Recent research suggests that anthropogenic global warming would be associated with an increase in the intensity of tropical cyclones. A recent statistical analysis of observed tropical cyclone intensity shows that its variability with location and season is strongly tied to the variability of the thermodynamic potential intensity (PI) of tropical cyclones, as calculated using a theory described in an earlier work by the authors. Thus it is of interest to look for possible trends in global measures of PI, which are far more stable than those of actual storm intensity. We estimate global trends of PI from 1958 to 1996, averaged over the region where it exceeds 40 m s-1, using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis and the NCEP Empirical Orthogonal Function (EOF) sea surface temperature (SST) analysis. We adjust the Reanalysis temperatures for a large, spurious temperature increase that occurred around 1979. We do this by subtracting from the Reanalysis the atmospheric temperature difference between pairs of years with similar tropical SST before and after 1979. The value of the global mean PI is very large for the SST of the corresponding region in the mid-1990s. Supported by a recent study on the effects of ozone decrease on tropospheric temperatures, we suggest that the ozone decrease might be one of the factors contributing to increase of PI during the 1990s.