Comment on cp-2022-26

Abstract

Numerous climate archives reveal alternating arid and humid conditions in North Africa during the last several million years. Most likely the dry phases resembled current hyper-arid landscapes, whereas the wet phases known as African Humid Periods (AHPs) sustained much more surface water and greater vegetated areas that "greened" a large part of the Sahara region. Previous analyses of sediment cores from the Mediterranean Sea showed the last five AHPs differed in strength, duration and rate of change. To understand the causes of such differences we perform transient simulations of the past 190,000 years with Earth system model of intermediate complexity CLIMBER-2. We analyse amplitude and rate of change of the modelled AHPs responses to changes in orbital parameters, greenhouse gases (GHGs) and ice sheets. In agreement with estimates from Mediterranean sapropels, we find the model predicts a threshold in orbital forcing for Sahara greening and occurrence of AHPs. Maximum rates of change in simulated vegetation extent at AHP onset and termination correlate well with the rate of change of the orbital forcing. As suggested by available data for the Holocene AHP, the onset of modelled AHPs happens usually faster than termination. A factor separation analysis confirms the dominant role of the orbital forcing in driving the amplitude of precipitation and vegetation extent for past AHPs. Forcing due to changes in GHGs and ice sheets is only of secondary importance, with a small contribution from synergies with the orbital forcing. Via the factor separation we detect that the threshold in orbital forcing for AHP onset varies with GHGs levels. To explore the implication of our finding from the palaeoclimate simulations for the AHPs that might occur in a greenhouse gas-induced warmer climate, we extend the palaeoclimate simulations into the future. For the next 100,000 years the variations in orbital forcing will be smaller than during the last hundred millennia, and the insolation threshold for the onset of late Quaternary AHPs will not be crossed. However, with higher GHGs concentrations the predicted threshold drops considerably. Thereby, the occurrence of AHPs in upcoming millennia appears to crucially depend on future concentrations of GHGs.