(Evolution of tropical storms (in blue), hurricanes (in green) and major hurricanes (category 3) (in red), in the North Atlantic.)
The scientific consensus in the 2007 IPCC report is that the intensity of tropical cyclones is likely to increase (with a probability greater than 66%).
A study published in 2005, not confirmed by a second study, indicated an overall increase in the intensity of cyclones between 1970 and 2004, with the total number of cyclones decreasing during the same period. The number of cyclones of intensity 4 and 5 nearly doubled in number and proportion between 1970 and 2004. According to this study, it is possible that this increase in intensity is related to global warming, but the observation period was too much short and the role of cyclones in atmospheric and oceanic fluxes was not sufficiently known for this relationship to be established with certainty. The second study, published a year later, did not show a significant increase in cyclone intensity since 1986. Ryan Maue of the University of Florida, in an article entitled Northern Hemisphere Tropical Cyclone Activity, observed a decrease in marked by cyclone activity since 2006 in the northern hemisphere compared to the last thirty years. He added that the decline was probably more pronounced, the measures dating back thirty years not detecting the weakest activities, which allow the measures of today. For Maue, it was possibly a low for fifty years that we observe in terms of cyclonic activity.
Moreover, computer simulations do not allow in the current state of knowledge to predict significant changes in the number of cyclones related to global warming. However, a 2014 US simulation shows that the number and intensity of cyclones is expected to grow in the tropics in all the world’s oceans except in the southwestern Pacific.
Ocean warming and sea level rise
(Rise of sea level. )
There is a warming of the oceans, which decreases in depth. The temperature rise since 1960 is estimated at 0.6 °C for surface water, and at 0.04 °C for the ocean as a whole.
It is estimated that oceans have so far absorbed 90% of the heat added to the climate system. This warming contributes 30% to sea-level rise by thermal expansion of the oceans, 60% of this rise being due to the melting of continental ice (half of which comes from the melting polar ice caps) and 10% to one flow of inland waters to the oceans. The data come from tide gauges set up since the middle of the 19th century, seconded from the 1990s by altimetry satellites. Their analysis suggests that the sea level rose during the twentieth century by a few tens of centimeters, and continues to rise steadily. Sea level is estimated to have risen by 1.8 mm per year between 1961 and 2003 and by 3.4 mm per year since 1993. This rise in sea level can also be observed indirectly by its impact on the environment, as is the case in New Brunswick.
In the framework of the ARGO system, 3,000 automatic beacons are distributed in all the oceans and make it possible to follow the temperature and the salinity of the oceans up to 2,000 meters deep. In the North Atlantic, Ifremer Brest researchers confirmed the warming trends in the surface layers.
The curve of the estimated amount of heat in the oceans is updated regularly by the US NOAA meteorological agency.
The rise in temperature is also detectable in rivers and lakes. Thus, between 1977 and 2006, the average annual temperature of the Rhône increased by 1.5 °C, and the average summer temperatures of the Loire from 1.5 °C to 2 °C. The deep waters of Lake Geneva have warmed by 1 °C in forty years.
The acidity of the oceans has increased by 26% due to the sharp rise in carbon dioxide emissions, whose water absorbs nearly a third of the quantities released into the atmosphere; this acidity contributes to the loss of biodiversity.