Is global climate change upon us? Indeed, a vast majority of scientists agree that global warming of the past century has been anthropogenically forced and will continue at earnest if the CO2 rise remains unabated. Yet, large uncertainties remain on the magnitude, regional patterns, sea level rise, and the response of extreme weather events, including tropical cyclones, to future warming. In attempt to constrain these uncertainties, one can look at the climate of the early Pliocene epoch, some 4-5 million years ago, the last time atmospheric CO2 concentrations were close to 400ppm, similar to present-day levels. At that time the Earth had a warm climate characterized by reduced zonal (east-west) and meridional (equator-to-pole) temperature gradients. Ocean surface temperatures in mid-latitudes were some 8oC warmer than today whereas land temperatures in the Arctic were nearly 20oC greater. The sea level was perhaps 20 meters higher than today.

Numerical simulations using a downscaling model and Pliocene climate as the boundary conditions indicate that the number of tropical cyclone doubled then as compared to today; stronger storms were more frequent as well. Further numerical simulations using a high-resolution cloud system resolving atmospheric model show that tropical cyclone activity changes non-monotonically with a reduction in the oceanic meridional temperature gradient – first the cyclone activity decreases slightly but then strongly increases. Such a non-monotonic behavior explains difficulties in assessing the effect of global warming on tropical storms. I discuss the implications of these and other related findings for our understanding of future climate change.


This seminar has been jointly organized by FEEM and CMCC.