An investigation of carbon cycle dynamics since the Last Glacial Maximum: Complex interactions between the terrestrial biosphere, weathering, ocean alkalinity, and CO2 radiative warming in an Earth system model of intermediate complexity
C. T. Simmons1, L. A. Mysak2, and H. D. Matthews11Department of Geography, Planning and Environment, Concordia University, 1445 Boul. de Maisonneuve Ouest, H1255-26, Montréal, Québec, Canada H3G 1M8 2Department of Atmospheric and Oceanic Sciences, McGill University, 805 Rue Sherbrooke Ouest, Montréal, Québec, Canada H3A 0B9
Received: 24 Feb 2016 – Accepted for review: 10 Mar 2016 – Discussion started: 17 Mar 2016
Abstract. Proxy reconstructions and modeling studies of the glacial-interglacial changes in the global carbon cycle have led to a stimulating debate in the paleoclimate literature about the mechanisms leading to a 90–100 ppmv increase in atmospheric CO2. In this paper, we used the University of Victoria Earth System Climate Model v. 2.9 to simulate the carbon cycle response to ice sheet retreat and Milankovitch (insolation) forcing from the Last Glacial Maximum (LGM) to the present. In addition, we conducted sensitivity studies to address the contributions of CO2 radiative forcing, atmospheric carbon content, and weathering rates to climate and carbon cycle changes since 21 kyr BP. The simulations show that ice sheet and orbital changes by themselves do not lead to a notable increase in atmospheric CO2 over the course of deglaciation. However, with the application of CO2 radiative forcing and different weathering rates, the simulated atmospheric CO2 variations ranged over ~ 35 ppmv. Virtually all of the simulated net global vegetation carbon uptake since the LGM is attributable to CO2 fertilization rather than greater land availability or warmer temperatures. Furthermore, the ‘greening’ from CO2 fertilization significantly enhances total deglacial warming (by 0.14°C) and contributes to warmer intermediate and deep ocean temperatures during the interglacial period. We also found that CO2 radiative forcing was the dominant factor allowing for greater outgassing at the ocean surface and an earlier ventilation of deep-ocean DIC. The downwelling of high-alkalinity surface waters stimulated by a stronger, earlier overturning circulation led to greater deep sedimentation (alkalinity removal), which, in turn, permitted CO2 to continue to increase through much of the simulation period.
Simmons, C. T., Mysak, L. A., and Matthews, H. D.: An investigation of carbon cycle dynamics since the Last Glacial Maximum: Complex interactions between the terrestrial biosphere, weathering, ocean alkalinity, and CO2 radiative warming in an Earth system model of intermediate complexity, Clim. Past Discuss., doi:10.5194/cp-2016-24, 2016.