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. During past geological times, the Earth experienced several intervals ofglobal warmth, but their driving factors remain equivocal. A carefulappraisal of the main processes controlling past warm events is essential toinform future climates and ultimately provide decision makers with a clearunderstanding of the processes at play in a warmer world. In this context,intervals of greenhouse climates, such as the thermal maximum of theCenomanian–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 tounravel the forcing parameters of the Cenomanian–Turonian greenhouseclimate. We perform six simulations with an incremental change in five majorboundary conditions in order to isolate their respective role on climatechange between the Cenomanian–Turonian and the preindustrial. Starting witha preindustrial simulation, we implement the following changes in boundaryconditions: (1) the absence of polar ice sheets, (2) the increase inatmospheric pCO2 to 1120 ppm, (3) the change in vegetation and soil parameters, (4) the 1 % decrease in the Cenomanian–Turonian value of thesolar constant and (5) the Cenomanian–Turonian palaeogeography. Between thepreindustrial simulation and the Cretaceous simulation, the model simulatesa global warming of more than 11  ∘ C. Most of this warming isdriven by the increase in atmospheric pCO2 to 1120 ppm.Palaeogeographic changes represent the second major contributor to globalwarming, whereas the reduction in the solar constant counteracts most ofgeographically driven warming. We further demonstrate that theimplementation of Cenomanian–Turonian boundary conditions flattensmeridional temperature gradients compared to the preindustrial simulation.Interestingly, we show that palaeogeography is the major driver of theflattening in the low latitudes to midlatitudes, whereas pCO2 rise and polar ice sheet retreat dominate the high-latitude response.

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