasted approximately 40,000 years roughly the same amount of time the Earth takes to cycle through the full range of its rotational axis' tilt. They were also spaced about 100,000 to 400,000 years apart, periods that are typical of other orbital variations. Thus, the authors conclude that orbital fluctuations were driving the climate in the Eocene, much like they have been in recent times.

Since the Eocene was so warm, however, those orbital forcings wouldn't have received any feedback from a retreating ice sheet, since no major ice sheets existed. What could drive the relatively large isotope swings?

A hint comes from the fact that the hyperthermals were associated with dark clays in the sediment sample, due to a lack of calcium carbonate. This suggested that the oceans were experiencing acidic conditions that dissolved carbonate out of the sediments. Ocean acidification, in turn, is associated with elevated atmospheric concentrations of CO 2 . Thus, like modern times, the orbital cycles seem to have triggered a carbon cycle feedback that enhanced the warming trends.

The amount of carbon required, however, is much larger than could be provided by sources like methane clathrates. After considering a number of other options, the authors settle on dissolved carbon in the abyssal oceans. Even that, however, may not be sufficient, unless the deep oceans happened to be nearly anoxic at the time, enabling lots of dissolved organic carbon to persist. There is some evidence to support this proposed source, but the authors can't explain how it would be suddenly liberated by changes in orbital forcings.

Even without a complete picture, however, the paper provides an indication that the same sorts of behavior we have seen in the Earth's recent past—orbital changes driving oceanic carbon release—have been in operation for tens of millions of years, and persisted even in a much warmer climate.

Nature, 2011. DOI: 10.1038/nature09826 (About DOIs).