AR4 concluded that there was “high confidence that the rate of global sea level rise increased from the 19th to the 20th century” but could not be certain as to whether the higher rate since 1993 was reflective of decadal variability or a further increase in the longer-term trend. Since AR4, there has been considerable effort to quantify the level of decadal and multi-decadal variability and to detect acceleration in GMSL and mean sea level at individual tide gauges. It has been clear for some time that there was a significant increase in the rate of sea level rise in the four oldest records from Northern Europe starting in the early to mid-19th century (Ekman, 1988; Woodworth, 1990, 1999; Mitchum et al., 2010). Estimates of the change in the rate have been computed, either by comparing trends over 100-year intervals for the Stockholm site (Ekman, 1988; Woodworth, 1990), or by fitting a quadratic term to all the long records starting before 1850 (Woodworth, 1990, 1999). The results are consistent and indicate a significant acceleration that started in the early to mid-19th century (Woodworth, 1990, 1999), although some have argued it may have started in the late 1700s (Jevrejeva et al., 2008). The increase in the rate of sea level rise at Stockholm (the longest record that extends past 1900) has been based on differencing 100-year trends from 1774–1884 and 1885–1985. The estimated change is 1.0 [0.7 to 1.3] mm yr –1 per century (1 standard error, as calculated by Woodworth, 1990). Although sites in other ocean basins do show an increased trend after 1860 (e.g., Figure 3.12), it is impossible to detect a change in the early to mid-1800s in other parts of the ocean using tide gauge data alone, as there are no observations.

Numerous studies have attempted to quantify if a detectable acceleration has continued into the 20th century, typically by fitting a quadratic to data at individual tide gauges (Woodworth, 1990; Woodworth et al., 2009, 2011; Houston and Dean, 2011; Watson, 2011) as well as to reconstructed time series of GMSL (Church and White, 2006; Jevrejeva et al., 2008; Church and White, 2011; Rahmstorf and Vermeer, 2011), or by examining differences in long-term rates computed at different tide gauges (Sallenger et al., 2012). Woodworth et al. (2011) find significant quadratic terms at the sites that begin before 1860 (all in the NH). Other authors using more numerous but significantly shorter records have found either insignificant or small negative quadratic terms in sea level around the United States and Australia since 1920 (Houston and Dean, 2011; Watson, 2011), or large positive quadratic values since 1950 along the U.S. east coast (Sallenger et al., 2012). However, fitting a quadratic term to tide gauge data after 1920 results in highly variable, insignificant quadratic terms (Rahmstorf and Vermeer, 2011), and so only studies that use data before 1920 and that extend until 2000 or beyond are suitable for evaluating long-term acceleration of sea level.

A long time scale is needed because significant multi-decadal variability appears in numerous tide gauge records during the 20th century (Holgate, 2007; Woodworth et al., 2009, 2011; Mitchum et al., 2010; Chambers et al., 2012). The multi-decadal variability is marked by an increasing trend starting in 1910–1920, a downward trend (i.e., leveling of sea level if a long-term trend is not removed) starting around 1950, and an increasing trend starting around 1980. The pattern can be seen in New York, Mumbai and Fremantle records, for instance (Figure 3.12), as well as 14 other gauges representing all ocean basins (Chambers et al., 2012), and in all reconstructions (Figure 3.14). It is also seen in an analysis of upper 400 m temperature (Gouretski et al., 2012; Section 3.3.2). Although the calculations of 18-year rates of GMSL rise based on the different reconstruction methods disagree by as much as 2 mm yr –1 before 1950 and on details of the variability (Figure 3.14), all do indicate 18-year trends that were significantly higher than the 20th century average at certain times (1920–1950, 1990–present) and lower at other periods (1910–1920, 1955–1980), likely related to multi-decadal variability. Several studies have suggested these variations may be linked to climate fluctuations like the Atlantic Multi-decadal Oscillation (AMO) and/or Pacific Decadal Oscillation (PDO, Box 2.5) (Holgate, 2007; Jevrejeva et al., 2008; Chambers et al., 2012), but these results are not conclusive.

While technically correct that these multi-decadal changes represent acceleration/deceleration of sea level, they should not be interpreted as change in the longer-term rate of sea level rise, as a time series longer than the variability is required to detect those trends. Using data extending from 1900 to after 2000, the quadratic term computed from both individual tide gauge records and GMSL reconstructions is significantly positive (Jevrejeva et al., 2008; Church and White, 2011; Rahmstorf and Vermeer, 2011; Woodworth et al., 2011). Church and White (2006) report that the estimated acceleration term in GMSL (twice the quadratic parameter) is 0.009 [0.006 to 0.012] mm yr –2 (1 standard deviation) from 1880 to 2009, which is consistent with the other published estimates (e.g., Jevrejeva et al., 2008; Woodworth et al., 2009) that use records longer than 100 years. Chambers et al. (2012) find that modelling a period near 60 years removes much of the multi-decadal variability of the 20th century in the tide gauge reconstruction time series. When a 60-year oscillation is modeled along with an acceleration term, the estimated acceleration in GMSL since 1900 ranges from: 0.000 [–0.002 to 0.002] mm yr –2 in the Ray and Douglas (2011) record, 0.013 [0.007 to 0.019] mm yr –2 in the Jevrejeva et al. (2008) record, and 0.012 [0.009 to 0.015] mm yr –2 in the Church and White (2011) record. Thus, while there is more disagreement on the value of a 20th century acceleration in GMSL when accounting for multi-decadal fluctuations, two out of three records still indicate a significant positive value. The trend in GMSL observed since 1993, however, is not significantly larger than the estimate of 18-year trends in previous decades (e.g., 1920–1950).

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