Data Annex: Climate Data - Sea Level Rise

Sea level is a function of numerous climatic and non-climatic factors, including ocean thermal expansion, melting from glaciers and ice sheets, land uplift, and groundwater depletion, among others. There is significant uncertainty regarding the extent of future sea level rise in particular locations and for particular decades.

Range of projections: The IPCC’s Fifth Assessment Report provides scenarios of global mean sea level rise that are below 1 meter by 2100; however, there is significant evidence for greater increases in sea level in the literature.⁴ The IPCC’s assessment relies on process-based projections and does not incorporate semi-empirical projections because of low confidence levels in the results. However, these semi-empirical models project greater rises in sea level, with an upper bound sometimes exceeding 1.5 meters by 2100.5 The U.S. National Climate Assessment projects rising of up to 2 meters in global mean sea level over this period, and the IPCC cites reports with estimates of up to 2.4 meters.⁶

Given the range of these sea level rise projections, project teams using this tool should apply the upper end of the estimates to provide a conservative screen for project managers.

Calculation of accelerating rise: A default Future time frame in the screening tool is 2040-2059, since many project lifetimes will not reach the end of the century. Sea level rise is not projected as a constant increase over time; rather, the rate accelerates from the current ~3 mm/year to up to roughly 15 mm/year by the end of the century.⁷

Note: To estimate an upper end of global mean sea level rise in the middle of the century, a quadratic relationship is used to calculate change from 1992 (the starting point) to 2050: E(t) = 0.0017t + bt² where E is global mean sea level rise in meters, t is the number of years since 1992, and b is a constant that ranges from 0 (lowest scenario) to 1.56 x 10^-4 (highest scenario).⁸ This equation yields an upper-end estimate of a rise of 0.62 meters in sea level by 2050, which is rounded to 0.6 meters for the sake of the tool.

Local vs. global rates: Caution should be used with such thresholds because local sea level change can vary significantly from global averages. For example, local sea level can be strongly influenced by local factors such as uplift or subsidence of the land surface, erosion (i.e., sediment removal), and accretion (i.e., sediment addition) in the project area. Rates of current local sea level rise data can be viewed in a “Tides and Currents” web page from the U.S. National Oceanic and Atmospheric Administration (NOAA). It is likely that these rates will increase in the future. They therefore generally represent a minimum rate of rise (or a maximum level of decrease in areas of rapid subsidence).

Upper bounds: An appropriate upper bound for a rate of sea level rise depends on the project lifetime. For projects with short lifetimes of 10-20 years, the rate of sea level change will resemble the Historical/Current rate. However, as mentioned above, the rate of global mean sea level rise is projected to increase over time. For projects with lifetimes that stretch beyond mid century, a maximum sea level rise rate of 15 mm/year can be applied over the project’s service life.⁹ Again, this is a conservative screen focused on the upper bound of projections.

As an example, if a project has an expected service life of 70 years, the maximum sea level rise that could be expected is roughly 1.05 meters x (0.015m/yr * 70 yrs).