We accomplish this by comparing the projections of a standard CMIP5 rcp4.5 scenario to those of the so-called "World Avoided" scenario, in which ODSs grow unabated in the absence of regulations. For this comparison, we use ouput from two 3-member ensembles of Whole Atmosphere Community Climate Model (WACCM), integrated between 2005 and 2065. WACCM is a configuration of CESM with resolved stratosphere and mesosphere, fully interactive stratospheric chemistry, in addition to coupled land, ocean, and sea-ice components: therefore, WACCM is the most comprehensive of the models in the CESM family.
We find that in the World Avoided projections the hurricane PI is substantially larger that in the standard rcp4.5 case. Specifically, over the decade 2056-2065, the increase in PI is comparable to the the one between rcp4.5 and rcp8.5 for the entire multi-model mean of the CMIP5 models. Similar to what is projected by the CMIP5 models for increasing CO2, in the World Avoided scenario the increase in PI is due to a combined increase in sea surface temperature and CAPE, and a decrease in the temperature at upper levels (above 100 hPa).
Our WACCM simulations indicate that the mitigating effect of the Montreal Protocol is highly significant: without ODSs regulations the PI would be twice as large as currently projected by the middle of this century.