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N°32 I quarterly I January 2014 Focus | 23 w q Studying the climate relies on continuous observations of land, oceans, and space. The resulting data is not only used to quantify global warming, but also to assess its consequences. “Our laboratory focuses on sea level measurements,” explains Anny Cazenave of LEGOS.1 “Satellite observations make it possible to determine how fast the global mean sea level is currently rising and detect differences in rate from one region of the planet to another.” On average and over the past 20 years, sea level has risen by three millimeters annually. “Much of our work aims to determine the reasons for this rise and evaluate their relative influence,” explains the researcher. About 30% of the sea level change can be attributed to ocean warming: water takes up more volume when its temperature is higher. “Another 30% is due to water from melting glaciers, and 20 to 25% to ice released from the Greenland and Antarctica ice sheets into the oceans,” adds Cazenave. The impact of other phenomena, such as changes in land water storage due to ground water extraction and building of dams along rivers, or the melting of permafrost at high latitudes, remains to be assessed. “We are also trying to explain natural variability. El Niño, the warm current that affects the tropical Pacific every 3 to 7 years, causes the global mean sea level to rise by several millimeters in response to modifications of the global water cycle. Conversely, during La Niña events, a temporary drop of the global mean sea level is observed. Before the first high-precision satellite altimetry measurements in the early 1990s, most ocean data was collected by volunteer merchant ships and oceanographic research vessels. Today, these have been supplemented by a number of satellites and, since 2000, by the Argo2 network. The international project Argo collects data from 3600 free- drifting profiling floats that cover the Earth’s oceans. They continually monitor sea water temperature and salinity, from the surface to a depth of 2000 meters. “This makes it possible to measure the heat stored in the oceans and its contribution to sea level rise,” Cazenave explains. 06 07 More than 3600 Argo profiling floats monitor climate data in the oceans. 08 The monitoring stations of the RAMCES network (LSCE) analyze air samples to measure greenhouse gas levels. Of course, climatic observations are not limited to the oceans. Researchers also study conditions on land, including air temperature, atmospheric pressure, humidity, nebulosity, etc. “This body of data is used in several ways,” explains Serge Planton,3 in charge of climate research at France’s national meteorological service. “It lets us analyze the climate of the 20th century in minute detail, see how accurately our models describe various phenomena, and compile a very precise inventory of the oceans and atmosphere— the starting point for future climate models.” Since the 1960s, satellites have significantly increased the amount of data available to climatologists, especially on the Earth’s uninhabited regions and oceans. “But this introduces heterogeneity in the data that can skew our conclusions if not analyzed carefully,” Planton warns. “For example, the increase in the number of cyclones recorded in the Southern Hemisphere over the 20th century is mostly due to a rise in satellite observations. This problem arises with each new generation of satellite sensors. It is essential to standardize the data so that it can be used to evaluate climate trends and interpret the evolutions observed using models.” 01. L aboratoire d’études en géophysique et océanographie spatiales (CNRS / CNES / IRD / UPS). 02. Array for Real-time Geostrophic Oceanography. 03. Groupe d’étude de l’atmosphère météorologique (CNRS / Météo France). contact information: Anny Cazenave > anny.cazenave@legos.obs-mip.fr Serge Planton > serge.planton@meteo.fr 06 07 08 © A.Che ziere - Directi on de la communicati on UVSQ © O. Dug ornay /Ifremer © Internati ona l Arg o Program Keeping an eye   on everything   eye everything


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