Élevation du niveau de la mer

• Average ocean levels have risen more than 20 cm over the past century. The rate of increase is accelerating in recent years. The average progression during the 20th century was +1.7 mm/year, while it has been twice as fast in the recent period, 1993-2011, with +3.2 mm/year. • This rise is nearly universal across all oceans. Only a few sectors are excluded, such as the eastern Pacific, in its northern and equatorial parts. • According to Church and White, progression was 1.7 ± 0.2 mm/year from 1901 to 2009, with the rate of change being approximately 0.5 mm/year over the past three millennia. Spatial data show that this increase has accelerated in recent years. The annual rate of change is 3.2 ± 0.4 mm/year for the period 1993-2011, nearly twice faster than the average over the last 100 years.

Ocean level rise has been verified for all French territories. In mainland France, it was 3.0 mm/year in Brest between 1980 and 2004, 2.6 mm for Marseille over 1980-2012, and 2.1 mm for the Charente straits between 1941 and 2011. In overseas territories, the progression was strong in French Polynesia, +2.9 mm from 1950 to 2010. It is more moderate in New Caledonia (+2 mm), in the Caribbean and Réunion, between +1.7 mm and +1.9 mm depending on the islands.

• From 2005 to 2013, the progression of global ocean mass was the primary cause of mean ocean level rise: melting of Greenland and Antarctic ice sheets and mountain glaciers, and modifications of the hydrological cycle. The increase in average ocean temperature, causing thermal expansion of water masses, was the second cause. • According to various scenarios from the Intergovernmental Panel on Climate Change (IPCC), oceans could rise another 26 to 82 cm by the end of the 21st century. • Over the studied period, global average ocean level rose more than 20 cm.

Society and the economy are also affected: increased exposure to climate hazards (heat waves, floods, cyclones, etc.) and health risks (vector-borne diseases, etc.), economic losses (lost working days, reduced agricultural yields, tourism, material damage, etc.). France could also indirectly suffer the consequences of climate impacts occurring abroad. Indeed, the emergence of geopolitical crises (migration, conflicts, etc.) triggered by tensions related to resource access (food, water) constitutes a plausible risk, highlighted by the IPCC and taken into account in international and national strategic considerations.

• There is great geographic variability in mean ocean level rise, ranging from -15 to 15 mm/year depending on the region. This disparity is confirmed by both tide gauge and satellite data. It is explained by fluctuations in ocean temperature rise, major ocean currents, and differences in water mass salinity. • This variability fluctuates both over time and across space. • As shown on the map, ocean level rise over the period 1993-2011 is fairly widespread. Some sectors are spared, such as the eastern Pacific on Central American coasts. Other sectors are particularly affected, such as the western equatorial Pacific or coasts near the South tropical zone in the Indian and Pacific oceans.

• The port of Brest and the roads of Marseille have very old tide gauges that allow reconstruction of mean sea level on Atlantic and Mediterranean coasts since the 19th century. They are operated by the Naval Hydrographic and Oceanographic Service (Shom), the national reference for sea level observation. • Other mainland French tide gauges have fairly long series, such as Cherbourg or Le Havre, but digitization and data calibration work would be necessary to fully exploit this information. • For usable tide gauge data, sea level rise is confirmed. According to the table, there is an acceleration of sea level rise in recent years in Brest, as well as in Marseille and the Charente straits. • Thus, in Brest, the progression is more than twice as fast between 1980 and 2004 as between 1890 and 1980.

• Recent work combining altimetric and tide gauge data and series reconstruction provides average sea level progressions for different French territories, despite the absence of long-term tide gauge data in these territories. • The results obtained differ from one region to another. • French Polynesia has the highest annual sea level variation rates over these past 60 years, 2.9 mm/year. They are 2 mm/year in New Caledonia. They are at average levels for the Caribbean and Réunion island.

Ongoing climate change has a direct impact on mean ocean level. The rise in ocean temperatures over the past century causes thermal expansion of water masses and a rise in mean ocean level. Furthermore, the global temperature increase on land causes an imbalance in water exchanges between continents and oceans, with melting of mountain glaciers and ice sheets in polar regions. This influx of fresh water also contributes significantly to ocean rise.

From 2005 to 2013, the contribution of increasing global ocean mass is the most important, with a value of 2 mm/year. The water mass of the world ocean varies according to the balance of exchanges between oceans, land masses (melting of ice sheets and mountain glaciers and exchanges with continental waters) and the atmosphere. This increase in ocean mass is calculated directly by satellite gravimetry. These studies have also shown that the El Niño phenomenon (abnormally high water temperatures in the eastern part of the South Pacific Ocean) causes a decrease in liquid water stocks on continents (more rainfall over oceans) and an increase in mean sea level.

Over this same period, ocean expansion due to warming represented a mean ocean level rise of nearly 1 mm/year.

• The latest report from the Intergovernmental Panel on Climate Change (IPCC), published in 2013, presented a new revision of sea level projections to 2100, following different global climate change scenarios. • The probable rise in mean level between 1986-2005 and 2081-2100 is between 26 and 82 cm, with significant uncertainties regarding the regional distribution of ocean level change.

• This rise will have consequences for risks of coastal submersion and erosion, in mainland France as well as overseas territories. It could also cause salt intrusions into fresh groundwater aquifers at the coast. • A 2013 study showed that ocean level rise could affect 6 to 12% of French islands, over 30% of which are in Polynesia, comprising 84 atolls, but also in New Caledonia (Bellard C., Leclerc C., Courchamp F.).

• Methodology • Two methods are implemented to reconstruct mean ocean level change since the late 19th century: • consider tide gauges located in geologically stable continental regions with several decades of continuous measurements; • use a larger number of tide gauges employing statistical methods to reconstruct series. • The results of this second method are presented. • More precise measurements have been available since 1992 with satellite altimetric data: Topex/Poseidon satellite since 1992, Jason-1 since late 2001, and Jason-2 since mid 2008. This information covers the entire ocean domain, not just coasts as with tide gauges. • Author: Data and Statistical Studies Service (SDES).