One of the more controversial discussions of the climate change question has been the subject of tropical cyclones, historical trends and expected shifts under global warming. The questions cannot be easily answered with either observations or climate models, and the subject has been one of heated debate in both scientific and political arenas.
Observational Record - Atlantic Basin
It is tempting to conclude that hurricane counts in the Atlantic Basin constitute a trend, but post-war aircraft reconnaissance and satellite technology must be considered before making such a bold statement. Basin detection capabilities in the Atlantic improved significantly in the post-war era with aircraft reconnaissance, yet only became truly robust around the year 1970 with satellite technology (Figure 3). furthermore, any possible long-term trend in the Atlantic Basin is masked by large interseasonal variability due to natural effects such as the Atlantic Multidecadal Oscillation (regardless of the cause) on the timescale of decades and ENSO on the timescale of three to five years.
Observational Record - Pacific Basin
In the western North Pacific, analyses of past observations do not show any increasing trend in tropical cyclone activity. Like the Atlantic Basin, activity undergoes large variations on multi-decadal time scales. If there is indeed a trend, it is actually a downward trend during the last 60 years, with the number of tropical cyclones in 2010 being the lowest during this period (Liu and Chan 2013). This latest study clearly shows that tropical cyclone activity in this ocean basin goes through large variations on multi-decadal time scales.
Observational Record - General
According to the IPCC (IPCC SREX, 2012), “there is low confidence in any observed long-term (40 years or more) increases in tropical cyclone activity (intensity, frequency and duration), after accounting for past changes in observing capabilities.”
In terms of intensity trends, changes in observation technologies (aircraft, satellite) in recent years have resulted in more accurate estimates of intensities. Landsea et al. (2006) have demonstrated that such changes can account for most of the perceived increases in the frequency of intense hurricanes in the Atlantic. In the western North Pacific,the frequency of occurrence of intense typhoons also undergoes large amplitude multi-decadal variations with no discernible trends (Chan, 2006, 2008). The “trend” discussed in Emanuel (2005) and Webster et al. (2005) is actually an upward branch of this multi-decadal signal.
Tropical Cyclones - A Look at the Future
It is tempting to conclude that because of global warming, tropical cyclone frequency will increase. The evidence suggests otherwise, however. Under global warming ocean temperatures should increase, but shear in the Atlantic Basin should also increase - the two factors have competing impacts on tropical cyclone frequency in the Atlantic Basin.
According to the latest climate model projections, global tropical cyclone activity in the future is likely to decrease although significant variations will still exist in individual ocean basins (Knutson et al., 2010). Despite a possible decrease in frequency of occurrence, the number of intense tropical cyclones will likely increase by about ten percent. Again, the extent of such an increase differs from basin to basin.
There is indeed a trend in raw losses under tropical cyclones. However, correcting for population density and inflation, these trends become indistinguishable. The increasing population density and property value concentrations in coastal areas implicate higher risk (Pielke et al., 2008). Furthermore, IPCC AR4 states that “failing to adjust for time-variant economic factors yields loss amounts that are not directly comparable and a pronounced upward trend through time for purely economic reasons.”
Hurricane impacts, notwithstanding any trends, are projected to become more severe with more frequent intense precipitation events and sea-level rise. The growing population density and escalating property values in coastal areas are cause for even greater concern.
Long-term adaptation strategies include development and improvement of codes and standards and land-use strategies that dissuade development in flood-prone coastal regions, as noted in the discussion above. Wind resilience measures of existing codes and standards for hurricane-prone regions such as the State of Florida have demonstrated their effectiveness under events such as Hurricane Charley (2004).