The annual number of tropical cyclones decreased by approximately 13% during the twentieth century, compared with the late nineteenth century, according to a study published in Nature Climate Change.
It is not clear how tropical cyclones change under human emissions because a warming ocean is expected to intensify storms, while some changes in atmospheric circulation are thought to prevent storm formation. Providing historical context is challenging, as the observational record is not complete, especially before 1950. This has led to conflicting assessments of past tropical cyclone trends.
Using historical records and model data, Savin Chand and colleagues reveal declining trends in the annual number of tropical cyclones since 1850 at both global and regional scales. The global annual number of storms decreased by approximately 13% in the twentieth century compared with the period between 1850 and 1900. For most tropical cyclone basins, this decline has accelerated since the 1950s, which the authors suggest is mainly the result of a weakening of tropical atmospheric circulation. The only exception to this trend is the North Atlantic basin, where the number of tropical cyclones has increased over recent decades. The authors suggest that this may be because the basin is recovering from a decline in tropical cyclone number due to human-related aerosol emissions in the late twentieth century. The number of annual storms is still, however, lower than in pre-industrial times, they state.
These findings support studies that suggest that current climate change leads to a decrease in the number of tropical cyclones. It should be noted, however, that frequency is only one aspect controlling the risks associated with tropical cyclones, as the intensity and geographical location are also expected to change. As these factors were not assessed herein, no direct conclusions on the overall changes in risk can be derived.
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