Benoit Meyssignac CNES/LEGOS Les variations du niveau de la mer en reponse au changement climatique vues depuis l’Espace Benoit Meyssignac CNES/LEGOS benoit.meyssignac@legos.obs-mip.fr Aitutaki, iles Cook In this talk I propose to give you a general overview of what we know about the contemporary sea level changes and how they respond to the recent climate change.

Evolution du Niveau de la Mer depuis 300 ans Satellite altimetry era (since 1993) 7 cm Today Historical tide gauge data and salt marsh data In summary tide gauge records have shown that sea level is rising fast since the late 19th century compared to past millenia (3 times faster) Satellite altimetry has confirmed this fast rise and even showed in agreement with tide gauge records that sea level has accelerated over the last decades to a rise of ~3 mm/yr ~24 cm IPCC 2013 2

Le Niveau de la mer mesuré par satellites More recently, since october 1992, sea level is precisely monitored with satellite radar altimetry. This space monitoring began with the CNES-NASA series of spacecrfat TOPEX and Jason.

Une couverture de l’Océan en 10 jours Thanks to their almost global coverage and high repetitivity, satellites give a sea level estimate every 100 km and every 10 days covering almost all the ocean in 10 days. What you see here is the ground track covered by TOPEX or Jason during one orbital cycle of 10 days. 4

Le principe de l’Altimétrie spatiale The satellite altimetry measurement is estimated thanks to a radar altimeter onboard the spacecraft which estimates the distance between the satellite and the instantaneous sea surface

Satellite altimetry has reveled that sea level was rising faster than before but it also revealed that sea level does not rise uniformly. There is a large regional variability in sea level rise with regions where sea level rises very fast like in the western tropical Pacific… I don’t have time to go much into the details of the regional variability in sea level here but if you are interested we can talk about this later on. Source: CLS, CNES, LEGOS 6

Tendances du niveau de la mer sur 1993-2014 La hausse du niveau de la mer n’est pas uniforme! Tendances du niveau de la mer sur 1993-2014 Satellite altimetry has reveled that sea level was rising faster than before but it also revealed that sea level does not rise uniformly. There is a large regional variability in sea level rise with regions where sea level rises very fast like in the western tropical Pacific… I don’t have time to go much into the details of the regional variability in sea level here but if you are interested we can talk about this later on. 7

Net =Energy in - Energy out Bilan énergétique actuel de la Terre Rayonnement reçu par la planète: 342 Watts/m2 30% (Rayonnement IR) Réchauffement de la Terre par les gaz à effet de serre 100% (visible+IR+UV) Infra Rouge 70% (243Watts/m2) If for some reason such as the motion of the Earth on its orbit, an increase of volcanic activity or a change in the atmospheric chemical concentrations, the Earth receives more Energy from the sun that it emits back to space then we have an energy imbalance at the top of teh atmosphere. The Earth starts storing energy and warms up increasing its emissions back to space. It warms up until the emissions back to space compensate the energy received from the sun and the climate gets to a new equilibrium (a warmer equilibrium). Between the initial cold equilibrium and the late warm equilibrium the climate goes through a transient phase called climate change The present day energy imbalance at the top of the atmosphere due to anthropogenic green house gazes emissions is of ~0.8 w/m2 Under climate change Net =Energy in - Energy out

Stockage de l’énergie dans le système climatique When the climate warms up to catch up with the Earth energy imbalance it stores the energy essentially in the ocean but also in the cryosphere, on land and in the atmosphere.

Conséquences attendues et observées: And the consequenses are numourous: global surface temperature rises, ocean temperature rises, land ice melts sea level rises etc

Hausse du niveau de la mer et bilan énergétique de la Terre: 2005-2013 Sea level rate sea level rise from Altimetry= 2.8 ± 0.4mm/yr 3 mm/yr 2.8 mm/yr Thermal expansion 0.9 ± 0.4 mm/yr 2 mm/yr Argo profiling floats GRACE space gravimetry 1.9 ± 0.3 mm/yr 1 mm/yr Over the last decade when the observing systems is mature enough, it is possible to track back some sea level changes to the earth energy imbalance caused by green house gazes emissions GRACE-based ocean mass (from Dieng et al. In revision) 11 11 11

Hausse du niveau de la mer et bilan énergétique de la Terre: 2005-2013 Sea level rate Heating rate sea level rise from Altimetry= 2.8 ± 0.4mm/yr 3 mm/yr 1 W/m2 2.8 mm/yr Thermal expansion 0.9 ± 0.4 mm/yr 2 mm/yr 0.5 W/m2 1.9 ± 0.3 mm/yr 1 mm/yr As we have seen before the increase in ocean mass over the recent years is essentially coming from land ice melt as GRACE measurement shows. Melting this ice needs 0.01 w/m2 in the heat budget of the Earth over this period which is very small GRACE-based ocean mass Land ice melt processes Land ice 0.01 W/m2 (from Dieng et al. In revision) 12 12 12

Hausse du niveau de la mer et bilan énergétique de la Terre: 2005-2013 Sea level rate Heating rate sea level rise from Altimetry= 2.8 ± 0.4mm/yr 3 mm/yr 1 W/m2 2.8 mm/yr Thermal expansion 0.9 ± 0.4 mm/yr 2 mm/yr Ocean heat uptake 0.5 W/m2 0.72 ± 0.5 W/m2 1.9 ± 0.3 mm/yr 1 mm/yr As for the thermal expansion we have seen that it is coming from an increase in the ocean heat content. This increase in ocean heat content represent 0.72 w/m2 in the heat budget of the eartn GRACE-based ocean mass Land ice melt processes Land ice 0.01 W/m2 (from Dieng et al. In revision) 13 13 13

Hausse du niveau de la mer et bilan énergétique de la Terre: 2005-2013 Sea level rate Heating rate sea level rise from Altimetry= 2.8 ± 0.4mm/yr Resulting Earth energy imbalance = 0.75 ± 0.52 W/m2 (from Dieng et al. In revision) 3 mm/yr 1 W/m2 2.8 mm/yr Thermal expansion 0.9 ± 0.4 mm/yr atmosphere + continent + sea ice 0.02 W/m2 2 mm/yr Ocean heat uptake 0.5 W/m2 0.72 ± 0.5 W/m2 1.9 ± 0.3 mm/yr 1 mm/yr If now we add the heat used to warm up the atmosphere, the continents and melt sea ice we get the heat budget f the earth which shows an imbalance of 0.75 w/m2 and an upper bound of 1.3 w/m2. This estimate is derived from Altimetry GRACE and Argo and is consistent with the knowledge on the recent Earth energy imbalance GRACE-based ocean mass Land ice melt processes Land ice 0.01 W/m2 14 14 14

* Monitoring long terme du niveau de la mer et des ses contributeurs Conclusion Le niveau de la mer actuel augmente rapidement comparé aux millénaires précédents Il augmente en réponse au réchauffement de l’océan et à la fonte des glaces continentales La hausse du niveau de la mer est cohérente avec le déséquilibre énergétique de la Terre du aux gaz à effet de serre!! Notre compréhension du phénomène est robuste mais il reste beaucoup de travail pour comprendre les processus en jeu (calottes polaires) Perspectives * Monitoring long terme du niveau de la mer et des ses contributeurs * Réduction des incertitudes sur le niveau de la mer et estimation des contributeurs mineurs * Estimer la variabilité interannuelle du déséquilbre énergétique de la Terre et sa relation au niveau de la mer (Quel role est joué par les glaces?) 15

Merci de votre attention 16