During 20th-century the average global sea level rose by around 17cm (Church and White 2006). Between 1961 and 2003 sea levels rose at an average rate of 1.8 mm per year. More recently the rate of sea level rise has accelerated, with global average increasing by 3.1 mm per year between 1993 and 2003 (Bindoff et al 2007). See Figure 4.

However, because the sea levels of the planet rise in an uneven fashion – unlike in a bathtub – in some regions sea levels are actually falling while in other areas the rate of sea level rise has been several times the global mean (Bindoff et al 2007). This regional variation reflects the complex interactions between the Earth’s climate and sea levels, but does not change the fact that there is an observed global trend of rising seas.

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Figure 4: Annual averages of global mean sea level based on reconstructed sea level fields since 1870 (red), tide gauge measurements since 1950 (blue) and satellites since 1992 (black). Units are in mm relative to the average for 1961 to 1990. (Bindoff et al 2007)

Sea level rise is driven by a number of different factors, including the melting of polar ice and the expansion of ocean water caused by rising sea temperatures (also known as “thermal expansion”). The contribution of each of these drivers to sea level rise since 1961, as reported by the IPCC, is shown in Table 1. 

Table 1: Estimates of the various contributions to global mean sea level rise (adapted from Bindoff et al 2007)

Church and White (2006), “A 20th century acceleration in global sea-level rise”, Geophysical Research Letters, Vol 33.

Bindoff, N.L., J. Willebrand, V. Artale, A, Cazenave, J. Gregory, S. Gulev, K. Hanawa, C. Le Quéré, S. Levitus, Y. Nojiri, C.K. Shum, L.D. Talley and A. Unnikrishnan, 2007: Observations: Oceanic Climate Change and Sea Level. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA