During past geological times, the Earth suffered several intervals of global warmth but their driving factors remain equivocal. A careful appraisal of the main processes involved in those past events is essential to evaluate how they can inform future climates, and thus to provide decision makers with a clear understanding of the processes at play in a warmer world. In this context, the greenhouse Earth of the Cretaceous era, specifically the Cenomanian-Turonian (~ 94 Ma), is of particular interest, as it corresponds to a thermal maximum. Here we use the IPSL-CM5A2 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 Cretaceous and the preindustrial. Starting with a preindustrial simulation, we implement: (1) the absence of polar ice sheets, (2) the increase in atmospheric pCO2 to 1120 ppm, (3) the change of vegetation and soil parameters, (4) the 1 % decrease in the Cenomanian-Turonian value of the solar constant and (5) the Cenomanian-Turonian paleogeography. Between the first (preindustrial) simulation and the last (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. Paleogeographic changes represent the second major contributor to the global warming while the reduction in the solar constant counteracts most of the geographically-driven global warming. We also demonstrate that the implementation of Cretaceous boundary conditions flattens the temperature gradients compared to the piControl simulation. Interestingly, we show that paleogeography is the major driver of the flattening in the low- to mid-latitudes whereas the pCO2 rise and polar ice sheet retreat dominate the high-latitudes response