The role of Southern Ocean processes in orbital and millennial CO2 variations – A synthesis

https://doi.org/10.1016/j.quascirev.2009.06.007Get rights and content

Abstract

Recent progress in the reconstruction of atmospheric CO2 records from Antarctic ice cores has allowed for the documentation of natural CO2 variations on orbital time scales over the last up to 800,000 years and for the resolution of millennial CO2 variations during the last glacial cycle in unprecedented detail. This has shown that atmospheric CO2 varied within natural bounds of approximately 170–300 ppmv but never reached recent CO2 concentrations caused by anthropogenic CO2 emissions. In addition, the natural atmospheric CO2 concentrations show an extraordinary correlation with Southern Ocean climate changes, pointing to a significant (direct or indirect) influence of climatic and environmental changes in the Southern Ocean region on atmospheric CO2 concentrations.

Here, we compile recent ice core and marine sediment records of atmospheric CO2, temperature and environmental changes in the Southern Ocean region, as well as carbon cycle model experiments, in order to quantify the effect of potential Southern Ocean processes on atmospheric CO2 related to these orbital and millennial changes. This shows that physical and biological changes in the SO are able to explain substantial parts of the glacial/interglacial CO2 change, but that none of the single processes is able to explain this change by itself. In particular, changes in the Southern Ocean related to changes in the surface buoyancy flux, which in return is controlled by the waxing and waning of sea ice may favorably explain the high correlation of CO2 and Antarctic temperature on orbital and millennial time scales. In contrast, the changes of the position and strength of the westerly wind field were most likely too small to explain the observed changes in atmospheric CO2 or may even have increased atmospheric CO2 in the glacial. Also iron fertilization of the marine biota in the Southern Ocean contributes to a glacial drawdown of CO2 but turns out to be limited by other factors than the total dust input such as bioavailability of iron or macronutrient supply.

Introduction

Carbon dioxide represents one of the most important greenhouse gases in the Earth's atmosphere, second only to the atmospheric content of water vapor (IPCC, 2007). Accordingly, significant variations in its atmospheric concentration lead to a respective change in radiative forcing. Due to anthropogenic emissions of CO2 by fossil fuel burning and land use changes, CO2 has increased over the last 150 years from a preindustrial level of approximately 280 ppmv to 385 ppmv in recent years, as documented in both atmospheric (http://www.cmdl.noaa.gov) and ice core records (Etheridge et al., 1996, Indermühle et al., 1999, MacFarling Meure et al., 2006). From these data one can confidently state that the recent rate of increase (up to 2 ppmv/year) has not been seen at decadal or longer timescales for at least the last 20,000 years (Joos and Spahni, 2008) and past CO2 concentrations in ice cores over the last 800,000 years have never come close to current CO2 levels (Petit et al., 1999; Siegenthaler et al., 2005, MacFarling Meure et al., 2006, Lüthi et al., 2008). Resulting from this anthropogenic CO2 increase is a change in radiative forcing of about 1.7 W/m2 which is in large parts responsible for the 0.7 °C warming observed over the last century (IPCC, 2007).

Today, the global carbon cycle is being driven out of equilibrium primarily by the burning of fossil fuels and deforestation leading to a continuing increase in atmospheric CO2 since the start of the industrialization. However, apart from the increasing anthropogenic emissions it is the carbon turnover and uptake capacity of the ocean and terrestrial biosphere that determines the long-term fate of this anthropogenic perturbation and its radiative forcing. In the pre-anthropocene, the natural shift from glacial to warm climate conditions initiated a significant increase in atmospheric CO2 and in terrestrial biospheric carbon stocks while reducing the carbon storage in the deep ocean. In return, higher natural CO2 levels lead an additional warming of the atmosphere via their feedback on the radiative balance (Fischer et al., 1999; Caillon et al., 2003). These natural CO2 changes are well archived in Antarctic ice cores (Neftel et al., 1982, Stauffer et al., 1998; Fischer et al., 1999, Indermühle et al., 1999; Petit et al., 1999; Monnin et al., 2001, Siegenthaler et al., 2005; Ahn and Brook, 2008, Lüthi et al., 2008) and show about 100 ppmv lower CO2 concentrations during glacial than interglacial times as well as significant millennial CO2 variations during glacial periods. Accordingly, the global carbon cycle is intimately coupled to long-term climate changes. Besides the necessity of an increased radiative forcing by CO2 and other greenhouse gases in the atmosphere to explain the glacial/interglacial temperature rise (Köhler et al., 2010) the climate induced changes in the carbon cycle and their consequence for atmospheric CO2 also represent an important testbed for hypotheses invoking changes in ocean circulation or marine and terrestrial biogenic productivity in the past. For instance any theory invoking changes in ocean circulation would affect the carbon storage in the ocean and, thus, would also have to consider its effect on atmospheric CO2.

Despite the importance for climate changes today and in the past, the glacial/interglacial CO2 changes could not be accounted for in carbon cycle models until recently. In recent model experiments, however, it has become possible to quantitatively explain the 100 ppmv glacial/interglacial shift in atmospheric CO2 concentrations by combining the effects of different processes acting on the global carbon cycle, such as sea surface temperature (SST) and salinity changes, gas exchange, ocean circulation, marine biological export production, terrestrial carbon storage, and carbonate compensation in the deep ocean (Archer et al., 2000a; Sigman and Boyle, 2000, LeGrand and Alverson, 2001; Paillard and Parrenin, 2004; Köhler et al., 2005; Köhler and Fischer, 2006, Brovkin et al., 2007). Although the total 100 ppmv change can now in principle be accounted for, the contributions of each individual process to the overall change still carry substantial uncertainties that do not allow for a unique solution to the problem. However, all the models agree that changes in the biological or physical carbon fluxes in the Southern Ocean (SO) connected to export production of organic material at the surface and SO circulation changes, together with their carbonate compensation feedback (Broecker and Peng, 1987) in the deep ocean, represent the most important factors influencing atmospheric CO2 on orbital time scales. Former comparisons of simulated atmospheric CO2 to changes in high latitude processes suggested that more complex models are in some aspects less sensitive on SO processes than simpler carbon cycle box models (Archer et al., 2000b, Broecker et al., 1999). However, the comparison of completely different types of models is not straightforward (Lane et al., 2006) and a given model maybe high latitude sensitive to certain parameters but not to others. The challenge remains to explain the low glacial CO2 in a self-consistent 3-dimensional dynamical model setting.

An important role of the SO in terms of the carbon cycle is also suggested by a very high correlation of atmospheric CO2 and Antarctic temperatures over the last 800,000 years (Wolff et al., 2005; Lüthi et al., 2008), the latter being reliably archived in the stable water isotope record in Antarctic ice cores (Jouzel et al., 2007). Despite this agreement on the role of the SO for atmospheric CO2 levels, disagreement exists on the importance of ocean circulation vs. export production changes, and about the physical processes that cause ocean circulation changes and affect carbon fluxes in the SO (Sigman and Boyle, 2000, Matsumoto et al., 2002, Köhler et al., 2005, Toggweiler et al., 2006, Watson and Garabato, 2006, Menviel et al., 2008, Parekh et al., 2008, Tschumi et al., 2008, Martínez-Garcia et al., 2009).

The goal of this paper is to discuss the potential SO processes that can lead to a glacial drawdown of atmospheric CO2 and to confront these hypotheses with the various marine sediment, ice core and modeling evidence. To this end, we will review the latest ice core observations on glacial/interglacial and millennial CO2 changes, the theoretical background of SO circulation changes and their role for carbon storage in the deeper ocean, as well as modeling exercises to constrain those changes and their effect on atmospheric CO2. We intentionally do not discuss carbon cycle processes outside the SO region, which are either rather well constrained (such as global SST and salinity) or beyond the scope of this paper (such as the terrestrial biosphere or carbonate compensation). However, it has to be kept in mind, that although SO processes dominate the atmospheric CO2 changes in the past, the full CO2 story can only be told when processes outside the SO are also taken into account.

Section snippets

Ice core data of atmospheric CO2 changes

Bubble enclosures in polar ice cores represent the only direct atmospheric archive that allows for reconstruction of the atmospheric composition over hundred thousands of years. In the case of CO2, Antarctic ice cores represent the only unaltered archive of past atmospheric CO2 concentrations, while in Greenland ice cores high carbonate and low pH in combination with higher concentrations of organic impurities in the ice leads to in situ production of CO2 (Anklin et al., 1995, Smith et al., 1997

Southern Ocean carbon cycle processes

Current understanding essentially suggests that in addition to the effect of the higher solubility of CO2 in SO surface waters during colder climate periods, which accounts for about 15 ppmv lower glacial atmospheric CO2 (Köhler and Fischer, 2006), two classes of SO processes could account for such a correlation: (i) changes in marine biological productivity in the SO, and (ii) changes in SO circulation (advection or mixing), that vary the exchange between carbon-enriched deep waters and the

Other evidence for SO hydrography changes

Both the iron fertilization as well as the westerly wind belt hypothesis have now been tested using climate and carbon cycle models, showing limited effect of a dust-induced increase in marine biological productivity and no change in zonal wind and the connected SOMOC. The surface buoyancy forcing hypothesis and the SO stratification hypothesis still lack a stringent model test, at least partly because of the limitation of global ocean circulation models in resolving eddy processes in the

Summary & conclusions

Already more than 20 years ago, the important role of high latitude oceans in controlling atmospheric CO2 was recognized (Sarmiento and Toggweiler, 1984, Siegenthaler and Wenk, 1984; Knox and McElroy, 1985). However, only recent ice core data from the EPICA and other ice cores, as well as improved model approaches, have allowed us to appreciate the dominant role of the SO to its full extent. Although none of the processes is able to explain the 100 ppmv glacial/interglacial CO2 change by itself,

Acknowledgments

This work is a contribution to the European Project for Ice Coring in Antarctica (EPICA), a joint European Science Foundation/European Commission scientific programme, funded by the EU (EPICA-MIS) and by national contributions from Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland and the United Kingdom. The main logistic support was provided by IPEV and PNRA (at Dome C) and AWI (at Dronning Maud Land). TFS acknowledges support by the Swiss National Science

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