The simulated climate of the Last Glacial Maximum and the insights into the global carbon cycle

Abstract. Global climate models (GCMs) provide an important tool for simulating the earth's climate. Here we present a GCM simulation of the climate of the Last Glacial Maximum (LGM), which was obtained by setting atmospheric greenhouse gas concentrations and the earth's orbital parameters to the values which prevailed at 21 000 years before present (BP). During the LGM, we simulate a significant cooling of the ocean and a dramatic expansion of the sea-ice extent. This behaviour agrees with reconstructions from paleoclimate archives. In the ocean, the LGM simulation produces a significant redistribution of dissolved oxygen and carbon. The oxygen levels rise and the volume of anoxic water declines by more than 50%, which is consistent with paleoclimate reconstructions of denitrification. The simulated LGM climate also stores more carbon in the deep ocean (below 2000 m), but with a reduced atmospheric CO₂ level the total carbon stored in the ocean declines by 600 Pg C. The LGM ocean circulation preconditions the ocean to store carbon in the deep; however, the ocean circulation and sea-ice changes are insufficient alone to increase the total carbon stored in the ocean and modifications to the ocean biogeochemical cycles are required. With modifications to organic and inorganic carbon export and organic carbon remineralization one can increase ocean carbon storage (240 Pg C) to a level that is sufficient to explain the reduction in atmospheric and land carbon during the LGM (520 ± 400 Pg C). With the modified biogeochemical cycling in the ocean, the simulated aragonite lysocline depth and dissolved oxygen become more consistent with paleo-reconstructions.