Probabilistic Projections of Future Sea-Level Change and Their Implications for Flood Risk Management: Insights from the American Climate Prospectus

Abstract:

Global mean sea level (GMSL) rise projections are insufficient for adaptation planning; local decisions require local projections that characterize risk over a range of timeframes and tolerances. We present a global set of local sea level (LSL) projections to inform decisions on timescales ranging from the coming decades through the 22nd century. We present complete probability distributions, informed by a combination of expert community assessment, expert elicitation, and process modeling [1]. We illustrate the application of this framework by estimating the joint distribution of future sea-level change and coastal flooding, and associated economic costs [1,2].

In much of the world in the current century, differences in median LSL projections are due primarily to varying levels of non-climatic uplift or subsidence. In the 22nd century and in the high-end tails, larger ice sheet contributions, particularly from the Antarctic ice sheet (AIS), contribute significantly to site-to-site differences. Uncertainty in GMSL and most LSL projections is dominated by the uncertain AIS component.

Sea-level rise dramatically reshapes flood risk. For example, at the New York City (Battery) tide gauge, our projections indicate a \emph{likely} (67% probability) 21st century LSL rise under RCP 8.5 of 65--129 cm (1-in-20 chance of exceeding 154 cm). Convolving the distribution of projected sea-level rise with the extreme value distribution of flood return periods indicates that this rise will cause the current 1.80 m `1-in-100 year' flood event to occur an expected nine times over the 21st century -- equivalent to the expected number of `1-in-11 year' floods in the absence of sea-level change.

Projected sea-level rise for 2100 under RCP 8.5 would likely place $80-$160 billion of current property in New York below the high tide line, with a 1-in-20 chance of losses >$190 billion. Even without accounting for potential changes in storms themselves, it would likely increase average annual storm damage by $2.6-$5.2 billion (1-in-20 chance of >$7 billion). Projected increases in tropical cyclone intensity would further increase damages [2].

References: [1] R. E. Kopp et al. (2014), Earth's Future, doi:10.1002/2014EF000239. [2] T. Houser et al. (2014), American Climate Prospectus, www.climateprospectus.org.