Sea level is undoubtedly one of the most fundamental variables for understanding how the planet is responding to climate change. Yet current methods for assessing the historical record offer conflicting and puzzling results, with the sum of the individual components of the sea level budget in some cases not matching the observed record of sea level rise.

Until very recently, the generally accepted estimate of twentieth century sea level rise was between 1.5 and 2.0 mm per year (e.g. [1]). However, the inability to account for this rate by summing the individual components for the period up to 1990 led to suggestions that the contribution from one or more components has been significantly underestimated [2].

A recent re-analysis of twentieth century sea level rise produced a lower average rate of sea level rise of around 1.2 mm/year between 1901 and 1990 [3], which does successfully close the sea level budget for this period. However, it provides no extra information about the contributions of individual sources and sinks, and perhaps most significantly, it implies that the observed rate of global mean sea level rise between 1993 and 2010 of around 3 mm/year represents a significantly larger acceleration than previously understood (nearer three times the twentieth century value rather than roughly double).

Understanding and constraining the causes of past sea level rise so that they are consistent with observations is key to improving our ability to predict future sea level rise. For example, the projections presented in the IPCC AR5 report [1] suggest thermal expansion of the oceans will make the largest single contribution to sea level rise to 2100. Yet recent observations show that ice sheets are currently making a larger contribution, and while this might not be a long term trend, it does suggest that our existing projections are not capturing all the physics, especially with regards to ice sheet melt.

References
[1] Church, J. A. et al. in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds Stocker, T. F. et al.) Ch. 13 (Cambridge Univ. Press, 2013).
[2] Gregory, J. M. et al. Twentieth-century global mean sea level rise: is the whole greater than the sum of the parts? J. Clim. 26, 4476–4499 (2013).
[3] Hay, C. C., E. Morrow, R. E. Kopp and J. X. Mitrovica (2015). “Probabilistic reanalysis of twentieth-century sea-level rise.” Nature.[1] Munk, W. (2002). “Twentieth century sea level: An enigma.” Proceedings of the National Academy of Sciences 99(10): 6550-6555.