WETTER SUBTROPICS IN A WARMER WORLD: CONTRASTING PAST AND FUTURE HYDROLOGICAL CYCLES

During the warm Miocene and Pliocene epochs, vast 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 established. 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? Here, we investigate this question by comparing a typical quadrupling-of-CO₂ experiment with a simulation driven by sea surface temperatures closely resembling available reconstructions for the early Pliocene. Based on these two experiments and a suite of other perturbed coupled climate simulations, we argue that this puzzle is explained by weaker atmospheric circulation in response to the different ocean surface temperature patterns of the Pliocene, specifically reduced meridional and zonal temperature gradients. Thus the Pliocene highlights that accurately predicting the response of the hydrological cycle to global warming requires predicting not only how global mean temperature responds to elevated CO₂ forcing (climate sensitivity) but also correctly quantifying how meridional SST patterns will change (structural climate sensitivity).