3.9 Tidal cycles as an explanation for Dansgaard–Oeschger triggering mechanism

ice core, unaffected by CO2 levels being constant until 115 ka and then falling by one third of the glacialinterglacial difference in the last 4 kyr (Fig. 2.18). It is hard to argue for an important role by CO2 on glacial terminations without evidence, when it clearly doesn't have one at glacial inceptions.

We know from ice core measurements that glacial termination I (the closest to us, starting at 18 ka) involved a change in CO2 atmospheric concentrations from 190 to 265 ppm, an increase of 75 ppm. Concurrently the temperature increased globally by an estimated 4–6 °C (Schneider von Deimling et al. 2006; Annan & Hargreaves 2013; Tierney et al. 2020). CO2 role has been estimated at causing directly 30% of the warming, and indirectly through forcing interactions another 20% (Gregoire et al. 2015), so between half and one third of the warming (or about 2–3 °C) is attributed to the CO2 increase.

A simple calculation tells us that the rise from 190 to 265 ppm is 48% of a doubling. This is true because we are dealing with a logarithmic scale, (ln(265)–ln(190))/(ln (190(2)–ln(190))=0.48. So 48% of a doubling is estimated to have produced c. 2–3 °C of warming between 18–10 ka. The rise from preindustrial to current levels of CO2 (280 to 415 ppm, or 135 ppm) constitutes 57% of a doubling. That is (ln(415)–ln(280))/(ln(280(2)–ln(280))=0.57, so it should be similar in terms of warming effect. Yet, even if CO2 is responsible for 100% of modern warming, as the IPCC claims (see Chap. 9), why has it produced only about 0.8 °C increase (HadCRUT4 1850–2020)? Something is not right. If our knowledge of past CO2 levels is correct, and the hypothesis that CO2 was responsible for one third to half of the warming at glacial termination is correct, at 18 ka CO2 was two to three times more potent than now. If anthropogenic CO2 is not responsible for all the warming observed since 1850, as it appears probable, then the situation is much worse. There is no way to reconcile the disparity that was already noticed by the late Marcel Leroux (2005). So we must accept, based on current data, that CO2 had a very minor role during the glacial cycle, responsible for, at most, one sixth of the warming at terminations, and therefore conclude that CO2 is not an important climate factor during the Plio–Pleistocene glacial–interglacial transitions.

A more pessimistic view is to consider that the recent CO2 increase entails more than 1 °C of committed warming not yet manifested, implying an equilibrium climate sensitivity of 4 or more. There is no evidence to support this belief. In fact, there is ample evidence against it: • The continued removal of anthropogenic CO2 via increasingly robust carbon sinks. The more we produce, the more is removed from the atmosphere. An increasing removal rate works against a hypothesized high warming commitment from current CO2 levels. • The lack of evidence for a climate sensitivity of 4 or more. Most experimentally deduced values for equilibrium climate sensitivity are between 1.5 and 2.5 (see

Sect. 9.5), half of the rate required for the claimed role of CO2 in deglaciation. • The lack of a significant increase in the rate of warming during the last century. If we had actually increased the committed warming significantly, the rate of warming should have increased proportionally, but that is not what has been observed (see Sect. 12.7).

• A great amount of committed warming should make periods of decades with little or no warming increasingly unlikely. Again this goes against observations.

The only reasonable way to reconcile the disparity in CO2 increases and temperature increases between glacial termination I and Modern Global Warming is to conclude that CO2 had a minor role in glacial termination. Further, it is reasonable to expect it will have a minor role in the next glacial inception.

2.12 Conclusions

2a. Obliquity is the main factor driving the glacial–interglacial cycle. Precession, eccentricity and 65°N summer insolation play a secondary role. The glacial cycle is the result of the interplay between two cycles operating simultaneously, the 41-kyr obliquity cycle and the global ice-volume 100-kyr cycle related to eccentricity. 2b. The Mid-to-Late Pleistocene pacing of interglacial periods is the consequence of the Earth being in a very cold state that prevents almost half of obliquity oscillations from successfully emerging from glacial conditions. The rate for the past million years has been 72.7 kyr/interglacial, or 1.8 obliquity oscillations between interglacials. This can be generally described as one interglacial every two obliquity oscillations except when close to the 405 kyr eccentricity peaks, when interglacials take place at every obliquity oscillation. 2c. Glacial terminations require, in addition to rising obliquity, the existence of very strong feedback factors recruited at very high global ice-volume levels. Increasing northern summer insolation once obliquity is rising is a positive factor, and if high enough during eccentricity peaks, it can drive the termination process on its own. 2d. The Mid-Pleistocene Transition was likely provoked by the continuous cooling of the planet causing too much accumulation of extrapolar ice. This change introduced the counter-intuitive requirement for large ice sheets build up to trigger an interglacial, starting the 100-kyr global ice-volume cycle and causing obliquity-driven interglacial generation to skip one oscillation unless high eccentricity removes the ice volume requirement. 2e. The most convincing hypothesis on the effect of obliquity is through changes to the summer latitudinal insolation gradient, that affects the latitudinal temperature gradient controlling energy and moisture transport to the poles. 2f. CO2 can only produce a minor effect in glacial terminations since its measured change in concentration (roughly a third of a doubling, which represents half of the warming effect of a doubling) is too small to account for any important contribution to the large observed temperature changes.

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