Scientists think they have a pretty good idea of how orbital variations drive the glacial cycles that have dominated Earth's recent history. Periodic changes in the Earth's rotational tilt and orientation, called Milankovitch cycles, alter how sunlight gets distributed over the planet's surface, driving the advance and retreat of ice sheets. But some of the details of how this system operates remain a bit hazy, and researchers have been puzzled by a transition called the Mid-Brunhes Event, which took place 430,000 years ago. Before the Mid-Brunhes, even the warm interglacials were colder than the present, with significant ice sheets left behind; afterwards, the conditions were similar to our current ones.

A paper published this weekend at Nature Geoscience examines a number of interglacial periods both before and after the Mid-Brunhes Event, and ties the climatic changes into differences in the Milankovich influences on the climate, enhanced by forcings from greenhouse gasses. The net result is a more moderate interglacial, with warmer winters and slightly cooler summers, with most of the changes happening during the Southern Hemisphere's summer.

Milankovitch cycles are driven by subtle changes in the Earth's orientation towards the sun. The Earth's axis of rotation isn't vertical relative to the plane of the solar system; instead, its tilt, or obliquity, is currently off by roughly 23°, and varies by over one degree in either direction over the course of tens of thousands of years. The direction of this axis, as well as the orientation of the long axis of the planet's elliptical orbit, also vary on similar time scales (called the precession). Combined, these factors influence how sunlight gets distributed across the surface of the Earth.

Warmer, interglacial periods are associated with the Northern Hemisphere summer seeing peak exposure to sunlight. The actual difference in energy terms is quite small, but feedbacks enhance the small effect. For example, the increased sunlight begins to melt the glacial ice sheets, exposing ground that absorbs far more radiation. Ocean circulation also increases, adding CO 2 to the atmosphere and increasing greenhouse forcings.

If the process is so similar for each interglacial, though, why the big switch at the Mid-Brunhes Event?

The authors at first attempted to tie it directly to orbital forcings by plotting the peak precession and obliquity against the peak temperature (temperature was estimated using oxygen isotope ratios obtained from ocean sediments). Unfortunately, the pattern they saw was essentially random, which sent them back to the drawing board.

The next thing they tried worked significantly better: they plotted the distribution of sunlight across the Earth's surface for each interglacial. It turns out that the post-Mid-Brunhes interglacials had a notably different distribution of warmth. In the Northern Hemisphere's summer, more recent interglacials are notable for receiving less sunlight, while the Southern Hemisphere remains largely unchanged. During the Northern Hemisphere's winter, however, the other pole gets a lot more sunlight.

The authors used a climate model to see how this would impact temperatures once other forcings and feedbacks, such as greenhouse gasses, are considered. It turns out that both poles and (to a lesser extent) all the continents heat up dramatically during the Northern Hemisphere's winter, while they cool off only slightly during the spring and summer. This should be sufficient to enhance both the ice and greenhouse gas feedbacks, producing the warmer interglacials that have characterized the last 430,000 years.

If the authors are right, then what looked to be a change of state—a distinct before-and-after centered on the Mid-Brunhes Event—is actually simply the result of subtle differences in the operation of a single, cyclical system.

If there's a frustration with the paper, it's that the authors don't run their system any further than is needed to evaluate that last million years. That's reasonable in the sense that this is the period for which ice cores provide good climate data, but it would be interesting to know whether the apparent changes in solar input that the authors see following the Mid-Brunhes will reverse in the future, or have flip-flopped in the more distant past. It would make their model into a predictive one, which could be confirmed if we get better data on climates older than our ice cores.

Nature Geoscience, 2010. DOI: 10.1038/NGEO771

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