With more than 30 explanations proposed for these glacial cycles, researchers at the Woods Hole Oceanographic Institution (WHOI) and the Massachusetts Institute of Technology (MIT) looked at the various possibilities to determine a more precise explanation. Some hypotheses suggested changes in Earth's orbit, others that glacial cycles are caused by random climate variability. The researchers found that the most plausible cause was that variations in the tilt of the Earth's axis control the timing of glaciations, acting as a planetary pacemaker of sorts.
Peter Huybers, a postdoctoral fellow in the WHOI Geology and Geophysics Department, and coauthor Carl Wunsch of MIT developed a simple model to look at the effects of changes in Earth's tilt, which determines climate belts around the planet and the seasons of the year. They also focused on rapid deglaciation events known as terminations, easily identified in climate records by their magnitude and abruptness. They first estimated the timing of glacial cycles using the rate at which sediment accumulates on the ocean floor as an indicator of time. The age estimates were then used to compare the timing of the glacial cycles with the timing of changes in Earth's orbit, known from the laws of motion and observations of the galaxy.
"Many studies have suggested a link between orbital variations and the approximately 100,000-year glacial cycles which occurred during the late Pleistocene, about 1 million to 10,000 years ago, but this is the first rigorous test of whether the glacial cycles are, in fact, paced by orbital variations," Huybers said. "We found that glaciations end near times when the Earth's tilt, or obliquity, is large. This narrows the number of possible explanations for the glacial cycles to those which can account for the tilt pacing of glacial cycles."
Obliquity, the angle between Earth's equatorial and orbital planes or the tilt in Earth's axis, varies between 22.5 and 24 degrees during a cycle of 41,000 years. As the tilt increases, so does the annual average sunlight reaching high latitudes, and these are the conditions under which Huybers and Wunsch find that glaciations end. Earth's tilt is currently 23.5 degrees and decreasing. Without the much more rapid anthropogenic or human influences on climate, Earth would probably be slowly moving toward glaciation.
"While we are confident that Earth's tilt paces the 100,000-year glacial cycles, we were not able to determine whether another orbital effect, the precession of Earth's equinoxes, also contributes to the pacing," Huybers said. Precession measures the slow change in the orientation of Earth's rotation axis, similar to a spinning top, and has been the favorite explanation amongst most scientists for the timing of the glacial cycles.
One major question is how can a 40,000-year tilt cycle produce 100,000-year glacial cycles? Huybers and Wunsch suggest that during the late Pleistocene glaciation did not end every time the tilt was large, but rather that glaciers grew over two (80,000 years) or three (120,000 years) obliquity cycles before ending. The average glacial duration then gives the 100,000-year time scale.
A possible explanation for why deglaciations do not occur every 40,000 years is that ice sheets must become large enough before they are sensitive to changes in Earth's tilt. Huybers and Wunsch developed a simple mathematical model to express this idea of changing sensitivity and showed that it gives the right timing for the glacial cycles.
Obliquity control of the recent glacial cycles provides a fresh view on the dramatic climate swings the Earth has been subjected to over the past one million years. "While the problem is far from solved, we are now one step closer to understanding the origins of the ice ages," Huybers said.
The research study was supported by the NOAA Postdoctoral Program inClimate and Global Change and the National Ocean Partnership Program (ECCO). Estimating the Circulation and Climate of the Ocean (ECCO) is a consortium of scientists focused on bringing experimental tools to practical and operational status for studying large-scale ocean dynamics, designing observational strategies, and examining the ocean's role in climate variability.