By considering the unique sea-level "fingerprint" created by a melting ice sheet, a team of geophysicists in North America has developed a new method for pinpointing the sources of global sea-level rise. Their approach could provide a way to measure the impact of the Greenland and West Antarctic ice sheets – the greatest sources of uncertainty in projections of future sea-level changes.
Long-term variations in sea level are caused by processes including thermal expansion of the water, changes in ocean circulation, and changes in the size of glaciers and ice sheets. Measurements from tide gauges indicate a global average sea-level rise of 1–2 mm/yr during the 20th century. However, this estimate ignores geographical variations in sea level, and provides no information about the contribution of different processes.
One possible way to pick apart the total sea-level change is to look for the distinct pattern, or fingerprint, of a melting ice sheet. Close to the ice sheet, for example, the sea level tends to fall. This is a result of both the local uplift of the Earth's crust after being relieved of the great weight of the ice and a reduction in the ice sheet's gravitational pull on the ocean. Moving further away from the ice sheet, however, the sea level rises progressively.
Melt rates of 0.3 and 0.5 mm/yr were assumed for the Greenland (GIS) and West Antarctic (WAIS) ice sheets, respectively, along with their predicted fingerprints. The researchers then applied the Kalman filter to this synthetic dataset, initializing the algorithm with melt rates of zero.The algorithm was found to estimate the melt rates most accurately when applied to the maximum number of tide gauges, providing enough information for the ice-sheet fingerprints to be separated from the globally uniform trend. The final estimated melt rates for the GIS and WAIS were 0.21 and 0.38 mm/yr, respectively, close to the values used in the synthetic dataset. The 1σ uncertainties associated with these values indicate the magnitude of ice-sheet melting that could potentially be detected in real sea-level records.