Thread by @ClimateAudit

@ScottAdamsSays 1/ Goddard responds to Myth #6 in Potholer video. Both misunderstand Milankovitch theory and thus exchange is uninformative. Milankovitch proposed that N Hemisphere ice sheets increased and disintegrated because of orbital-scale variations in high NH latitude summer insolation.

@ScottAdamsSays 2/ in 1970s, orbital scale variations in d18O (temperature proxy) were observed in long ice and ocean cores, lending strong support to Milankovitch theory, but important puzzles, esp very strong 100 kyr cycle in d18O.

@ScottAdamsSays 3/ later ice cores showed strong co-variability in long ice cores between CO2 and O18 - famous diagram used by Al Gore. Subsequent disputes over whether CO2 leads or lags. Even if CO2 leads, unclear what would initiate CO2 cycles.

@ScottAdamsSays 4/ in potholer video (#6), Potholer argued that orbital fluctuations in insolation were only ~0.7 wm-2, far too little to initiate and disintegrate NH continental ice sheets. However, Milankovitch had argued high-latitude NH summer insolation, where fluctuations MUCH greater.

@ScottAdamsSays 5/ Potholer also argued that there had been no relevant changes in insolation in past 15000 years and that such changes could reasonably excluded from models. This is untrue in respect to Milankovitch insolation: high latitude NH summer insolation.

@ScottAdamsSays 6/ Goddard correctly pointed out that Milankovitch theory applied high NH latitude insolation (but neglected SUMMER insolation - an important element of theory.) While his criticisms of Potholer were sort of correct, the implication that Milankovitch theory might explain recent

@ScottAdamsSays 7/ recent warming is incorrect. Recent Milankovitch forcing promotes cooling and cannot explain warming. But I think that the topic is really interesting anyway as I'll show in next diagram.

@ScottAdamsSays 8/ this diagram shows 60N summer (JJA) insolation over past 30000 years and next 2000 years (similar for similar lats). The decrease in 60N JJA insolation since Holocene Optimum is more than 35 wm-2. Decrease in 60N annual insolation ~10 wm-2. Compare to ~1.5 wm-2 anthropogenic.

@ScottAdamsSays 9/ noticeable decreases in 60N summer insolation since Roman times (~6 wm-2) and even since medieval times (~2.5 wm-2). Esper et al 2012 made important observation that tree ring proxies respond to summer temperature (insolation) but lack variability on millennial scale.

@ScottAdamsSays 10/ the above graphic also shows that modern 60N summer insolation - the Milankovitch "prime driver" - has returned to levels of the last Ice Age. This is what prompted concerns in the 1970s (when orbital-scale d18O information became available) of a return to Ice Age.

@ScottAdamsSays 11/ Lamb 1982, Climatic Changes in our Own Times and Future Threats, showed Berger's projected changes in d18O (based on Milankowitch theory) for next 60 kyr, a descent back into ice age.

@ScottAdamsSays 12/ I think that the Little Ice Age is best interpreted as incipient neo-glaciation, consistent with very low levels of summer high NH insolation, and presaging worse NH climate in centuries to come.

@ScottAdamsSays 13/ I think that 20th century warming might well be something different than advertised by either skeptics or climate activists. I think that the warming is real enough, but, instead of being "bad", mitigates descent into much more dangerous Milankovitch ice age or near ice age

@ScottAdamsSays 14/ Abe-Ouchi et al (2013), Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume, (Nature, 2013) moraymo.us/wp-content/upl… , appears to me to provide a brilliant explanation of long-standing 100 kyr puzzle in Milankovitch theory.

@ScottAdamsSays 15/ they observe that glacio-isostatic adjustment modulates isolation cycles through depression/rebound. They did simulations combining both effects in which ~100 kyr cycles emerged - see excerpt from their Fig 1. Ice sheet accumulation begins at high eccentricity, full rebound,

@ScottAdamsSays 16/ then continues through 4-5 precession cycles at lower eccentricity, growing larger and depressing continent in Canada. At full depression, disintegration occurs at high eccentricity. eg, Early Holocene 13K-8K BP. Isostatic rebound still going on in Canada. Very elegant.

@ScottAdamsSays 17/ Abe-Ouchi et al (whose ice age model appears definitive to me) weigh in on CO2 role as follows. That it "does not drive" glaciation cycles. That CO2 co-variability results from ice sheet variation which is forced by Milankovitch forcing modulated by glacio-isostasy.

@ScottAdamsSays 18/ here's another remarkable Abe-Ouchi result. In their calculations, the strong ~100 kyr cycle is not present when CO2 levels above 230 ppm. Some readers may recall that a 41 kyr obliquity cycle was dominant prior to Pleistocene (i.e. prior to 1 million BP). Transition from

@ScottAdamsSays 19/ pre-Pleistocene 41 kyr regime to 100 kyr regime of Pleistocene has been another huge mystery. Abe-Ouchi appears to have explained this as well. I just encountered this paper and do not know what objections have been raised to it. But on its face, seems like big deal