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Atmospheric CO₂ Concentrations Over the Last 800,000 Years with a Constant Lower Limit

Over the glacial cycles of the past 800,000 years global temperatures and atmospheric concentrations of CO₂ varied widely. It has been shown by Antarctic ice cores that the concentrations of CO_2­ were very similar during all the coldest points of these cycles, in spite of this variability. It was observed by Galbraith & Eggleston that the recurring minimum concentrations of CO₂, 190 ± 7 ppm all fall on the lower bound of any known throughout the history of the Earth. In this paper it is shown by Galbraith & Eggleston that although the distribution of terrestrial ice sheet was normal over the past 800,000 years, as might have been expected by approximately normal distribution of the orbital forcing that drove the glacial cycles, temperatures of Antarctica have a strong mode, though the concentrations of CO₂ have a central mode as well as a cold mode. The recurring minima and pronounced cold modes are consistent with feedback that is strongly negative to decreasing CO­_2­ that resisted further cooling on timescales of less than 10,000 years, though there are many possible explanations. It is suggested by Galbraith & Eggleston that CO₂ limiting of photosynthesis could possibly be a negative feedback, either directly or by co-limitation of N₂ fixation, which could have inhibited any further lowering of CO₂ by the reduction of the storage of carbon.

A simple unifying mechanism has been sought by investigators for decades to explain the correlation of the atmospheric CO₂ and temperature in Antarctic ice cores over the glacial cycles that has been observed. A shift in the partitioning of carbon between the ocean and the atmosphere (Broecker & Denton, 1989), which accounts for a small change in the terrestrial biosphere, though no other change in the size of the carbon inventory (Jaccard et al., 2013; Sigman & Boyle, 2000; Sigman, Hain & Huag, 2010), in general, is called for by these explanations. According to Galbraith & Eggleston there is, however, a growing realisation that the ocean-atmosphere inventory could possibly have varied more significantly (Broecker & Putnam, 2015) as a result of rates of outgassing changes due to changes driven by isostasy (Huybers & Langmuir, 2009; Lund et al., 2016; Ronge et al., 2016), changes in permafrost peatlands (Zech, 2012) and marine carbon burial variations (Cartapanis, Blanchi & Galbraith, 2016). The ability of a single unified mechanism to explain the history of CO₂ over glacial cycles is being chipped away by the importance of geological changes in the carbon inventory. It would appear instead that the history of CO₂ was actually the product of a complex chain of events that involved many factors, which includes ocean circulation, the marine ecosystem, the terrestrial biosphere and interactions between the lithosphere and ice sheets.

One of the Antarctic ice core records which has, given this apparent complexity, received little attention, takes on a new significance. Though each of the past 8 cycles had its own peculiarities, the minima of δD (a proxy for Antarctic atmospheric temperature) and CO₂ were always very similar: -440 ± 5‰ and 190 ± 7 ppm, respectively. This similarity is in sharp contrast with the interglacials, during which δD and CO₂ varied more broadly. It is also in contrast with the different temporal pathways followed by δD and CO₂; over periods that span many thousands of years, though their overall correlation is striking.

When compared with reconstructions of atmospheric CO₂ prior to the ice ages (Royer, 2014; Pagani, Caldeira Berner & Beerling, 2009), the CO₂ minima are also remarkable. There are no reconstructions that show unambiguously lower values than the glacial CO₂ minima, though CO₂ has often been much higher than during interglacials which exceed 1,000 ppm. That minimum values of ~200 ppm were reached repeatedly during the glaciations of the Carboniferous, indistinguishable (within uncertainty) from the minima that were recorded in ice cores that spanned the past 800,000 years is shown by a recent reconstruction of atmospheric CO₂ during the last prolonged ‘icehouse’ of the Carboniferous. These observations taken together raise an important question: is there a lower limit to the atmospheric CO₂ on the timescale, which is geologically short, of glacial cycles?

The analysis presented in this paper highlights the CO₂ minima consistency, and the strong CO₂ low mode, and aspects of glacial cycles of the Late Pleistocene. The lower bound on CO₂ that has been proposed could have implications for understanding glacial cycle periodicity (Crucifix, 2012), as well as the average CO₂ consistency over the past 800,000 years. Galbraith & Eggleston say they cannot currently provide a definitive test of hypotheses that have been proposed, given a lack of observational constraints that are suitable, though their discussion points towards a feedback that stabilises CO₂ which respond directly to the concentration of CO₂. Notably, it was shown there was a single brief excursion of CO₂ below the typical glacial minimum at about 667 ka, the ice core from EPICA Dome C measurements reached a minimum of 174 ppm (Lüthi et al., 2008; Bereiter et al., 2015). The absolute value of this minimum should be regarded with care, as specific methods of measurement in technically challenging sections of the ice core can also lead to results that are erroneous (Bereiter et al., 2015; 4 Bereiter et al., 2012). If it is real, however, its short time period of a few thousand years could possibly indicate the timescale on which the feedback operates, and the testing of possible mechanisms with regard to what appears to be a fundamental, though unexplained, homeostatic regulation of the Earth system might be helped by further study.  

Galbraith, E. D. and S. Eggleston (2017). "A lower limit to atmospheric CO2 concentrations over the past 800,000 years." Nature Geosci 10(4): 295-298.