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–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–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 pCO2 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 ∘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–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.