Aerosol vs. Greenhouse Gas Influences on Tropical Cyclone Intensity

We interpret future and recent historic trends in tropical cyclone intensity using the theory of potential intensity (PI), which predicts the maximum intensity achievable by a tropical cyclone in a given local environment. Greenhouse gas warming increases PI while aerosol cooling decreases it. On a global basis, climate model simulations suggest that aerosol cooling has largely canceled that effect over most of the historical record (until the last couple of decades, when aerosol cooling appears to have reached a plateau while greenhouse gases continue to increase). This is so despite the fact that the aerosol cooling has been smaller than the greenhouse gas warming – by something like a factor of two – over the entire period, so that the climate has warmed considerably. This is possible because aerosols act on the atmosphere primarily through their effect on shortwave radiation while greenhouse gases act through the longwave, and changes in shortwave radiation have about twice the impact on PI as changes in longwave radiation do. Thus aerosols can approximately cancel greenhouse gases in their effect on PI while nonetheless greenhouse gases continue to dominate in their effect on surface temperature. We will present analyses of PI from comprehensive climate models and a single column model, forced by greenhouse gases and aerosols, together and separately, in order to understand this compensation in more depth. Our analysis will focus on how shortwave and longwave forcings influence the surface temperature and the atmospheric profiles of temperature and humidity in order to generate their different effects on PI.