Climate Science Special Report: Sea Level Rise (Invited Presentation)

Historic and possible future changes in global mean sea level (GMSL), local relative sea level (RSL) and associated impacts assessed in the CSSR chapter are presented. Since 1900, GMSL has risen about 16-21 cm with about half of that amount (7 cm) in the last 2.5 decades. Thermal expansion, melt of land-based ice and to a lesser extent changes in land water storage are primarily responsible for the GMSL rise, with ice-melt increasingly the dominant factor. Due to increasing RSL, the annual frequencies of high tide flooding causing minor (nuisance) impacts along U.S. coastlines have increased 5- to 10-fold in the last 50 years. GMSL is very likely to rise by an additional 0.3 to 1.2 meters by 2100 (relative to year 2000) under future potential emissions (RCP2.6, 4.5 and 8.5), though a rise of upwards of 2.5 meters is considered a plausible, though low-probability, outcome under the RCP8.5 emission scenario. Under a range (0.3 to 2.5 m) of GMSL scenarios for 2100, RSL rise along the U.S. coastline will vary from that of GMSL due to changes in the Earth’s gravitational field and rotation from melting of land ice, changes in ocean circulation, and vertical land motion. Under the Intermediate Scenario (1 m GMSL rise by 2100), RSL along the U.S. coasts of the Northeast Atlantic and the western Gulf of Mexico are projected to rise about 30-50% more than the global average, whereas much of Pacific Northwest and Alaska coasts are projected to rise significantly less than the global average. Impacts of future RSL rise will occur well before 2100. For example, with only about 0.35 m of additional RSL rise experienced locally, coastal flooding associated with moderate impacts that today trigger NOAA coastal flood warnings will increase 25-fold by around 2040 (+/- 5 years) under the Intermediate scenario at the majority of 90 U.S. tide gauge locations outside Alaska.