Reviewer 4

a strong increase in GHG to imagine that in the long run Milankovitch forcing might win. The Romans that for many centuries lived in a warm world characterized by technical progress could not imagine that their mighty empire could fall amid a cooling and worsening climate and terrible plagues into a millennial dark age of lost knowledge and declining civilization. A new glacial period would constitute humankind's biggest test and clearly has the potential to constitute its worst catastrophe. The world is so populated that an important cooling would place us in overshooting conditions. The precautionary principle requires that we start preparing for that possibility over the next decades and centuries while we are in a warm optimum, as cooling periods are rife with troubles. For the past 2 million years, when obliquity declined enough, a glacial period always followed. Obliquity is declining fast, and we should not place too much confidence in computer models that tell us this time will be different.

14.9 Conclusions

14a. Typical interglacials last on average 13 kyr from their start to glacial inception, when their temperature decrease significantly accelerates towards glacial levels.

The Holocene is astronomically, and according to its

Antarctica temperature evolution, a typical interglacial. 14b. The glacial cycle fits a model of a stable glacial state that reaches an excitable point where fast excitation (rapid warming) takes it to a quasi-stable interglacial state that slowly degrades to an unstable point (glacial inception) where a slow relaxation takes it back to the glacial state. This dynamic system is defined as a fastslow excitable system around a two-branch slow manifold. 14c. Due to the c. 6000-yr lag between orbital forcing and ice-volume effect, the orbital threshold for glacial inception is crossed several millennia before glacial inception takes place. Analysis of the past 800 kyr indicates the orbital threshold to terminate the Holocene was crossed 1400–2400 years ago. 14d. In the absence of sufficient anthropogenic forcing, glacial inception might take place in 1500–4500 years as determined by orbital parameters, average interglacial length, Neoglaciation length, and solar variability periodicities. 14e. The long interglacial hypothesis rests on the wrong astronomical parameter, high equilibrium climate sensitivity to CO2, and uncertain model predictions of very long-term CO2 decay rates. The virtual certainty by the IPCC that a glaciation is not possible for the next 50 kyr if CO2 levels remain above 300 ppm is unsupported by evidence.

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