Enlarge /. A mass of stalactites in the Italian Corchia cave from which the new record comes.
The general outline of the ice ages of the past million years is quite well understood. The timing of the glacier cycles was controlled by cycles in the Earth's orbit, the temperature fluctuations being amplified by feedback in the Earth system via increased and decreased greenhouse gas concentrations. However, a general overview does not mean that there are no interesting questions in this overall picture.
Some of these questions relate to a remarkable change in the ice age rhythm that occurred about a million years ago. Before this transition, the glacier cycles came and went every 41,000 years. This corresponds to a cyclical pattern that changes the inclination of the earth's rotation axis and strengthens and weakens the contrast between summer and winter. After the transition, the glacier cycles extended to around 100,000 years. This is about as long as a cycle that changes the shape of the earth's orbit around the sun.
The latter cycle is known to be too weak to really run the show. But there is a third influence: an approximately 23,000 year "precession cycle" that changes in which section of the earth's orbit each season occurs. So which combination of these cycles explains the heartbeat of 100,000 years? And how could something suddenly take over the dominant role of tilt in the last 41,000-year heartbeat?
Climate from caves
A new study by Petra Bajo and Russel Drysdale from the University of Melbourne addresses these issues using stalagmites from a cave in Italy. The drip-drip deposit of carbonate, which slowly builds up into stalagmites, can produce identifiable annual records of the chemistry of rainwater seeping into the cave. In combination with the radiometric dating of uranium, which is also included in the carbonate, researchers can obtain high-resolution climate records from these cave features.
The longest available ice core climate records date back less than a million years, so everything that has passed is generally based on sediment cores on the ocean floor. The timeline on these sediment cores is not as accurate as that of cave records or ice cores, which leads to a small dilemma. Since we know that the climate fluctuations in the sediment records essentially match the orbital cycles, the orbital cycles are used to determine the time axis. This means that these schedules are not necessarily helpful in testing detailed hypotheses about the orbital cycles.
Enter the new Italian cave record. It spans from 970,000 to 810,000 years, with the age of each data point known within 7,000 years. The cave record was also used to set the timeline for a current record of seabed sediment. With both records available, the team compared the changes in the glaciers to the timing of the orbital cycles.
A commonly used composition of the seabed core put the length of the ice age at this period at 92,000 years, but this new record shortens it to around 85,000 years. With this shortened timing, the end of both ice ages (ie the beginning of warming) coincides with a peak in the slope cycle, but only one of them coincides with a peak in the 23,000 year precession cycle. So this shows that the slope cycle was still the dominant factor as it had been in the shorter ice ages.
Other cave records have filled up over the past 640,000 years, so the researchers have expanded their analysis over this period. They found that the time between warming events was consistently a multiple of the 41,000 year grade and 23,000 precession cycles. As others have found, it is a coincidence that the average length is around 100,000 years to match the third orbital cycle.
When considering all warming events together, the researchers argue that it is not the case that precession has taken control and write that the slope "clearly played an equal, if not greater role". While some events started with a low phase precession, all started with a slope at a high point or on the way up.
But instead of looking at two overlapping stacks of spaghetti, they blended the effects of tilt and precession in a single curve, contrasting this with the complete climate record. The result shows fairly obvious peaks that correspond very well to large warming events. And that fits the different lengths of the last glacier cycles, which were between 85,000 and 120,000 years ago.
Enlarge /. The upper field shows the inclination cycle in blue and the precession cycle in the black curve. The lower field shows them summarized in a single metric. Red vertical lines mark the time of significant warming events (including some minor events that have not completely ended the ice age). Age is shown at the bottom in thousands of years.
All of this confirms that the transition to longer ice ages was driven by something in the Earth's climate system rather than a change in the orbital pacemaker. It seems that the world's glacier configuration – with huge areas covered by ice sheets – has passed a kind of critical threshold that made it resilient enough to withstand some weaker orbital impacts. If you ever sleep long enough that you have successfully slept through an alarm undisturbed, you may be able to refer to it.
Science, 2020. DOI: 10.1126 / science.aaw1114 (About DOIs).