Cyclones made more powerful and destructive by climate change

« A cyclone is a low-pressure system that forms at tropical latitudes, in an area warm enough for it to develop, » explains Emmanuel Cloppet, from Météo-France. “It will be characterized by rain-storm clouds which will rotate and generate both very heavy rain and strong winds, as well as a wind-generated swell”, he adds . These large phenomena – hundreds of kilometers wide – are also formidable because they can cross great distances.

They are classified according to the intensity of the winds: tropical depression (less than 63 km/h), tropical storm (between 63 and 117 km/h) and cyclone (beyond). They are called differently depending on the region where they evolve: we speak of cyclone (or tropical cyclone) in the Indian Ocean and the South Pacific, of hurricane in the North Atlantic and in the Northeast Pacific and finally of typhoon in the Pacific. North West. Meteorologists classify them according to their intensity according to scales which differ according to the regions.

For hurricanes, the Saffir-Simpson scale has for example 5 levels. « The overall number of tropical cyclones has not changed globally, but climate change has increased the occurrence of the most intense and destructive storms », summarizes the World Weather Attribution (WWA), a group of scientists who seek to establish the link between certain extreme events and global warming. The total number of cyclones has therefore not changed, but the most violent ones (categories 3 to 5 on the Saffir-Simpson scale), which cause the majority of the damage, are thus becoming more frequent.

Climate change caused by human activity affects on the one hand the rainfall of cyclones, reinforced by the increase in atmospheric temperatures.  » A three-degree increase in air temperature is potentially a 20% increase in the amount of rain generated by a hurricane episode « , underlines Emmanuel Cloppet. However, it is these intense rains that cause sometimes deadly floods and mudslides, as in the case of Cyclone Freddy, which killed hundreds in Malawi and Mozambique in early 2023.

The warming of the oceans also « feeds » tropical cyclones, which can thus become more violent. “ Climate change therefore creates the conditions under which more powerful storms can form, rapidly intensify and persist to reach land, while carrying more water ,” conclude the WWA experts.

Cyclones generate very strong swells that can cause coastal flooding. And now storm surges are getting higher due to sea level rise as a result of climate change. In addition, global warming risks widening the zone conducive to the formation of cyclones, which could thus affect new regions.  » It’s as if the tropics were widening: in the decades to come, the cyclones will find favorable conditions in terms of sea temperatures over wider areas than today,  » says Emmanuel Cloppet.  » Areas that are very little impacted today could be much more so tomorrow , » he warns.

So far, scientists say they can only definitively attribute the warming to a northward shift of the western North Pacific cyclones, which have hit East and Southeast Asia. « These can therefore strike places that are relatively unprepared, with no historical reason to expect such an occurrence, » notes the WWA.