That the onset of glacial (cold) and interglacial (warm) periods on earth are a consequence of the Milankovitch cycles is almost certain. Researchers have now developed a model which seems to be able to explain why and when glacial periods end to give interglacial conditions. Exactly what cause glacial conditions to be triggered remains to be discovered.
P. C. Tzedakis, M. Crucifix, T. Mitsui, E. W. Wolff. A simple rule to determine which insolation cycles lead to interglacials. Nature, 2017; 542 (7642): 427 DOI: 10.1038/nature21364
Abstract: The pacing of glacial–interglacial cycles during the Quaternary period (the past 2.6 million years) is attributed to astronomically driven changes in high-latitude insolation. However, it has not been clear how astronomical forcing translates into the observed sequence of interglacials. Here we show that before one million years ago interglacials occurred when the energy related to summer insolation exceeded a simple threshold, about every 41,000 years. Over the past one million years, fewer of these insolation peaks resulted in deglaciation (that is, more insolation peaks were ‘skipped’), implying that the energy threshold for deglaciation had risen, which led to longer glacials. However, as a glacial lengthens, the energy needed for deglaciation decreases. A statistical model that combines these observations correctly predicts every complete deglaciation of the past million years and shows that the sequence of interglacials that has occurred is one of a small set of possibilities. The model accounts for the dominance of obliquity-paced glacial–interglacial cycles early in the Quaternary and for the change in their frequency about one million years ago. We propose that the appearance of larger ice sheets over the past million years was a consequence of an increase in the deglaciation threshold and in the number of skipped insolation peaks.
Science Daily reports:
…. In a new study published today in Nature, researchers from UCL (University College London), University of Cambridge and University of Louvain have combined existing ideas to solve the problem of which solar energy peaks in the last 2.6 million years led to the melting of the ice sheets and the start of a warm period.
During this interval, Earth’s climate has alternated between cold (glacial) and warm (interglacial) periods. In the cold times, ice sheets advanced over large parts of North America and northern Europe. In the warm periods like today, the ice sheets retreated completely.
It has long been realised that these cycles were paced by astronomical changes in the Earth’s orbit around the Sun and in the tilt of its axis, which change the amount of solar energy available to melt ice at high northern latitudes in summer.
However, of the 110 incoming solar energy peaks (about every 21,000 years) only 50 led to complete melting of the ice sheets. Finding a way to translate the astronomical changes into the sequence of interglacials has previously proved elusive.
Professor Chronis Tzedakis (UCL Geography) said: “The basic idea is that there is a threshold for the amount of energy reaching high northern latitudes in summer. Above that threshold, the ice retreats completely and we enter an interglacial.”
From 2.6 to 1 million years ago, the threshold was reached roughly every 41,000 years, and this predicts almost perfectly when interglacials started and the ice sheets disappeared. Professor Eric Wolff (University of Cambridge) said: “Simply put, every second solar energy peak occurs when the Earth’s axis is more inclined, boosting the total energy at high latitudes above the threshold.”
Somewhere around a million years ago, the threshold rose, so that the ice sheets kept growing for longer than 41,000 years. However, as a glacial period lengthens, ice sheets become larger, but also more unstable.
The researchers combined these observations into a simple model, using only solar energy and waiting time since the previous interglacial, that was able to predict all the interglacial onsets of the last million years, occurring roughly every 100,000 years.
Dr Takahito Mitsui (University of Louvain) said: “The next step is to understand why the energy threshold rose around a million years ago — one idea is that this was due to a decline in the concentration of CO2, and this needs to be tested.”
The results explain why we have been in a warm period for the last 11,000 years: despite the weak increase in solar energy, ice sheets retreated completely during our current interglacial because of the very long waiting time since the previous interglacial and the accumulated instability of ice sheets. …..
What would cause the current interglacial to end remains to be discovered. It’s only my speculation of course but I suspect that a trigger event is probably needed. Possibly 2 or 3 major (VEI >6) volcanic eruptions over a short period, with large amounts of dust, which in turn led to a a few “years without summers”, could provide such a trigger for an unstoppable process. However the onset of full glacial conditions would still take a few thousand years. The availability of high energy densities would probably make it (relatively) easy for humans to continue to thrive and prosper (as they have done through other glacial periods with much lower energy availability).