Peter Harris provided us with a challenging debate here on this blog where the similarity between the current situation and the 400 kya glacial transition was discussed. I sought more information from Jan Hollan – but some background here first.
So if the present time was really analogous to 400,000 years ago (kya) transition, the advice to the IPCC that no ice age was due for many 10,000s of years would be incorrect and a major concern to humanity.
Augustin et al in Nature 429, 623-628 (10 June 2004) | doi:10.1038/nature02599 discuss 8 glacial cycles from Antarctic ice cores. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.
Jan Hollan reported here that the first ever pronounced fall of summer insolation happens some 130 thousands years from now, but it is not at all so deep as those ones that started the last two Ice Ages. So, we can say there is no conceivable cause for another glaciation for at least those 130,000 years. Quite probably, another glaciation cannot come sooner that 620 thousand years from now.
Berger and Loutre argue in their Science paper that with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth’s orbit around the Sun.
There are three interacting aspects of the Earth’s orbit that need to be accounted for in making the Milankovitch radiation calculations – discussed here. The mechanisms are orbit eccentricity (roundness to elliptical); obliquity (axial tilt) and precession (wobble around the axis).
The calculations of solar radiation at 65 degree North from Milankovitch mechanisms are based on a derivative of Laskar et al.
Jan Hollan now provided an additional graph from 500,000 years ago to 200,000 years from present.
Made for a rather low 1366.3 W/m2 solar constant again. -400 ka summer insolation minimum was some 10 W/m2 lower than that one we have almost reached already. No pronounced decline of solar orbital summer forcing at 65° N is ahead of us next 50,000 years. Jan Hollan suggests we should not extrapolate past trends (like decline in summer insolation, or the shape of the past glaciation cycles). We should look at reliably computed past, current and future forcings instead (see Laskar et al for the mathematics involved). Hollan states that it is evident we have almost reached the near-future insolation minimum already. Before the atmosphere returns to normal (thousands of years), we will be on the increasing part of the insolation curve again.
Which all means – no Milankovitch based ice age predicted for 50,000 years and more likely 130,000 or 620,000 years hence according to Jan Hollan.