# What caused peak CO2 to rise, starting about 400,000 years ago?

This question assumes the accuracy of this (and similar) charts.

From the image, CO2 peaked at about 260, perhaps 265 ppm during the interglacial periods between 800,000 years ago to the interglacial which ended about 480,000 years ago. Beginning around 400,000 years ago, CO2 ppm reached about 280 ppm and repeated or exceeded that peak during the next 4 inter-glacial periods.

I understand that CO2 can play a feedback role, where as the oceans warm, less CO2 is absorbed and that can lead to higher CO2 ppm in the atmosphere, but whether the question is focused on the CO2 ppm or ocean temperatures, it's basically the same. What was the cause for lower peak temperature and/or lower ocean temperature leading to lower peak CO2 in the atmosphere during that time period.

Northern Hemisphere insolation doesn't explain it. That underwent even wilder swings during the lower peak CO2 period.

• It's a bit sad that they say today we have 390 ppm – Gimelist Nov 11 '19 at 10:42
• @Gimelist It's probably an old chart more than an inaccuracy. – userLTK Nov 11 '19 at 12:46
• I know, exactly my point. It's "old". – Gimelist Nov 11 '19 at 12:47
• @Gimelist - The paper from which the $\text{CO}_2$ portion of the graph was taken was published in 2008, which was when $\text{CO}_2$ levels had reached 390 ppm. What's sad is that we're well past that now, to over 407 ppm (the average for 2018). What's sadder yet is that not much is being done to address the problem. – David Hammen Nov 11 '19 at 12:49
• @DavidHammen yes, yes. Exactly my point, right? – Gimelist Nov 11 '19 at 19:44

First things first: There's nothing per se wrong in science with answering "We don't know" to a vexing problem. This might well be one of those cases.

The question you are asking was asked by Imbrie and Imbrie in 1980. The problem you have noticed (a very strong interglacial despite low Milankovitch forcing) is now known as the "stage 11 problem", and this appears to be closely associated with the transition from a ~41000 year glacial cycle to the ~100000 year cycle we have seen for the last 900000 years.

Why "stage 11"? This is a reference to the concept of Marine Isotope Stages, depicted below. The Marine Isotope Stages refer to the abundance of oxygen 18 relative to oxygen 16. Water made of two protons (hydrogen) and one oxygen 16 atom evaporate more readily than does the slightly heavier water centered on an oxygen 18 atom. This makes the oxygen 18 levels in oceanic deposits a very nice proxy for climate.

Three key transitions can be seen in the above graph. The glacial cycle started to change from a ~41000 year period to a ~100000 year period a bit less than one million years ago. The second transition occurred after MIS 17 when glacials shifted to being more extreme. The third transition, the one that you noticed, occurred after MIS 12 when interglacials shifted to being more extreme. This latter shift is the core of the "stage 11 problem".

The key issue with explaining the observed ~100000 year cycle is that the Milankovitch forcings for a 100000 cycle are rather low while the Milankovitch forcings for a ~41000 year cycle are very strong. The stage 11 problem gets at the very heart of the 100000 problem because stages 12 and 11, which mark an extremely large swing in climate, occurred when Milankovitch forcings were very low. One non-solution of this stage 11 problem is that the onset of stage 11 (or possibly the onset of the preceding stage 12) represents when the 100000 year cycle finally took full control. This is essentially the argument made by Berger and Wefer.

So, rhetorically, what did cause this shift from a glacial cycle of ~41000 years to ~100000 years, and why did it take half a million years for this transition to become complete? A partial explanation by Abe-Ouchi et al. is that something happened between 0.4 and 1.0 million years ago that enabled hysteresis loops. Instead of disappearing on a 41000 cycle, ice instead built up to such thick levels that all of the ice didn't melt at what would have ended a 41000 year glaciation. Once that happened, it wasn't much of a jump to survive two 41000 year cycles. But by then the ice had become so very thick that all that was needed was a mild forcing from warming to make the ice melt catastrophically.

This still falls in the "and then magic happens" category. One very plausible hypothesis that has gainied a lot of traction (e.g., Bintanja and Van de Wal) is that those massive ice sheets eventually pushed the built-up regolith from the 200+ million years since the previous ice age (the Karoo) either out to sea in the north or in huge piles of glacial drift to the south.

The initial glaciation events of the current ice age that started about 2.6 million years ago were rather weak in part because regolith makes for a rather nice ice lubricant. Those comparatively weak glaciation events continued for the next 1.6 million years, at a 41000 year interval. Much of the regolith had been pushed out of the way by a million years ago. While regolith is slippery (to ice), bare rock is anything but. This stickiness enabled ice to build up to the extent that those hysteresis loops could start to form, sporadically. It took another half a million years for the transition from a purely Milankovitch-driven 41000 year cycle to a Milankovitch+hysteresis 100000 year cycle.

## References:

Imbrie, John, and John Z. Imbrie. "Modeling the climatic response to orbital variations." Science 207, no. 4434 (1980): 943-953.

Berger, W. H., and G. Wefer. "On the dynamics of the ice ages: stage-11 paradox, mid-Brunhes climate shift, and 100-ky cycle." GEOPHYSICAL MONOGRAPH-AMERICAN GEOPHYSICAL UNION 137 (2003): 41-60.

Abe-Ouchi, Ayako, Fuyuki Saito, Kenji Kawamura, Maureen E. Raymo, Jun’ichi Okuno, Kunio Takahashi, and Heinz Blatter. "Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume." Nature 500, no. 7461 (2013): 190.

Bintanja, R., and R. S. W. Van de Wal. "North American ice-sheet dynamics and the onset of 100,000-year glacial cycles." Nature 454, no. 7206 (2008): 869.

• it is worth noting many natural things can increase CO2, (like sea level dropping), we just are not seeing any of them now. CO2 is also part of a feedback loop, so warming can be caused by CO2 but warming can also increase CO2. which is why adding CO2 to the system is bad. – John Nov 15 '19 at 2:50
• Vostok is an Antarctic ice core. Correlation with northern hemisphere analogues is not trivial. A broader picture is necessary that includes interplay between orbital forcing, circulation patterns and ice sheet dynamics. I don't mean to corner anybody. Interesting read including more recent pubs as well: sciencedirect.com/topics/earth-and-planetary-sciences/… – user18411 Dec 8 '19 at 14:12
• There is some evidence the difference in the cycles is caused by Milankovitch cycles aligning with advantageous conditions due to the earths axial tilt. agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL071307 – John Dec 8 '19 at 15:01
• What if an enormous amount of methan was released at the time? – Lehs Dec 19 '19 at 20:29

Prof Malte Jansen and his team at Chicago University have found that when the sea ice surrounding Antarctica increases, it cuts off the supply of CO2 from sea to air, which would explain the lows. When the sea ice retreats, that might explain the highs.

You will immediately be thinking that the normal course of events is that CO2 moves from air to sea, so why should things be different in Antarctica? The Southern Ocean is extremely deep and cold. Deep, cold water holds more CO2 than shallow warmer waters. In the area surrounding Antarctica there are rising currents from vast depths, laden with CO2. Normally when these currents reach the surface, they deliver some of their excess CO2 to the atmosphere, then dive back down into the abyss where hydrothermal vents replace the CO2. When the surface is covered with ice, this gas delivery is halted, so atmospheric CO2 is reduced. Conversely, when the sea ice recedes the delivery of CO2 resumes, perhaps at an accelerated pace because of the build up of CO2 when deliveries were halted.

This mechanism could explain or partly explain the highs and lows on your chart, but does not exclude the possibility there may be other mechanisms at work as well. Prof Jansen is convinced that the scenario set out above explains ice ages and why they do not always fit comfortably with the Milankovitch Cycles. Obviously the increased albedo of advancing ice cover is another factor which helps to bring on an ice age.

• This is not the right answer. You are once again regurgitating the rather distorted Fox News presentation of Jansen's work. Jansen's most recent article, the one distorted by Fox News, was published only a month ago, far too soon for the scientific process to determine correctness. (continued) – David Hammen Nov 11 '19 at 2:35
• Moreover, the article does not say what you think it does. From Jansen's article, Here, we show that glacial ocean-sea ice numerical simulations with a single-basin general circulation model, forced solely by atmospheric cooling, can predict ocean circulation patterns associated with increased atmospheric carbon sequestration in the deep ocean. In other words, this is a feedback rather than a cause. – David Hammen Nov 11 '19 at 2:36
• @ David Hammen You should write to Prof Jansen and tell him he's wrong. The poor fellow thinks he and his team have discovered something! You might also mention that he should write his papers more carefully and clearly, because people are constantly misinterpreting them. – Michael Walsby Dec 8 '19 at 20:18