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

==== 4.2.2.1 Global Mean Sea Level Changes During the Instrumental Period ====

<div id="section-4-2-2-1global-mean-sea-level-changes-during-the-instrumental-period-block-1"></div>

Observational estimates of the sea level variations over past millennia rely essentially on proxy-based regional relative sea level reconstructions corrected for GIA. Since AR5, the increasing availability of regional proxy-based reconstructions enables the estimation of GMSL change over the last ∼ 3 kyr. The first statistical integration of the available reconstructions shows that the GMSL experienced variations of ±9 [±7 to ±11] cm (5–95% uncertainty range; Kopp et al., 2016 <sup>[[#fn:r110|110]]</sup> ) over the 2400 years preceding the 20th century ( ''medium confidence'' ). This is more tightly bound than the AR5 assessment which indicated a variability in GMSL that was &lt;±25 cm over the same period. This progress since AR5 confirms that it is ''virtually certain'' that the mean rate of GMSL has increased during the last two centuries from relatively low rates of change during the late Holocene (order tenths of mm yr <sup>–1</sup> ) to modern rates (order mm yr <sup>–1</sup> ; Woodruff et al., 2013 <sup>[[#fn:r111|111]]</sup> ) .

Over the last two centuries, sea level observations have mostly relied on tide gauge measurements. These records, beginning around 1700 in some locations (Holgate et al., 2012 <sup>[[#fn:r112|112]]</sup> ; PSMSL, 2019 <sup>[[#fn:r113|113]]</sup> ) , provide insight into historic sea level trends. Since 1992, the emergence of precise satellite altimetry has advanced our knowledge on GMSL and regional sea level changes considerably through a combination of near global ocean coverage and high spatial resolution. It has also enabled more detailed monitoring of land ice loss. Since 2002, high precision gravity measurements provided by the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On missions show the loss of land ice in Greenland and Antarctica, and confirm independent assessments of ice sheet mass changes based on satellite altimetry (Shepherd et al., 2012 <sup>[[#fn:r114|114]]</sup> ; The Imbie team, 2018) and InSAR measurements combined with ice sheet SMB estimates (Noël et al., 2018 <sup>[[#fn:r115|115]]</sup> ; Rignot et al., 2019 <sup>[[#fn:r116|116]]</sup> ) . Since 2006, when the array of Argo profiling floats reached near-global coverage, it has been possible to get an accurate estimate of the ocean thermal expansion (down to 2000 m depth) and test the closure of the sea level budget. T he combined analysis of the different observing systems that are available has improved significantly the understanding of the magnitude and relative contributions of the different processes causing sea level change. In particular, important progress has been achieved since AR5 on estimating and understanding the increasing contribution of the ice sheets to SLR.

<div id="section-4-2-2-1global-mean-sea-level-changes-during-the-instrumental-period-block-2"></div>

<span id="tide-gauge-records"></span>
===== 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.

<div id="section-4-2-2-1global-mean-sea-level-changes-during-the-instrumental-period-block-3"></div>

<span id="satellite-altimetry"></span>
===== 4.2.2.1.2 Satellite altimetry =====

High precision satellite altimetry started in October 1992 with the launch of the TOPEX/Poseidon and Jason series of spacecraft. Since then, 11 satellite altimeters have been launched providing nearly global sea level measurements (up to ±82° latitude) over more than 25 years. Six groups (AVISO/CNES, SL_cci/ESA, University of Colorado, CSIRO, NASA/GSFC, NOAA; Nerem et al., 2010; <sup>[[#fn:r138|138]]</sup> Henry et al., 2014 <sup>[[#fn:r139|139]]</sup> ; Leuliette, 2015 <sup>[[#fn:r140|140]]</sup> ; Watson et al., 2015 <sup>[[#fn:r141|141]]</sup> ; Beckley et al., 2017 <sup>[[#fn:r142|142]]</sup> ; Legeais et al., 2018 <sup>[[#fn:r143|143]]</sup> ) provide altimetry-based GMSL time series. Since AR5, several studies using two independent approaches based on tide gauge records (Watson et al., 2015 <sup>[[#fn:r144|144]]</sup> ) and the sea level budget closure (Chen et al., 2017 <sup>[[#fn:r145|145]]</sup> ; Dieng et al., 2017 <sup>[[#fn:r146|146]]</sup> ) identified a drift of 1.5 (0.4–3.4) mm yr <sup>–1</sup> in TOPEX A from January 1993 to February 1999. Accounting for this drift leads to a revised GMSL rate from satellite altimetry of 3.16 (2.79–3.53) for 1993–2015 (WCRP Global Sea Level Budget Group, 2018 <sup>[[#fn:r147|147]]</sup> ; see Table 4.1) compared to 3.3 mm yr <sup>–1</sup> (2.7–3.9) for 1993–2010 in AR5. Compared to AR5, the revised satellite altimetry GMSL estimates now show with ''high confidence'' an acceleration of 0.084 (0.059–0.090) mm yr <sup>–1</sup> over 1993–2015 (5–95% uncertainty range; Watson et al., 2015 <sup>[[#fn:r148|148]]</sup> ; Nerem et al., 2018 <sup>[[#fn:r149|149]]</sup> ) . This acceleration is due to an increase in Greenland mass loss since the 2000s (Chen et al., 2017 <sup>[[#fn:r150|150]]</sup> ; Dieng et al., 2017 <sup>[[#fn:r151|151]]</sup> ) and a slight increase in all other contributions probably partly due to the recovery from the Pinatubo volcanic eruption in 1991 (Fasullo et al., 2016 <sup>[[#fn:r152|152]]</sup> ) and partly due to increased GHG concentrations e.g., (Slangen et al., 2016 <sup>[[#fn:r153|153]]</sup> ; ''high confidence'' ). The current sea level rise is 3.6 ± 0.3 mm yr <sup>–1</sup> over 2006–2015 (90% confidence level). This is the highest rate measured by satellite altimetry (Ablain et al., 2019 <sup>[[#fn:r154|154]]</sup> ; ''medium confidence'' ). Before the satellite altimetry era, the highest rate of sea level rise recorded was reached during the period 1935–1944. It amounted 2.5 ± 0.7 mm yr <sup>–1</sup> (estimate at the 90% confidence level from sea level reconstructions; Church and White, 2011 <sup>[[#fn:r155|155]]</sup> ; Ray and Douglas, 2011 <sup>[[#fn:r156|156]]</sup> ; Jevrejeva et al., 2008 <sup>[[#fn:r157|157]]</sup> ; Hay et al., 2015 <sup>[[#fn:r158|158]]</sup> ; Dangendorf et al., 2017 <sup>[[#fn:r159|159]]</sup> ). This is expected to be smaller than the current rate of sea level rise, making the current sea level rise the highest on instrumental record ( ''medium confidence'' ).

<div id="section-4-2-2-2contributions-to-global-mean-sea-level-change-during-the-instrumental-period"></div>

<span id="contributions-to-global-mean-sea-level-change-during-the-instrumental-period"></span>