Quantifying the ocean's role in glacial CO<sub>2</sub> reductions

Abstract

Abstract.A series of Last Glacial Maximum (LGM) marine carbon cycle sensitivity experiments is conducted to test the effect of different physical processes, as simulated by two atmosphere-ocean general circulation model (AOGCM) experiments, on atmospheric pCO 2 . One AOGCM solution exhibits an increase in North Atlantic Deep Water (NADW) formation under glacial conditions, whereas the other mimics an increase in Antarctic Bottom Water (AABW) associated with a weaker NADW. None of these sensitivity experiments reproduces the observed magnitude of glacial/interglacial pCO 2 changes. However, to explain the reconstructed vertical gradient of dissolved inorganic carbon (DIC) of 40 mmol m −3 a marked enhancement in AABW formation is required. Furthermore, for the enhanced AABW sensitivity experiment the simulated stable carbon isotope ratio (δ 13 C) decreases by 0.4 ‰ at intermediate depths in the South Atlantic in accordance with sedimentary evidence. The shift of deep and bottom water formation sites from the North Atlantic to the Southern Ocean increases the total preformed nutrient inventory, so that the lowered efficiency of Southern Ocean nutrient utilization in turn increases atmospheric pCO 2 . This change eventually offsets the effect of an increased abyssal carbon pool due to stronger AABW formation. The effects of interhemispheric glacial sea-ice changes on atmospheric pCO 2 oppose each other. Whereas, extended sea-ice coverage in the Southern Hemisphere reduces the airsea gas exchange of CO 2 in agreement with previous theoretical considerations, glacial advances of sea-ice in the Northern Hemisphere lead to a weakening of the oceanic carbon uptake through the physical pump. Due to enhanced gas solubility associated with lower sea surface temperature, both glacial experiments generate a reduction of atmospheric pCO 2 by about 20-23 ppmv. The sensitivity experiments presented here demonstrate the presence of compensating effects of different physical processes in the ocean on glacial CO 2 and the difficulty of finding a simple explanation of the glacial CO 2 problem by invoking ocean dynamical changes.

Deglacial ice sheet meltdown: orbital pacemaking and CO<sub>2</sub> effects

“…This supports previous model studies, which have identified GHG variations as an important contributor to Pleistocene glacial cycles (Gallée et al, 1992;Yoshimori et al, 2001;Ganopolski and Calov, 2011;AbeOuchi et al, 2013). And the results emphasize how important it is to identify the mechanisms responsible for the orbitalscale modulation of atmospheric CO 2 concentrations (Kohfeld and Ridgwell, 2009;Tagliabue et al, 2009;Chikamoto et al, 2012;Menviel et al, 2012;Brovkin et al, 2012).…”

“…While the changes of earth's orbital and tilt parameters and the resulting shortwave radiative forcing at the top of the atmosphere are well understood, the carbon cycle feedbacks that led to the reconstructed atmospheric CO 2 changes are subject to ongoing research (Kohfeld and Ridgwell, 2009;Tagliabue et al, 2009;Brovkin et al, 2012;Chikamoto et al, 2012;Menviel et al, 2012). It was demonstrated, however, that the carbon cycle feedbacks played an important role during the last deglaciation and the Quaternary Period (last ∼ 2.6 million years), amplifying glacial-interglacial cycles (Gallée et al, 1992;Yoshimori et al, 2001;Ganopolski and Calov, 2011;Abe-Ouchi et al, 2013).…”

Responses of Basal Melting of Antarctic Ice Shelves to the Climatic Forcing of the Last Glacial Maximum and CO2 Doubling

“…The MIROC 3.2 contributed to the Coupled Model Intercomparison Project phase 3, which was extensively cited in the Intergovernmental Panel on Climate Change Fourth Assessment Report. The coefficient of the isopycnal layer thickness diffusivity was 7:0 3 10 26 cm 2 s 21 instead of the value of 3:0 3 10 26 cm 2 s 21 used in the original MIROC 3.2 (Oka et al 2011;Chikamoto et al 2012). The resolution of the atmospheric component was T42 (about 2.88 3 2.88) with 20 vertical levels, and that of the ocean component was about 1.48 3 18 with 43 vertical levels.…”

Coupled climate-carbon cycle simulation of the Last Glacial Maximum atmospheric CO<sub>2</sub> decrease using a large ensemble of modern plausible parameter sets

“…Brovkin et al, 2002;Bopp et al, 5 2003;Brovkin et al, 2007;Chikamoto et al, 2012;Palastanga et al, 2013;Schmittner and Somes, 2016;Buchanan et al, 2016) (Fig. 12).…”

“…The most commonly accepted mechanisms to explain the atmospheric CO 2 decrease include lower sea surface temperatures (Martin et al, 2005;Menviel et al, 2012), iron fertilisation (Bopp et al, 2003;Oka et al, 2011;Jaccard et al, 2013;Ziegler et al, 2013;Martínez-Garcia et al, 2014;Lambert et al, 2015), sea-ice capping of air-sea gas exchange (Stephens and Keeling, 2000;Sun and Matsumoto, 2010;Chikamoto et al, 2012) and ocean circulation/stratification changes (Adkins et al, 2002;20 Lynch-Stieglitz et al, 2007;Skinner et al, 2010;Lippold et al, 2012;Gebbie, 2014;Skinner et al, 2014;Tiedemann et al, 2015;de la Fuente et al, 2015;Freeman et al, 2015), due to a range of possible mechanisms such as increased brine rejection (Shin et al, 2003;Bouttes et al, 2010Bouttes et al, , 2011Zhang et al, 2013;Ballarotta et al, 2014), a shift in/weakening of the westerly wind belt over the Southern Ocean (Toggweiler et al, 2006;Anderson et al, 2009;Völker and Köhler, 2013), stronger westerly winds over the North Atlantic (Muglia and Schmittner, 2015), and a reduced or reversed buoyancy flux from the 25 atmosphere to the ocean surface in the Southern Ocean (Watson and Garabato, 2006;Ferrari et al, 2014). A process that is conversely assumed to have contributed to increasing atmospheric CO 2 is increasing salinity and ocean total dissolved inorganic carbon (DIC) concentration in response to decreasing sea level (Ciais et al, 2013).…”