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.