An ESA-funded research instills confidence in the understanding of worldwide mean sea-level rise by placing precise numbers and accuracy measures on individual reasons for sea-level rise.
The research was directed by TU Dresden and has been published in Earth System Science Data on February 7th, 2022. It demonstrates how the sum of sea-level contributions measured on a month-to-month basis equals the total sea-level change detected by satellites.
The global mean sea level has risen by over 3 cm every decade since precise satellite measurements commenced in the 1990s. A good third of this increase is as a result of thermal expansion of the warming ocean.
Approximately two-thirds are because of water masses added to the ocean, mostly through the melting of glaciers and the two ice sheets in Antarctica and Greenland. The melting has risen since the 1990s, thereby accelerating sea-level rise.
Another reason for the addition of water to the ocean is a decline in water storage on land, largely because of groundwater depletion caused by human water abstractions.
Researchers test their comprehension of sea-level-change processes by comparing the detected sea-level change with the sum of measured contributions, that is, by evaluating the sea-level budget.
Ocean mass change can be established from the sum of individual contributions: from the Antarctic Ice Sheet, the Greenland Ice Sheet, the glaciers around the world, and variations in land water storage.
Alternatively, it can be assessed directly by satellites that detect minute changes of Earth’s gravitational pull caused by regional variations of ice or water masses.
The Climate Change Initiative (CCI) of ESA has produced high-quality and nonstop space-based records of Essential Climate Variables (ECV) climate variables, including several variables associated with sea level.
The “CCI Sea Level Budget Closure” project run by a consortium of 10 European research institutions has currently evaluated these CCI ECV records together concerning the sea-level budget. To this end, the project advanced and extended the examination of data from earth observation satellites as well as from numerical modeling and oceanographic measurements.
Background on Measurement Methods and Model Calculations
Ocean water density varies, and therefore its thermal expansion was measured by a new combination of measurements of the Argo network of ocean profiling floats with CCI sea surface temperature records.
Estimates of Greenland Ice Sheet and Antarctic Ice Sheet mass variations were extracted from ice surface elevation deviations assessed by four different ESA satellite missions. This needed innovative approaches to translate the ice-sheet volume change to mass change by considering the variable snow and ice densities involved.
Glacier mass alterations were evaluated by a global glacier model aided by satellite data. Changes in the land water storage were evaluated by a global hydrological model enhanced by an upgraded representation of groundwater withdrawal, among other enhancements.
Averaged over the period between 1993 and 2016, the mean rate of assessed global mean sea-level rise was 3.0 mm annually. The thermal expansion effect is measured at 1.1 mm annually (38% of the total rise), while the incorporation of water masses is measured at 1.7 mm annually (57%).
The mass component comprises 0.6 mm annually (21% of total sea-level rise) from glaciers beyond Greenland and Antarctica, 0.6 mm annually (20%) from Greenland, 0.2 mm annually (6%) from Antarctica, and 0.3 mm annually (10%) from variations in land water storage.
In the more recent sub-period between 2003 and 2016, the sea-level rise was 3.6 mm annually, higher than in the total period. This is because of an increase of the mass contributions, currently, about 2.4 mm annually (66% of the rise), with the largest increase contributed from Greenland, while the contribution from thermal expansion stayed similar at 1.2 mm annually (currently 33%).
These results are in keeping with earlier studies. They gain more confidence through the progress of data analysis involved.
The progress includes the stable approach to stipulating the accuracy boundaries across all elements of the budget. These constitute approximately 10% of the total sea-level rise. This accuracy is the margin within which a discrepancy between the sum of the parts and the whole may be anticipated. Certainly, the discrepancy matches those margins.
The results summon additional improvements in the interpretation of the satellite measurements and the physical processes under consideration. For example, slow deformations of the solid Earth beneath the ocean impact satellite observations.
These effects have to be isolated from changes within the ocean itself and they are presently a vital source of the remaining ambiguity in the sea-level budget.
Assembling this coherent picture of sea-level and ocean-mass budgets not only required advanced datasets from satellite earth observation and modeling. It also required the experts from various disciplines to arrive at a common framework. Part of our results went into the recent Sixth Assessment Report of the IPCC. Now we provide the full set of time series and their documentation.
Martin Horwath, Study Science Lead, Technische Universität Dresden
Jérôme Benveniste (ESA/ESRIN) who commenced and supervised the study, adds: “This is a fruit of the continuity in research and development on Earth observation data analysis enabled by ESA’s Climate Change Initiative. The beauty of the results lies in the coherence of all the CCI Essential Climate Variables, which, when well-prepared and assembled, give a precise picture of our climate and its trend. The work doesn’t stop at this impressive milestone, there are still questions to be answered regarding the climate variability and its evolution.”
It was also important for to show that results change to some extent when methods are updated. New approaches in how we account for mass displacements in the solid Earth have led to an improved understanding of how reliable the results for ocean-mass change really are. More research is needed to further separate the different processes of mass transports in the Earth system.
Benjamin Gutknecht, Project Scientist, Technische Universität Dresden
Petra Döll (Goethe Universität Frankfurt), consortium member responsible for the assessment of land-water contributions: “Global land water storage fluctuates seasonally and from year to year, and this translates to short-term ups and downs of sea level-level by several millimetres. In the long run, however, land water storage has decreased over the last decades and has thereby aggravated sea-level rise.”
Horwath, M., et al. (2022) Global sea-level budget and ocean-mass budget, with a focus on advanced data products and uncertainty characterisation. Earth System Science Data. doi.org/10.5194/essd-14-411-2022.