• Les #océans continuent de monter

    Qui dit vrai ? Il semblerait que le « ralentissement » observé dans la dernière décennie soit en fait le résultat d’une erreur de calcul, remontant à la façon de mesurer d’un satellite, Topex A, entre 1993 et 1999. Les appareils appelés marégraphes, employés à travers le monde pour mesurer le niveau des eaux se sont par ailleurs multipliés ces dernières décennies et permettent des mesures régionales plus précises.


    #climat #mer #changement_climatique

    • L’article reprend un article du New Scientist qui se réfère à cette étude.

      Unabated global mean sea-level rise over the satellite altimeter era : Nature Climate Change : Nature Publishing Group

      The rate of global mean sea-level (GMSL) rise has been suggested to be lower for the past decade compared with the preceding decade as a result of natural variability1, with an average rate of rise since 1993 of +3.2 ± 0.4 mm yr−1 (refs 2, 3). However, satellite-based GMSL estimates do not include an allowance for potential instrumental drifts (bias drift4, 5). Here, we report improved bias drift estimates for individual altimeter missions from a refined estimation approach that incorporates new Global Positioning System (GPS) estimates of vertical land movement (VLM). In contrast to previous results (for example, refs 6, 7), we identify significant non-zero systematic drifts that are satellite-specific, most notably affecting the first 6 years of the GMSL record. Applying the bias drift corrections has two implications. First, the GMSL rate (1993 to mid-2014) is systematically reduced to between +2.6 ± 0.4 mm yr−1 and +2.9 ± 0.4 mm yr−1, depending on the choice of VLM applied. These rates are in closer agreement with the rate derived from the sum of the observed contributions2, GMSL estimated from a comprehensive network of tide gauges with GPS-based VLM applied (updated from ref. 8) and reprocessed ERS-2/Envisat altimetry9. Second, in contrast to the previously reported slowing in the rate during the past two decades1, our corrected GMSL data set indicates an acceleration in sea-level rise (independent of the VLM used), which is of opposite sign to previous estimates and comparable to the accelerated loss of ice from Greenland and to recent projections2, 10, and larger than the twentieth-century acceleration2, 8, 10.

      Figure 3: Adjusted and unadjusted satellite altimeter GMSL time series (each arbitrarily offset and corrected for ocean-basin expansion).


      La page de l’auteur principal à l’Université de Tasmanie

      Christopher Watson - Profiles - University of Tasmania, Australia

      Sea level rise estimates derived by Dr Watson and his colleagues can be compared with other observations that measure the different contributions to the overall sea level change signal.

      Understanding for example the contributions from the earth’s ice sheets is really important – the collaborations available here in Hobart on this topic are really exciting.

      It is now clear that as a society on this planet, we have tweaked one of the controlling inputs into the earth system–heat. This has given the system a kick and the different components of the system are responding.

      There are many unanswered questions in this field and we need answers to best underpin mitigation and adaptation strategies

      One goal is to determine the regional impact of sea level rise. The rise won’t be consistent across the globe. Some places will experience more sea level rise than others. Different coastal landforms will respond in very different ways. 

      I enjoy trying to provide the most accurate data possible to allow us to make intelligent and informed decisions. For society to respond to the challenge ahead, we need to be informed about what we’re facing.


      La fin de l’article du New Scientist

      Apparent slowing of sea level rise is artefact of satellite data - environment - 11 May 2015 - New Scientist

      The IPCC figure, though, is based on the questionable assumption that there will be no sudden break-up of the great ice sheets. Some researchers expect the break-up to happen faster, so the sea level could rise by 2 metres or even more by 2100.

      What’s more, since the IPCC report was written, there has been a series of very grim findings.

      Last year, we heard that the collapse of part of the West Antarctic ice sheet containing enough water to raise the sea level 1.2 metres could not be stopped. This year it emerged that part of the East Antarctic ice sheet, which was not expected to lose much ice for many centuries, may be unstable too. It holds enough ice to raise the sea level by 3.2 metres.

      The new science seems to be strengthening the possibility of ’much worse’,” says Alley. In other words, we are on course for a rise in the sea level of many metres.

      That means many low-lying coastal areas and cities are doomed.

      What is not clear yet is whether it will take centuries for the sea to drown them or whether it could happen much sooner. Most think it will be many centuries, but there is no way to be sure. “We cannot exclude the possibility of an abrupt change, a ’drunk driver’ in the climate system,” says Alley.