What level of CO2 Prevents Ice Ages From Happening?

[wili]

A paper last fall by A. Abe-Ouchi, F. Saito, K. Kawamura, M. E. Raymo, J. Okuno, K. Takahashi, and H. Blatter addresses this question.


Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume

Nature, Vol 500, August 8, 2013, p. 190.

Modeling produces a time plot of ice volume changes over the last 400,000 years that is a close approximation of the usual temperature or [CO2] plots over that period. An especially interesting feature of the modeling results is that if [CO2] is kept constant at 220 ppm across the entire time period, the model still produces the 100 ka ice age cycle with nearly the same curve shape.

 If [CO2] is kept constant 260 ppm, the ice ages completely disappear.

If [CO2] is kept constant at 160 ppm, the ice age frequency is much higher and interglacials are much colder.

These results indicate that [CO2] is critical to gross climate behavior, but that whether [CO2] precedes or lags temperature is not particularly important in ice age-interglacial periodicity and intensity. It also suggests that at 400 ppm of [CO2] descent into another ice age will not be possible.

So the answer seems to be that, if CO2 does not go below 260 ppm, iceages cannot form, no matter what part of the Milankovitch cycle you're in.

(Apologies if this has already been posted here. Thanks to TCFlood at RealClimate for pointing this paper out.)

[ChrisReynolds]

Thanks Wili.

[Pmt111500]

Thanks Wili, Should have guessed this had been solved on some level already, the poll at http://forum.arctic-sea-ice.net/index.php/topic,709.0.html is then pretty much obsolete.

[Steven]

On the Nature webpage of the paper mentioned by wili, there is a video showing the simulated ice sheet change for the last 400,000 years.

Here is a link to the video.  From the text accompanying the video:

"The 100,000 year glacial cycles and the fast terminations at the end of each glacial cycle are the prominently visible patterns."

Moreover, the paper discusses some numerical experiments with constant CO2 levels (as mentioned upthread):

A striking feature of our results is that, in the experiments with constant CO2 levels, the strong 100,000 year cycle with a large amplitude appears only for the North American ice sheet within a particular range of CO2 levels (Fig. 1g)...  The Eurasian ice sheet responds only to insolation forcings at 41,000 year and 23,000 year periodicities, with small amplitudes in all cases (Fig. 1h).

link to Fig. 1.  Fig. 1g is for the North American ice sheet, Fig. 1h for the Eurasian ice sheet, and they both show the modelled ice-volume change with a constant CO2 concentration (blue, 160 ppm; black, 220 ppm; red, 260 ppm) expressed in Sea Level Equivalent relative to present.

[FLwolverine]

Wili - with the help of another poster at Dr Rood's blog at WU, I was able to read a copy of the Nature paper.  What looks like a quote in tcford's RealClimate comment,  is not a quote.  Those statements do not appear in the paper.  The only place I have found those conclusions online are in a comment by tcford at SkepticalScience and the comment you quoted from RealClimate.

It appears that the statements are based on tcford's reading of the charts of model runs included in the paper.  Someone besides me will need to decide whether the conclusions are justified.  However, (a) the highest concentration of CO2 shown in a model run is 260ppm, and (b) it doesn't look to me like the graph for that run shows the "ice age" completely disappearing (but I could be wrong about that).

Here is the introduction to the paper:

The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern1,2 (gradual growth and fast termi- nation). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccent- ricity, obliquity and precession cycles4,5. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal cli- matic feedbacks may also be at work4–7. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’5,8–10, but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis11–13, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity mini- mum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14–16, which is the lithosphere–asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.

[FLwolverine]

I should add that the charts I'm referring to can be viewed at the link in Steven's post, so people here can reach their own conclusions.

[wili]

Thanks tons, Steven and FLW.

[Pmt111500]

Ah-ha! thanks. Steven.

On the Nature
link to Fig. 1.  Fig. 1g is for the North American ice sheet, Fig. 1h for the Eurasian ice sheet, and they both show the modelled ice-volume change with a constant CO2 concentration (blue, 160 ppm; black, 220 ppm; red, 260 ppm) expressed in Sea Level Equivalent relative to present.

So in their model constant 220ppm (e) is enough to drop the sea level almost as much as in true simulation with variable [co2] (100m vs.125m)! This is a great result since the low limit in the poll does not have to be changed. If reading the chart correctly, constant 260 ppm (sum of g & h) still produces almost 50m of sea level change. That's like half an Ice Age. Or, equivalent of tripling the Greenland Ice Sheet to Canada Arctic Archipelago and adding some serious amounts of mountain glaciers. Maybe their model does not answer the poll after all! Was Little Ice Age a prelude to glacial conditions (which humans interrupted) or not?

[tcflood]

It was my posting on Real Climate that Wili quoted at the top of this thread. I did not mean to claim that my posting there was a direct quote of interpretations from the paper.

My comments were my interpretations. I was hoping to generate discussion about the paper so as to make its implications more clear to me and, if my interpretations were correct, to draw attention to what seemed to me like important work. 

It is true that I missed the point that, while there are large effects of differing but constant CO2 concentrations, [CO2], on the North American ice sheet in the computations, there was almost no effect on the Eurasian ice sheet.

I apologize for my error.

However, I still wonder what the overall effect of [CO2] in excess of 400 ppm would be in these models (the number I originally mentioned in my comments). 

I also wonder about the implications that the dramatic effects of different but CONSTANT CO2 concentrations have on either of the two ice sheets (BIG effects or NO effects) have regarding the omnipresent debate about whether [CO2] trends precede or follow temperature trends in the ice age-interglacial cycles. The results in this paper suggest to me that the precise timing of these two quantities may be unimportant.

What do you think?

[Steven]

tcflood, for what it's worth, here's a quote from the paper that may be relevant to your second question:

This result supports the idea that the crucial mechanism for the ~100,000 year cycles is the delayed glacial isostatic rebound, which keeps the ice elevation low, and, therefore, the ice ablation high, while the ice sheet retreats. We note, however, that CO2 variations can result in amplification of the full magnitude of ice-volume changes during the ~100,000 year cycles, but do not drive the cycles.

(source)

This seems to be consistent with their Figure 1(d-e).  For constant 220 ppm CO2 level, the sea level changes are similar, but with a smaller amplitude, than in the simulation that uses historical CO2 concentrations from Vostok ice core as model input; note the comparison of the first glacial cycle is hard due to different initial conditions.  So the historical CO2 variations tend to amplify the peaks and troughs in the glacial cycle to some extent. 

[EDIT: given that the model is forced by CO2, probably no strong conclusions can be drawn from the discussion in the previous paragraph...]

It seems hard and somewhat subjective to specify the CO2 level that prevents glacial periods from happening.  Anyway it's interesting that in the simulation with constant 260 ppm CO2 level, the amplitude of the 100,000 year glacial cycle is strongly reduced compared to the 220 ppm case.  This may be relevant to the transition that occurred about 1 million years ago, when the dominant glacial periodicity changed from 41,000 years to 100,000 years, e.g. shown here.  Precise causes of this transition are unknown, partly because there is no sufficiently detailed CO2 record from the relevant time period.  However it should be possible to retrieve an Antarctic ice core from before this mid pleistocene transition.  It would be very interesting to have such a CO2 record, and feed it into the model of the paper.

[Steven]

Perhaps of interest: below is a figure showing proxy CO2 data, from several recent studies.  Time is expressed in millions of years before present, and the estimated CO2 concentration in parts per million.  The black line is based on data from the EPICA ice core spanning the last ~800,000 years.  The source of the figure below is here.





And here is a figure showing a proxy temperature record:

[ChrisReynolds]

Thanks Steven,

Maybe we should state the CO2 increase in terms of 'millions of years of CO2 removal undone'.

I just popped in to link to AGW Observer: Papers on AGW and the next glaciation.
http://agwobserver.wordpress.com/2014/02/18/papers-on-anthropogenic-global-warming-and-next-glaciation/

The answer to the original question of this thread may be in there somewhere.