Globally sea levels are on the rise. Now researchers from TU Delft and other European universities report a reconstruction of global mean sea level since 1902 that yields a slower average rise before 1990 than previously thought, but shows similar high rates as independent satellite observations from 1993-2012. This suggests that global mean sea level has been accelerating much faster than previously assumed in the past two decades. The results appear this week in the Proceedings of the National Academy of Sciences (PNAS).
While altimeters on board of satellites are now monitoring sea levels consistently around the globe, before their launch in 1992 the only information on global mean sea level rise comes from a network of tide gauges along the coast. “These tide gauges measure sea level at specific locations relative to the ground and are therefore contaminated by vertical land motion of the Earth and regional variability patterns resulting from wind redistribution, ocean circulation changes, or gravitationally triggered mass redistribution effects after freshwater pulses from land-locked water. This makes it very complicated to estimate a global average, which is not contaminated by local features from better covered regions”, says first author dr. Sönke Dangendorf of the University of Siegen, Germany.
Dangendorf and his colleagues took a closer look at available tide gauge records and ancillary information of potential contamination factors. “We selected only the longest and high quality tide gauges, which best cover certain regions of the global ocean. Then, we corrected each of these tide gauges for all potential contamination factors before averaging them into a global mean”, explains dr. Marta Marcos, of the University of the Balearic Islands at Mallorca.
Researchers from TU Delft - ir. Thomas Frederikse and dr. Riccardo Riva - looked into a specific factor in sea level rise. ‘The global ice mass loss also has an effect on the distribution of mass on Earth. This results in a slight change in gravitational forces and the rotation (speed and angle) of the Earth’ , says Riva. ‘This in turn has an effect on sea levels, with very large regional differences. The mean effect turns out to lead to only a relatively small correction globally, but of course you have to take it into account to get a complete picture.’
After applying all the different corrections, the new reconstruction shows significantly slower global mean sea level rise before 1993, while afterwards the reconstruction goes in concert with the independent observational record from satellites. “The most sensitive factor was the vertical land motion correction”, professor Guy Wöppelmann from the University of La Rochelle, France, adds. “While former reconstructions could only correct for one particular process of vertical land motion, namely glacial isostatic adjustment, we were now able to add other local effects”, Guy Wöppelmann further explains. In most regions these contributions have, together with some methodological adjustments, resulted in a significant downward correction of relative sea level rates before 1990 (1.1 mm/yr).
As a result of this downward correction before 1990, the entire shape of the global mean sea level curve changed significantly. Due to the flatter character at the beginning and the unaltered high rates during the last two to three decades the 20th century acceleration becomes more pronounced. “Such acceleration is now more consistent with our understanding of individual processes contributing to the observed sea level rise.
More information
Article #16-16007: “Reassessment of 20th century global mean sea-level rise,” by Sönke Dangendorf, Marta Marcos, Guy Wöppelmann, Clint Conrad, Thomas Frederikse, and Riccardo Riva. www.pnas.org/content/early/2017/05/16/1616007114.abstract
Contact Riccardo Riva: https://www.tudelft.nl/staff/r.e.m.riva/?no_cache=1
Science Information Officer TU Delft Roy Meijer, r.e.t.meijer@tudelft.nl, +31 15 278175