03 Global warming
At the same time, the natural fluctuation between these phases seems to be intensified by a superimposed long-term warming process so that sea surface temperature and the level of hurricane activity increase from warm phase to warm phase (Fig. 4). The increase in the number of strong hurricanes per year from 2.6 to 4.1 from the previous warm phase to the current warm phase means an increase of 58%.* There are strong arguments in favour of climate change as the long-term warming agent. The current unusually high level of activity is most probably due to the warm phase prevailing since the mid-1990s, which is supposed to continue for several years and intensified by the relatively linear process of global warming.
There is a clear indication that both the natural climatic cycle and global warming influence not only overall frequency but also landfall frequency. Between the last warm phase (approx. 1926 to approx. 1970) and the current warm phase since approx. 1995, the average annual number of landfalls increased as follows (Fig. 5):
| Cat. 3—5 hurricanes |
+67% (from 0.6 to 1.0) |
| Cat. 1—5 hurricanes |
+33% (from 1.8 to 2.4) |
| Trop. storms and Cat. 1—5 hurricanes |
+47% (from 3.4 to 5.0) |
This comparison has to be seen as being primarily linked to the influence of global warming.
The change in level between the last cold phase (approx. 1971 to approx. 1994) and the current warm phase since 1995 has the following impact on the number of landfalls (Fig. 5):
| Cat. 3—5 hurricanes |
+233% (from 0.3 to 1.0) |
| Cat. 1—5 hurricanes |
+100% (from 1.2 to 2.4) |
| Trop. storms and Cat. 1—5 hurricanes |
+100% (from 2.5 to 5.0) |
This comparison has to be seen as being primarily indicative of the natural climatic oscillation.
* The records of the period before aircraft reconnaissance started in the mid-1940s are not as reliable as the records since then. This applies primarily to intensity attributions, because one has to rely on observations made by ships.