Using sea-level data to constrain the contribution of the Greenland ice sheet to contemporary and recent sea-level change
Wake, Leanne Mary
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Due to the potentially wide-reaching impacts on climate and sea-level change of a declining Greenland Ice Sheet (GrIS), the mass balance of the past decade has caused concern that the ice sheet is reacting to increased temperatures of the industrial era and that the ice sheet is in the initial stages of deglaciation. Global mean sea-level has been rising at a rate of 1.8 +/- 0.5 mm/yr over the past 50 years (Bindoff et al. 2007), and this has accelerated to 3.1 +/-0.1mm/yr (Cazenave et al., 2008) over the past decade. This study shows that although the surface mass balance of the GrIS can react quickly to changes in temperature, overall the ice sheet is in near balance over the period 1866-2005. During 1866-2005, the contribution from the GrIS to eustatic sea-level change is not larger than the error attached to current estimates of global mean sea-level rise. A novel type of relative sea-level data gathered from salt marshes in the south west of Greenland cover the period from ~1200 to 1800AD and show that a major slowdown in local sea-level rise from ~3mm/yr to ~0mm/yr occurred around 1500-1600 AD, with no significant departure from a 0mm/yr trend thereafter. Large contributions to sea-level change from steric changes and cryospheric sources outside of Greenland are ruled out as major drivers of this deceleration in sea-level fall. Modelling results indicate that the slowdown in relative sea-level is most likely due to the combined contribution of dynamic-related ice loss from Jakobshavn Isbrae and a delayed earth response to mass loss during a period of elevated temperatures from ~1000-1500AD. When considering the saltmarsh sea-level data for the 20th century within the context of the complete time series, the magnitude of ice loss in west Greenland for the past decade does not appear to be anomalous. This analysis suggests that similar mass loss has been sustained for several centuries prior to 1500AD.