Executive Summary

This chapter considers changes in global mean sea level, regional sea level, sea level extremes, and waves. Confidence in projections of global mean sea level rise has increased since the Fourth Assessment Report (AR4) because of the improved physical understanding of the components of sea level, the improved agreement of process-based models with observations, and the inclusion of ice-sheet dynamical changes.

Past Sea Level Change

Paleo sea level records from warm periods during the last 3 million years indicate that global mean sea level has exceeded 5 m above present (very high confidene) when global mean temperature was up to 2°C warmer than pre-industrial (medium confidence). There is very high confidence that maximum global mean sea level during the last interglacial period (~129 to 116 ka) was, for several thousand years, at least 5 m higher than present and high confidence that it did not exceed 10 m above present, implying substantial contributions from the Greenland and Antarctic ice sheets. This change in sea level occurred in the context of different orbital forcing and with high latitude surface temperature, averaged over several thousand years, at least 2°C warmer than present (high confidence) {5.3.4, 5.6.1, 5.6.2,13.2.1}

Proxy and instrumental sea level data indicate a transition in the late 19th century to the early 20th century from relatively low mean rates of rise over the previous two millennia to higher rates of rise (high confidence).