Editing IPCC:AR6/SROCC/Chapter-4 (section)

===== 4.2.2.1.1 Tide gauge records =====

The number of tide gauges has increased over time from only a few in northern Europe in the 18th century to more than 2000 today along the world’s coastlines. Because of their location and limited number, tide gauges sample the ocean sparsely and non-uniformly with a bias towards the Northern Hemisphere. Most tide gauge records are short and have significant gaps. In addition, tide gauges are anchored on land and are affected by the vertical motion of Earth’s crust caused by both natural processes (e.g., GIA, tectonics and sediment compaction; Wöppelmann and Marcos, 2016 <sup>[[#fn:r117|117]]</sup> ; Pfeffer et al., 2017 <sup>[[#fn:r118|118]]</sup> ) and anthropogenic activities (e.g., groundwater depletion, dam building or settling of landfill in urban areas; Raucoules et al., 2013 <sup>[[#fn:r119|119]]</sup> ; Pfeffer et al., 2017 <sup>[[#fn:r120|120]]</sup> ) . When estimating the GMSL due to the ocean thermal expansion and land ice melt, tide gauges must be corrected for this VLM, where VLM = GIA + anthropogenic subsidence + (tectonics, natural subsidence). This is possible with stations of the Global Positioning System (GPS) network when they are co-located with tide gauges (Santamaría-Gómez et al., 2017 <sup>[[#fn:r121|121]]</sup> ; Kleinherenbrink et al., 2018 <sup>[[#fn:r122|122]]</sup> ) . However, this approach provides information on the VLM over the past two to three decades and has limited value over longer time scales for places where the VLM has varied significantly through the last century (Riva et al., 2017 <sup>[[#fn:r123|123]]</sup> ) .

AR5 assessed the different strategies to estimate the 20th century GMSL changes. These strategies only accounted for the inhomogeneous space and time coverage of tide gauge data and for the VLM induced by GIA (Figure 4.5). Since AR5 two new approaches have been developed. The first one uses a Kalman smoother which combines tide gauge records with the spatial patterns associated with ocean dynamic change, change in land ice and GIA. It enables accounting for the inhomogeneous distribution of tide gauges and the VLM associated with both GIA and current land ice loss (Hay et al., 2015 <sup>[[#fn:r124|124]]</sup> ; Figure 4.5). The second approach uses ad hoc corrections to tide gauge records with an additional spatial pattern associated with changes in terrestrial water storage to account for the inhomogeneous distribution in tide gauges. It also accounts for the total VLM (Dangendorf et al., 2017 <sup>[[#fn:r125|125]]</sup> ; Figure 4.5). Both methods yield significantly lower GMSL changes over the period 1950–1970 than previous estimates, leading to long-term trends since 1900 that are smaller than previous estimates by 0.4 mm yr <sup>–1</sup> (Figure 4.5). Different arguments including biases in the tide gauge datasets (Hamlington and Thompson, 2015 <sup>[[#fn:r126|126]]</sup> ) , biases in the averaging technique and absence of VLM correction (Dangendorf et al., 2017 <sup>[[#fn:r127|127]]</sup> ) , or in the spatial patterns associated with the sea level contributions (Hamlington et al., 2018 <sup>[[#fn:r128|128]]</sup> ) have been proposed to explain these smaller GMSL rates. There is no agreement yet on which is the primary reason for the differences and it is not clear whether all the reasons invoked can actually explain all the differences across reconstructions. As there is no clear evidence to discard any reconstruction, this assessment considers the ensemble of AR5 sea level reconstructions augmented by the two recent reconstructions from Hay et al. (2015) <sup>[[#fn:r129|129]]</sup> and Dangendorf et al. (2017) <sup>[[#fn:r130|130]]</sup> to evaluate the GMSL changes over the 20th century. On this basis, it is estimated that it is ''very likely'' that the long-term trend in GMSL estimated from tide gauge records is 1.5 (1.1–1.9) mm yr <sup>–1</sup> between 1902 and 2010 for a total SLR of 0.16 (0.12–0.21) m (see also Table 4.1). This estimate is consistent with the AR5 assessment (but with an increased uncertainty range) and confirms that it is ''virtually certain'' that GMSL rates over the 20th century are several times as large as GMSL rates during the late Holocene (see 4.2.2.1). Over the 20th century the GMSL record also shows an acceleration ( ''high confidence'' ) as now four out of five reconstructions extending back to at least 1902 show a robust acceleration (Jevrejeva et al., 2008 <sup>[[#fn:r131|131]]</sup> ; Church and White, 2011 <sup>[[#fn:r132|132]]</sup> ; Ray and Douglas, 2011 <sup>[[#fn:r133|133]]</sup> ; Haigh et al., 2014b <sup>[[#fn:r134|134]]</sup> ; Hay et al., 2015 <sup>[[#fn:r135|135]]</sup> ; Watson, 2016 <sup>[[#fn:r136|136]]</sup> ; Dangendorf et al., 2017 <sup>[[#fn:r137|137]]</sup> ) . The estimates of the acceleration ranges between -0.002–0.019 mm yr <sup>–1</sup> over 1902–2010 are consistent with AR5.

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