In the context of climate change, a clear understanding of the processes and factors driving global warming is a major concern. During past geological times, Earth suffered several intervals of global warmth but the priming factors remain equivocal. Yet a careful appraisal of all processes being implied during those past events is essential to evaluate how they can inform future climates, in order to provide decision makers a clear understanding of the processes at play in a warmer world. In this context, the global warmth of the Cretaceous era, specifically during the Cenomanian-Turonian, is of particular interest. Here we use the IPSL-CM5A2 Earth System model to unravel the forcing parameters of the Cenomanian-Turonian greenhouse climate. We perform six simulations, from the preindustrial to the Cretaceous by implementing one additional boundary condition change at a time, i.e. (1) polar ice cap retreat, (2) pCO2 increase to 1120 ppm, (3) vegetation and soil parameters, (4) solar constant reduction (~ -1%) and (5) paleogeography (90Ma). Between the first preindustrial simulation and the last Cretaceous simulation, a global warming of more than 11°C is simulated. Most of this warming is driven by the increase in pCO2 to 1120 ppm. Paleogeographic changes represent the second major contributor to the warming while the solar constant reduction counteracts most of this geographically-driven warming. Finally, changes in vegetation and soil parameters as well as the retreat of polar ice caps have a minor impact at the global scale. A full assessment of the processes driving warming or cooling under each boundary condition change will be presented. Ultimately, our work supports the overarching influence of atmospheric carbon dioxide in driving the Earth’s global climate and global warming.

The Cenomanian-Turonian period recorded one of the largest disruptions to the oxygen and carbon cycles, the Oceanic Anoxic Event 2 (OAE2, 94 Ma). This event is global, yet paleo-reconstructions document heterogeneous ocean oxygenation states and sedimentary carbon contents, both temporally and spatially, suggesting that several mechanisms are at play. To better understand the long-term controls on oceanic oxygen and the initial oxygenation conditions prevailing at the beginning of OAE2, we perform numerical simulations of the Cenomanian using the IPSCL-CM5A2 Earth System Model, which includes a marine biogeochemistry component. We examine the control of the biogeochemical states of the global and Central Atlantic oceans by the depth of the Central American Seaway (CAS). The simulations show that a vigorous ocean circulation existed during the Cenomanian and that dysoxia/anoxia was caused by paleogeography rather than by ocean stagnation. The existence of restricted basins, disconnected from the deep global circulation and supplied with oxygen-depleted waters from Oxygen Minimum Zones of the surrounding basins, played a key role in the development of dysoxic/anoxic regions. A comparison with redox-proxy data suggests that a deep connection existed between the Pacific and Central Atlantic prior to OAE2. A shallowing of the CAS may have contributed to the establishment of enhanced anoxia in the Central Atlantic during OAE2. The paleogeographic configuration and that of gateways and submarine topographic barriers appear as major long-term controllers of the oceanic circulation and oxygen distribution, leading to low-oxygen concentrations in extended parts of the ocean as prerequisite conditions for OAEs to occur.