A Revised Estimate of 20th Century Global Mean Sea Level

Abstract:

One of the primary goals of paleo-sea level research is to assess the stability of ice sheets and glaciers in warming climates. In this context, the 20^th century may be thought of as the most recent, recorded, and studied of all past episodes of warming. Over the past decade, a consensus has emerged in the literature that 20^th century global mean sea level (GMSL), inferred from tide gauge records, rose at a mean rate of 1.6-1.9 mm/yr. This sea-level rise can be attributed to multiple sources, including thermal expansion of the oceans, ice sheet and glacier mass flux, and anthropogenic changes in land water storage. The Fifth Assessment Report of the IPCC summarized the estimated contributions of these sources over 1901-1990 and computed a total rate, using a bottom-up approach, of ~1.0 mm/yr, which falls significantly short of the rate inferred from tide gauge records. Using two independent probabilistic approaches that utilize models of glacial isostatic adjustment, ocean dynamics, and the sea-level fingerprints of rapid land-ice melt to analyze tide gauge records (Kalman smoothing and Gaussian process regression), we are able to close the 20^th century sea-level budget and resolve the above enigma. Our revised estimate for the rate of GMSL rise during 1901-1990 is 1.1–1.3 mm/yr (90% credible interval). This value, which is ~20-30% less than previous estimates, suggests that the change in the GMSL rate from the 20^th century to the last two decades (2.7 ± 0.4 mm/yr, consistent with past estimates) was greater than previous estimates. Moreover, since some forward projections of GMSL change into the next century are based in part on past estimates of GMSL change, our revised rate may impact projections of GMSL rise for the 21^st century and beyond.