2. Background

#### 2.1 Sea level changes

Mean sea level (MSL) is increasing nearly everywhere on Earth, with a globallyaveraged rise of +1.7 0.2 mm year<sup>1</sup> as estimated from coastal and island tide gauge measurements from 1900 to 2009 [2, 12, 13], and at a rate +3.4 0.4 mm year<sup>1</sup> for 1993–2016 as estimated from satellite altimetry (http://sealevel.colorado.edu/; [14]). In the twentieth century, the most rapid increase in MSL over the last three millennia has been observed, based on a semi-empirical estimate of sea-level rise [3], finding that without global warming, the observed increases in global sea levels would have been much less. Furthermore, since 1970, global mean sea level rise has been dominated by anthropogenic forcing [15]. Some climate models predict that MSL rates will accelerate in future decades via global climate change mechanisms [13] such as ice sheet melt and thermosteric MSL rise; both of these are induced by upper-ocean warming [4, 12, 16–19].

However, there is a wide spatial variability to these rates [3, 20], attributed to the combined effects of spatially variable wind and warming, and different vertical rates of land subsidence. In the Western Pacific, MSL rise often is larger than +10 mm year<sup>1</sup> in some locations, whereas Eastern Pacific rates are near zero or sometimes slightly negative because of tectonic and weather factors [21]. The anomalously rapid sea level rise observed in the western Pacific tends to be underestimated in many models. This may be because of low variability in tropical zonal wind stress [22]. However, the extreme rate in the Western tropical Pacific is unlikely to persist unabated [23], and a reversal of this Pacific asymmetry may be imminent soon.
