Global mean sea level has risen by more than 3 centimeters per decade since precise satellite measurements began in the 1990s. A full third of this rise is due to thermal expansion of the warming ocean. Almost two-thirds are due to water masses added to the ocean, mainly by the melting of glaciers and the two ice sheets of Greenland and Antarctica. Melting has increased since the 1990s, accelerating sea level rise. Another addition of water to the ocean results from a decrease in water storage on land, mainly due to the groundwater depletion caused by human abstraction.
Scientists test their understanding of the processes of sea level change by comparing the observed sea level change with the sum of the assessed contributions, i.e. by estimating the sea level budget. of ocean mass can be determined from the sum of the individual contributions: from the Greenland ice sheet, the Antarctic ice sheet, glaciers around the world, and changes in terrestrial water storage. Alternatively, it can be measured directly by satellites that observe tiny changes in Earth’s gravitational pull driven by regional changes in masses of ice or water.
ESA’s Climate Change Initiative (CCI) has generated continuous, high-quality spatial records of Essential Climate Variables (ECVs), including a number of sea level-related variables. Sea Level Budget Closure” led by a consortium of ten European research institutes has now jointly assessed these CCI ECV records with respect to the sea level budget. To this end, the project has advanced and extended the analysis of data from Earth observation satellites as well as oceanographic measurements and numerical modelling.
General information on measurement methods and calculation models
Changes in ocean water density, and therefore its thermal expansion, were assessed by a novel combination of measurements from the Argo network of ocean profiler floats with the CCI sea surface temperature records. Estimates of the mass changes of the Greenland Ice Sheet and the Antarctic Ice Sheet were derived from ice surface elevation changes measured by four different ESA satellite missions. This required innovative methods to convert the change in ice sheet volume into change in mass taking into account the varying densities of snow and ice involved. Glacier mass changes were assessed by a global glacier model supported by satellite data. Changes in terrestrial water storage were assessed by a global hydrological model enhanced by better representation of groundwater abstraction, among other improvements.
Averaged over the period from 1993 to 2016, the average rate of measured global mean sea level rise was 3.0 millimeters per year (mm/year). The thermal expansion effect is estimated at 1.1 mm/year (38% of the total elevation), while the contribution of water bodies is estimated at 1.7 mm/year (57%). The mass component includes 0.6 mm/year (21% of total sea level rise) from glaciers outside Greenland and Antarctica, 0.6 mm/year (20%) from Greenland, 0.2 mm/year (6%) from Antarctica and 0.3 mm/year (10%) from terrestrial water storage changes. In the most recent sub-period from 2003 to 2016, sea level rise was 3.6 mm/yr, higher than the full period. This is due to an increase in mass contributions, now about 2.4 mm/yr (66% of the increase), with the largest increase coming from Greenland, while the contribution from thermal expansion has remained similar to 1.2 mm/year (now 33%). These results are in agreement with previous studies. They gain additional confidence through advances in the analysis of the data involved. Progress includes the consistent approach to specifying precision limits across all budget items. These represent about 10% of the total sea level rise. This precision is the margin within which one can expect a shift between the sum of the parts and the whole. Indeed, the shift is consistent with these margins.
The results call for further improvements in the understanding of satellite measurements and the physical processes involved. For example, slow deformations of the solid Earth under the ocean affect satellite observations. These effects must be separated from changes within the ocean itself and are currently an important source of the remaining uncertainty in the sea level budget.
Martin Horwath (Technische Universität Dresden), lead scientist of the study, summarizes that “assembling this coherent picture of sea level and ocean mass budgets not only required advanced datasets from observation and modeling of the earth from satellites. It also required experts from various disciplines to come to a common framework. He adds that “some of our results were incorporated into the recent Sixth Assessment Report of the IPCC Now we provide the complete set of time series and their documentation.”
Jérôme Benveniste (ESA/ESRIN), who initiated and supervised the study, adds that “it is the result of the continuity of research and development on the analysis of Earth observation data made possible by the Initiative ESA Climate Change. The beauty of the results lies in the coherence of all the Essential Climate Variables of the CCI which, well prepared and assembled, give an accurate picture of our climate and its evolution. The work does not stop at this impressive stage, there are still unanswered questions regarding climate variability and its evolution.
Benjamin Gutknecht (Technische Universität Dresden), project scientist, adds that “it was also important to show that the results change to some extent when the methods are updated. New approaches in how we account for mass shifts in the solid Earth have led to a better understanding of the true reliability of results for ocean mass change. Further research is needed to further separate the different mass transport processes in the Earth system. »
Petra Döll (Goethe Universität Frankfurt), member of the consortium responsible for the assessment of land-water contributions, concludes that “the global storage of terrestrial water fluctuates seasonally and from year to year, and this results in short-term highs and lows in sea level by several millimeters. In the long term, however, the storage of terrestrial waters has diminished in recent decades and thus aggravated sea level rise.”
The research has been published in Earth System Science.
Greenland ice sheet loses enough water in 20 years to cover US: study
Martin Horwath et al, Global Sea Level Budget and Ocean Mass Budget, with a focus on advanced data products and characterization of uncertainty, Earth System Science (2022). DOI: 10.5194/essd-14-411-2022
Dresden University of Technology
The budget is closed: individual contributors to global sea level rise match (2022, February 7)
retrieved 8 February 2022
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