Obliquity pacing of the late Pleistocene glacial terminations

The 100,000-year timescale in the glacial/interglacial cycles of the late Pleistocene epoch ( the past similar to 700,000 years) is commonly attributed to control by variations in the Earth's orbit(1). This hypothesis has inspired models that depend on the Earth's obliquity (similar to 40,000 yr; similar to 40 kyr), orbital eccentricity (similar to 100 kyr) and precessional (similar to 20 kyr) fluctuations(2-5), with the emphasis usually on eccentricity and precessional forcing. According to a contrasting hypothesis, the glacial cycles arise primarily because of random internal climate variability(6-8). Taking these two perspectives together, there are currently more than thirty different models of the seven late-Pleistocene glacial cycles(9). Here we present a statistical test of the orbital forcing hypothesis, focusing on the rapid deglaciation events known as terminations(10,11). According to our analysis, the null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level, whereas the corresponding null hypotheses for eccentricity and precession cannot be rejected. The simplest inference consistent with the test results is that the ice sheets terminated every second or third obliquity cycle at times of high obliquity, similar to the original proposal by Milankovitch(12). We also present simple stochastic and deterministic models that describe the timing of the late-Pleistocene glacial terminations purely in terms of obliquity forcing.