Stripping back the modern to reveal the Cenomanian-Turonian climate and temperature gradient underneath

During past geological times, the Earth experienced several intervals of global warmth, but their driving factors remain equivocal. A careful appraisal of the main processes controlling past warm events is essential to inform future climates and ultimately provide decision makers with a clear understanding of the processes at play in a warmer world. In this context, intervals of greenhouse climates, such as the thermal maximum of the Cenomanian-Turonian (similar to 94 Ma) during the Cretaceous Period, are of particular interest. Here we use the IPSL-CM5A2 (IPSL: Institut Pierre et Simon Laplace) Earth system model to unravel the forcing parameters of the Cenomanian-Turonian greenhouse climate. We perform six simulations with an incremental change in five major boundary conditions in order to isolate their respective role on climate change between the Cenomanian-Turonian and the preindustrial. Starting with a preindustrial simulation, we implement the following changes in boundary conditions: (1) the absence of polar ice sheets, (2) the increase in atmospheric pCO(2) to 1120 ppm, (3) the change in vegetation and soil parameters, (4) the 1% decrease in the Cenomanian-Turonian value of the solar constant and (5) the Cenomanian-Turonian palaeogeography. Between the preindustrial simulation and the Cretaceous simulation, the model simulates a global warming of more than 11 degrees C. Most of this warming is driven by the increase in atmospheric pCO(2) to 1120 ppm. Palaeogeographic changes represent the second major contributor to global warming, whereas the reduction in the solar constant counteracts most of geographically driven warming. We further demonstrate that the implementation of CenomanianTuronian boundary conditions flattens meridional temperature gradients compared to the preindustrial simulation. Interestingly, we show that palaeogeography is the major driver of the flattening in the low latitudes to midlatitudes, whereas pCO(2) rise and polar ice sheet retreat dominate the highlatitude response.