%0 Journal Article %T Stripping back the modern to reveal the Cenomanian每Turonian climate and temperature gradient underneath %A Bopp %A Laurent %A Donnadieu %A Yannick %A Green %A J. A. Mattias %A Ladant %A Jean-Baptiste %A Laugi谷 %A Marie %A Raisson %A Franˋois %J - %D 2020 %R https://doi.org/10.5194/cp-16-953-2020 %X

Abstract. 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每Turonian (‵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每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每Turonian and the preindustrial. Starting with a preindustrial simulation, we implement the following changes in boundary conditions: (1)ˋthe absence of polar ice sheets, (2)ˋthe increase in atmospheric pCO2 to 1120 ppm, (3)ˋthe change in vegetation and soil parameters, (4)ˋthe 1 % decrease in the Cenomanian每Turonian value of the solar constant and (5)ˋthe Cenomanian每Turonian palaeogeography. Between the preindustrial simulation and the Cretaceous simulation, the model simulates a global warming of more than 11 ∼C. Most of this warming is driven by the increase in atmospheric pCO2 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 Cenomanian每Turonian 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 pCO2 rise and polar ice sheet retreat dominate the high-latitude response.