Propagating uncertainty in future sea levels increases anticipated coastal adaptation costs

Global sea-level rise and hazards from extreme flooding pose severe risks to millions of people around the world who live in coastal zones. Common strategies to manage these risks include adapting existing infrastructure or retreating further inland or to higher ground. Models for adaptation and impacts from future coastal hazards offer an important computational laboratory in which to examine the efficacy of these risk management strategies. These models need to be flexible and modular so that they may be coupled with other geophysical and socioeconomic models to examine how uncertainties propagate to affect coastal impacts. Here, we present an open-source and modular Coastal Impacts and Adaptation Model and demonstrate the impacts of socioeconomic and geophysical parametric uncertainties on projected coastal adaptation costs over the next century. We find that when uncertainty in future sea-level rise is characterized by quantiles representing low, moderate, and high global mean sea-level rise, high-end net present value of global least-cost adaptation costs over the 2010-2150 period total US$5.6 trillion (95th percentile). However, when we use a full ensemble of projections to characterize sea-level rise and socioeconomic model parametric uncertainties, these high-end global risks increase to US$7.4 trillion, a 31% increase. Similarly, propagating uncertainty via the full ensemble increases the moderate risk estimate from US$4.1 trillion to US$4.7 trillion (median; 15% increase). Further, we find that adaptation measures reduce losses of property and lives from flooding by over US$1 trillion annually in 2100, even in a low-emissions scenario. This work underscores the need for adaptive measures to protect property and peoples around the worlds coasts, and demonstrates that such action is the economically prudent strategy.