Detecting a forced signal in satellite-era sea-level change

Kristin Richter,Cécile Agosta,Angélique Melet,Nicolas Champollion,Ben Marzeion,Matthew D Palmer,Christopher D Roberts,Benoit Meyssignac,Giorgio Spada,Stefan R M Ligtenberg,John A Church,Aimée B A Slangen,Xavier Fettweis

doi:10.1088/1748-9326/ab986e

Abstract

In this study, we compare the spatial patterns of simulated geocentric sea-level change to observations from satellite altimetry over the period 1993–2015 to assess whether a forced signal is detectable. This is challenging, as on these time scales internal variability plays an important role and may dominate the observed spatial patterns of regional sea-level change. Model simulations of regional sea-level change associated with sterodynamic sea level, atmospheric loading, glacier mass change, and ice-sheet surface mass balance changes are combined with observations of groundwater depletion, reservoir storage, and dynamic ice-sheet mass changes. The resulting total geocentric regional sea-level change is then compared to independent measurements from satellite altimeter observations. The detectability of the climate-forced signal is assessed by comparing the model ensemble mean of the ‘historical’ simulations with the characteristics of sea-level variability in pre-industrial control simulations. To further minimize the impact of internal variability, zonal averages were produced. We find that, in all ocean basins, zonally averaged simulated sea-level changes are consistent with observations within sampling uncertainties associated with simulated internal variability of the sterodynamic component. Furthermore, the simulated zonally averaged sea-level change cannot be explained by internal variability alone—thus we conclude that the observations include a forced contribution that is detectable at basin scales.

Highlights

During the altimetry period (1993–2018) global mean sea level (GMSL) has been rising at a rate of about 3 mm yr−1 (WCRP Global Sea Level Budget Group 2018)
The spatial pattern of change, is mainly related to ocean dynamic sea-level change, which dominates over the barystatic fingerprints (e.g. Spada and Galassi 2016) that result from changes in Earth gravity, rotation and viscoelastic solidEarth deformation (GRD, Gregory et al 2019) due to shrinking land ice from glaciers and ice sheets as well as changes in terrestrial landwater storage
Detecting a forced trend in sea level becomes more challenging on smaller space- and shorter time-scales as regional modes of internal variability have a larger influence on sea level at these scales

Summary

Introduction

During the altimetry period (1993–2018) global mean sea level (GMSL) has been rising at a rate of about 3 mm yr−1 (WCRP Global Sea Level Budget Group 2018). Several studies have shown that the total GMSL change (Dangendorf et al 2015, Slangen et al 2016) as well as the component contributions (Marcos and Amores 2014, Slangen et al 2014, Marzeion et al 2014) are partly driven by external climate forcings (e.g. increasing greenhouse gas concentrations). These studies usually consider time periods of at least 40 years or more. A forced signal is related to external drivers, both natural (e.g. volcanic eruptions) as well as anthropogenic (e.g. greenhouse gas emissions)

Results

Discussion

Conclusion