153.7.6 Conclusions

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It is virtually certain that globally averaged sea level has risen over the 20th century, with a very likely mean rate between 1900 and 2010 of 1.7 [1.5 to 1.9] mm yr –1 and 3.2 [2.8 and 3.6] mm yr –1 between 1993 and 2010. This assessment is based on high agreement among multiple studies using different methods, and from independent observing systems (tide gauges and altimetry) since 1993. It is likely that a rate comparable to that since 1993 occurred between 1920 and 1950, possibly due to a multi-decadal climate variation, as individual tide gauges around the world and all reconstructions of GMSL show increased rates of sea level rise during this period. Although local vertical land motion can cause even larger rates of sea level rise (or fall) relative to the coastline, it is very likely that this does not affect the estimates of the global average rate, based on multiple estimations of the average with and without VLM corrections.

It is virtually certain that interannual and decadal changes in the large-scale winds and ocean circulation can cause significantly higher or lower rates over shorter periods at individual locations, as this has been observed in tide gauge records around the world. Warming of the upper 700 m of the ocean has very likely contributed an average of 0.6 [0.4 to 0.8] mm yr –1 of sea level change since 1971. Warming between 700 m and 2000 m has likely been contributing an additional 0.1 mm yr –1 [0 to 0.2] of sea level rise since 1971, and warming below 2000 m likely has been contributing another 0.1 [0.0 to 0.2] mm yr –1 of sea level rise since the early 1990s.

It is very likely that the rate of mean sea level rise along Northern European coastlines has accelerated since the early 1800s and that this has continued through the 20th century, as the increased rate since 1875 has been observed in multiple long tide gauge records and by different groups using different analysis techniques. It is likely that sea level rise throughout the NH has also accelerated since 1850, as this is also observed in a smaller number of gauges along the coast of North America. Two of the three time series based on reconstructing GMSL from tide gauge data back to 1900 or earlier indicate a significant positive acceleration, while one does not. The range is –0.002 to 0.019 mm yr –2 , so it is likely that GMSL has accelerated since 1900. Finally, it is likely that extreme sea levels have increased since 1970, largely as a result of the rise in mean sea level.

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