Interannual variability in the rate of the global mean sea level rise

Abstract:

Recent studies have shown that the global mean sea level (GMSL) is accelerating. For improved process understanding, it is crucial to precisely estimate the GMSL acceleration due to externally-forced global climate change. For that purpose, the internal climate variability-related signal of the GMSL components, in particular the terrestrial waters and ocean thermal expansion, needs to be removed from the GMSL record. In the present study, we first estimate how the observed GMSL rate has evolved with time over the altimetry era (1993-present). We find that this rate computed over 5-year moving windows, displays significant inter-annual variability around 7-year and 12-year. In a next step, we remove from the observed GMSL time series, the inter-annual variability of all components of the sea level budget and find that an important signal, of 7-8-year periodicity, still remains in the GMSL rate. Looking at each component individually, we find that such a cycle exists in the glaciers component, but in the published model-based and in situ glacier mass balance times series used in this study, its amplitude is too small to explain what we observe in the GMSL and GMSL rate. Estimating the glaciers inter-annual variability using GRACE solutions over the period 2003-2015 further confirms the presence of a 7-8-year cycle, with an amplitude about 7 times larger than in the model-based and 3 times larger than in the in situ glacier mass balance time series. Recomputing the GMSL rate over the GRACE period removing the inter-annual variability of the GRACE-based glaciers (in addition of that of all other components) allows to significantly remove the 7-8-year cycle in the GMSL rate. Prior to GRACE, i.e., for the time span starting in 1993, we cannot use GRACE-based glaciers nor GRACE-based ocean mass inter-annual variability. Thus, we empirically remove an 8-year cycle in addition to all other inter-annual contributions. The corresponding GMSL rate is found to have increased by a factor of two over the altimetry era, from 2 mm/yr in 1995 to more than 4 mm/yr in 2015. About half of this acceleration is due to Greenland ice mass loss, the other half being equally shared between glacier melting, Antarctica mass loss and ocean thermal expansion.