Contrasting the Hydrological Cycle in Past and Future Warm Climates

Warm epochs of the past, from the Cretaceous to the Eocene to the Pliocene, are believed to have had generally wetter climate conditions than cold epochs. In particular, 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 did the broad pattern of arid and semi-arid subtropical regions become fully established. However, current projections for future global warming caused by CO2 rise suggest the strengthening of such dry conditions over subtropical regions, rather than the return to a wetter state. What makes these past warm climates so different from future projections? Here, we investigate this question by comparing a typical quadrupling-of-CO2 experiment with an early Pliocene simulation that closely reproduces the available observations. We argue that different cloud properties, maintaining low-latitude ocean temperature patterns with reduced zonal and meridional temperature gradients and hence weaker atmospheric circulation, could explain this puzzle.