After the dramatic consequences of hurricanes Harvey, Irma, Maria and Jose, this year in 2017, understanding the origins of such extreme events is of growing importance. What is the nexus between cyclones, the Ocean, and climate change?
Tropical cyclones, also called hurricanes in the North Atlantic Ocean, are violent perturbations, or atmospheric depressions. The “eye” at the centre of the cyclone, is a calm 30-60km wide zone. It is surrounded by a “wall”, an area struck by diverse phenomena: strong winds (up to 300 kilometres per hour); as well as a rise in the sea level, and strong swells which, added to strong rainfalls, causes severe floods.
How do cyclones form?
Several factors are necessary to the forming of cyclones: strong and homogeneous winds in altitude (up to 15km), high humidity and sea surface temperature higher than 26°C in the 60 metres upper layer. This temperature heats the air and provokes great water evaporation from the Ocean to the atmosphere.
For instance, in the case of Irma, sea surface waters (on average warmer by 0.5 to 2°C in most of the tropical North Atlantic Ocean) caused particularly important evaporation. The situation for Harvey, Maria and Jose was comparable. Heat received at the surface is then transformed in mechanical energy, higher in altitude. When this source of energy is not available any more, the cyclone dissipates. Cyclones weaken when they penetrate in the land, as they are not fuelled by warm water and therefore humidity and energy anymore.
What nexus can we establish with climate change?
Regarding the link between these hurricanes and climate change, scientists remain prudent. It seems that warmer sea surface temperatures tend to generate longer and more intense cyclones.
However, this causal link can be established based on a high number of cyclones: on average, these extreme disasters might be more frequent and intense as planetary temperature increases. Harvey, Irma, Jose and Maria occurred during the same year. They are not numerous enough and too recent to be designated as a direct consequence of climate change.
2016 for instance, was characterised by temperatures warmer than during 2017 in the tropical Atlantic. Despite a situation that seems more favourable to cyclones intensification, only two force 4 or 5 cyclones were recorded during the whole season.
More time and a finer analysis will then be necessary to establish and measure such a link.
Yet, these events draw our attention to this question of a potential link between climate change and natural disasters. Irma, Harvey, Jose and Maria must be considered in that context and we should retain the hypothesis of climate as a probable factor. At the same time these catastrophes have to be seen as an example of what the future can hold.
Victor Brun, Xavier Capet, Françoise Gaill, Sabrina Speich and Gilles Reverdin
 « Cyclone » is a generic term. We call them hurricanes in the North Atlantic, in the Gulf of Mexico, and in the east of the North Pacific; they are called typhoons in the west of the North Pacific and in the South China Sea.
 Essentially through the water evaporation process, we talk about “latent heat”.
 Kerry Emmanuel, “Increasing destructiveness of tropical cyclones over the past 30 years”, Nature 2005
 IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.