During the warm Miocene and Pliocene epochs, subtropical regions had enough precipitation to support rich vegetation and fauna. Only with global cooling and the onset of glacial cycles some 3 million years ago, towards the end of the Pliocene, did the broad patterns of arid and semi-arid subtropical regions become fully developed. 

However, current projections of future global warming caused by CO₂ rise generally suggest the intensification of dry conditions over these subtropical regions, rather than the return to a wetter state. What makes future projections different from these past warm climates? In an effort to answer this question, this talk focuses on the warm Pliocene – the most recent time in Earth’s history during which reconstructed atmospheric CO₂ concentrations are estimated to have been in the region of 400ppm. 

We investigate this question by comparing a typical quadrupling-of-CO₂ GCM experiment with a simulation driven by sea surface temperatures closely resembling available reconstructions for the early Pliocene, as well as a range of perturbed cloud forcing or feedback GCM sensitivity experiments. The results point to the competition between (1) global warming induced specific humidity changes that act to enhance subtropical moisture divergence, and (2) the relaxation of the meridional SST gradient that acts to reduce the strength of atmospheric meridional circulation. Under the weak meridional SST gradients of the Pliocene the latter effect more than compensates, leading to reduced subtropical moisture divergence and wetter subtropics. 

Therefore, when it comes to predicting the response of the hydrological cycle within the subtropics under global warming, the warm Pliocene highlights the importance of correctly quantifying how meridional SST patterns will change.