3.1 Regional Sea Level

The important question arises here about how does the interplay between the internal climate variability and anthropogenic influence manifest itself in regional variations of SLC?

The Hurst exponent α varies significantly from one region to another reflecting mainly local/regional phenomena as demonstrated by Barbosa et al. [2008], Hughes and Williams [2010], and Bos et al. [2013]. The largest observed values of α ~0.9 have been found in the north Pacific sea level records from Balboa, in Panama, to Victoria, in Canada and somewhat smaller α ~0.8 in Seattle and San Diego (Table 1). An unnatural SLC is observed in four out of six sea level records: Seattle, San Francisco, San Diego, and Honolulu (Figure 2a). In San Diego, the MASLT is 0.9 mm/yr, or about 50% of the observed sea level trend. For other tide gauges, the estimated part of the MASLT is significantly lower and represents only 10–20% of the observed sea level trend. These results confirm Meyssignac et al. [2012] observations that the sea level fluctuations in the Tropical Pacific are due mostly to the internal modes of ocean variability.

In the Western North Atlantic, two regions can be distinguished: The first one includes the Trois‐Rivieres and Batiscan tide gauges both located on the northern shore of the Saint Lawrence River, Canada, and Portland tide gauge, in the U.S. For these tide gauges, α is large (ranging between 0.8 and 1, Table 1). None of these three records shows a significant unnatural SLC over their period of operation (Figure 2a). It is important to note here that the tide gauges located in the Saint Lawrence River estuary are influenced by strong discharge from upland runoff, in addition to sea level influences. Thus, the sea level variability enhanced by the estuarine dynamics makes the external trends to go undetected even with a rather conservative choice of the 99% confidence level. The second region stands out as a cluster of unnatural SLC in the largest urban areas of the US East Coast: New York, Philadelphia, Atlantic City, and Baltimore (Figure 2a). In New York, the minimum externally driven sea level trend is 1.2 mm/yr, or about 67% of the observed total sea level trend. At the Atlantic City and Baltimore tide gauges, the estimated part of the MASLT is 1.1 mm/yr and represents 39% and 52% of the total sea level trend, respectively. For the tide gauge at Philadelphia, the estimated part of the MASLT is lower and represents 24% of the observed sea level trend. Here again, this lower value can be explained by the location of Philadelphia tide gauge at about 100 km upstream from the Delaware River mouth. These cities correspond to the sea level rise “hot spot” detected by Sallenger et al. [2012]: a region where the sea level is rising between three and four times faster than the global average. Our analysis indicates unnatural origin of this fast sea level rising due probably to the anthropogenic warming that induces, among other consequences, significant changes in the strength of the Atlantic Meridional overturning circulation and of the Gulf Stream [Sallenger et al. 2012].

In the northern Gulf of Mexico, a strong significant unnatural SLC is found at Galveston (Figure 2a). The MASLT is 4.1 mm/yr (i.e., 68%) against 6 mm/yr of the observed sea level trend. This tide gauge is known to be dominated by land subsidence due to the extraction of subsurface fluids, hydrocarbons, and groundwater withdrawal [Morton et al., 2006; Kolker et al., 2011]. In contrast, the Key West sea level station is situated on a stable coral reef, at the edge of the Florida shelf [Maul and Martin, 1993; Davis, 2011], and little affected by the post‐glacial rebound. The MASLT at Key West station is 1 mm/yr and represents 45% of the observed sea level trend. Detecting a significant unnatural SLC at Key West indicates probably unnatural trend in dynamics of the Loop Current flowing out the Gulf of Mexico to the western North Atlantic.

Along the Atlantic coasts of Iberia and UK the tide gauges of Newlyn and Cascais present α in range [0.7; 0.8] and significant unnatural SLC (Figure 2b). The MASLT is 0.6 mm/yr at Newlyn tide gauge and 0.3 mm/yr at Cascais tide gauge, or about 40% and 21% of the observed sea level trend, respectively. It is tentative to make a parallel with the observed “hot spot” at the east coast of U.S., following Miller and Douglas [2007] and Woodworth et al. [2010] who have suggested a link between sea level at the eastern and western boundaries of the North Atlantic through changes in the strength of the oceanic subtropical gyre. It is remarkable that the Brest record, French Atlantic Coast, being coherent with the regional value α ~0.7 does not show a significant SLC. This might be explained by the earlier period of the Brest record (1846–1943) considered in the study, as compared to the Newlyn and Cascais records. Also of interest is the comparison of Portland (Maine) and Cascais sea level records that have been shown [Miller and Douglas, 2007; Stammer et al., 2013] to display similar sea level variations but shifted in time by about 10 years. Whereas the sea level trend at Cascais is identified as having an unnatural component, that at Portland can still be explained by its natural power law variability.

In the Mediterranean Sea, the century‐scale tide record from Marseille reveals a strong unnatural SLC (Figure 2b). The MASLT is 0.7 mm/yr, or more than a half (58%) of the total sea level trend. Contrarily, the Trieste tide‐gauge record shows no significant unnatural SLC over the period of operation although its fluctuation exponent (α ~0.8) is close to that in Marseille (α ~0.7). This difference indicates a stronger regional/local driver of SLC at Marseille.

In the North Sea, the relative sea level increase is observed at all tide gauges (Figure 2b), and it is significantly larger than can be expected from the natural sea level variability. Although, there are no long‐term Norwegian records used in our analysis, it seems reasonable to suppose that the whole North Sea has a large contribution of unnatural SLC. In the straits between the North Sea and the Baltic Sea (Hornbaek, Smogen, Hirtshal, Frederikshavn, Aarhus, Fredericia, Korsor, and Slipshavn), we find homogeneous distribution of α in range [0.6; 0.7] (Table 1).

The Baltic Sea is a semi‐enclosed sea consisting of three well‐defined sub‐basins: The Bothnian Bay, the Bothnian Sea, and the Baltic proper with the Gulf of Finland. In the extreme western part of the Baltic Sea, the tide gauges of Travemunde, Wismar 2, Warnemunde 2, Gedser, and Swinoujscie show α in range [0.5; 0.6] and strong significant unnatural SLC (Figure 2b). In this region, the MASLT represent more than 80% of the observed sea level trends, except for Swinoujscie and Kobenhavn, where the MASLT represent 50% and 63% of the observed sea level trend, respectively. The central region that includes the Bothnian Sea, the Finland Gulf, and the proper Baltic Sea shows, in average, coherent Hurst exponents in the range [0.5; 0.6]. In these regions, no significant change in SLC was found, except for the station of Kungsholmsfort. In the Bothnian Bay, the tide‐gauge records of Oulu, Ratan, and Vaasa show coherent Hurst exponents close to 0.6 and strong significant unnatural change of trends. The MASLT are 2.7 mm/yr at Oulu tide gauge, 0.9 mm/yr at Ratan tide gauge, and 0.8 mm/yr at Vaasa tide gauge, or about 73%, 56%, and 57% of the observed sea level trend, respectively.

The Mumbai tide gauge (Figure 2c), the unique more than a century long sea level record in the Indian Ocean, presents a significant unnatural SLC (α ~0.6). The MASLT is 0.7 mm/yr and represents 64% of the observed sea level trend. In this megacity, no significant subsidence caused by groundwater pumping, oil/gas extraction from coastal reservoirs, or by sedimentary compaction in the Gulf of Cambay has yet been demonstrated, and the unnatural sea level trend is probably induced by externally driven changes in the Indian Ocean currents.

On the west side of Australia (Figure 2c), the tide‐gauge record of Fremantle has a α ~0.8 and shows a strong significant unnatural SLC. The MASLT is 0.9 mm/yr, or about 47% of the observed sea level trend. The Fremantle tide gauge is located in the Perth Basin, where land subsidence has been reported in previous studies [Belperio, 1993; Featherstone et al., 2012; Burgette et al., 2013]. In contrast to Fremantle, the Sydney tide‐gauge record from the eastern coast of Australia does not present statistically significant unnatural SLC (α ~0.9). In this site, there is no noticeable tectonic activity, and the coastal subsidence has a slow rate [Murray‐Wallace and Belperio, 1991].