Mitigation of hurricane potential intensity by the Montreal Protocol

In the last decade, it has become apparent that the regulation ofozone depleting substances (ODS) by the Montreal Protocol has hadprofound impacts on the climate system, affecting not only surfacetemperatures but also the atmospheric circulation and the entirehydrological cycle. In this study we demonstrate that he MontrealProtocol will also be very effective in mitigating the potentialintensity (PI) of hurricanes in the coming half century. We accomplish this by comparing the projections of a standard CMIP5RCP4.5 scenario to those of the so-called "World Avoided" scenario, inwhich ODS grow unabated in the absence of regulations. For thiscomparison, we use ouput from two 3-member ensembles of WholeAtmosphere Community Climate Model (WACCM), integrated between 2005and 2065. WACCM is the most comprehensive member of the CommunityEarth System Models (CESM), and includes interactive stratosphericchemistry, in addition to coupled land, ocean, and sea-ice components, In the World Avoided projections we find that the hurricane PI issubstantially larger that in the standard RCP4.5 case. Specifically,over the decade 2056-2065, the increase in PI is comparable to the onebetween RCP4.5 and RCP8.5 for the entire multi-model mean of the CMIP5models. Similar to what is projected by the CMIP5 models forincreasing CO2, in the World Avoided scenario the increase in hurricanPI is due to a combined increase in sea surface temperature and CAPE,and a decrease in the temperature in upper levels, near 70hPa. Our WACCM simulations indicate that the mitigating effect of theMontreal Protocol is highly significant: without ODSs regulations thePI would be twice as large as currently projected by the middle ofthis century.