Recent Progress in Understanding and Projecting Regional and Global Mean 1 Sea - Level Change

17 Considerable progress has been made in understanding present and future regional and 18 global sea level in the two years since publication of the Fifth Assessment Report (AR5) of the 19 Intergovernmental Panel on Climate Change. Here we evaluate how the new results affect the 20 AR5’s assessment of (i) historical sea-level rise, including attribution of that rise and 21 implications for the sea-level budget, (ii) projections of the components and of total global mean 22 sea level (GMSL), and (iii) projections of regional variability and emergence of the 23 anthropogenic signal. In each of these cases, new work largely provides additional evidence in 24 support of the AR5 assessment, providing greater confidence in those findings. Recent analyses 25 confirm the 20 century sea-level rise, with some analyses showing a slightly smaller rate before 26 1990 and some a slightly larger value than reported in the AR5. There is now more evidence of 27 an acceleration in the rate of rise. Ongoing ocean heat uptake and associated thermal expansion 28 have continued since 2000, and are consistent with ocean thermal expansion reported in the AR5. 29 A significant amount of heat is being stored deeper in the water column, with a larger rate of heat 30 uptake since 2000 compared to the previous decades and with the largest storage in the Southern 31 Ocean. The first formal detection studies for ocean thermal expansion and glacier mass loss since 32 the AR5 have confirmed the AR5 finding of a significant anthropogenic contribution to sea-level 33 rise over the last 50 years. New projections of glacier loss from two regions suggest smaller 34 contributions to GMSL rise from these regions than in studies assessed by the AR5; additional 35 regional studies are required to further assess whether there are broader implications of these 36 results. Mass loss from the Greenland Ice Sheet, primarily as a result of increased surface 37 melting, and from the Antarctic Ice Sheet, primarily as a result of increased ice discharge, has 38 accelerated. The largest estimates of acceleration in mass loss from the two ice sheets for 200339