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Coastal land use in the USA is regulated by a chain of integrated federal to local policies that emphasize preservation of open space, public access, and the environment. Are these policies prepared to achieve lawfully stated objectives under accelerating sea level rise? To test the efficacy of these policies during the past century of slowly rising sea level, and thus their potential effectiveness under future accelerated sea level rise, we quantify land use on a section of windward Oʻahu coast, 1928–2015. Data show a shift from stable/accreting shorelines and wide beaches, to expanding erosion and beach loss concurrent with increasing development and seawall construction; trends at odds with policy objectives. Shoreline hardening increased 63%. Net shoreline change shifted from stable/accreting to erosional on 74% of the coast. More than 45% of this shift was due to flanking (erosion triggered by nearby hardening). Five geomorphic settings were analyzed. Prior to local coastal policy, from 1928 to 1975, headland beaches were mildly erosional with average change rates − 0.07 ± 0.1 m/yr. Beaches in all other geomorphic categories accreted. Between 1975 and 2015, following enactment of coastal zone management policy, average change in all geomorphologies shifted to erosional, coastal development and seawall construction expanded, average beach width declined. Today, nearly 20% of beach length has been lost; 55% of beaches have narrowed. Failure to achieve policy goals under slow, historic sea level rise (1.2 to 1.4 mm/yr) implies preserving beaches, open space, and public access will require new policies, or more effective ways for implementing existing policies, in a future characterized by accelerated sea level rise.

The ongoing emission of greenhouse gases (GHGs) is triggering changes in many climate hazards that can impact humanity. We found traceable evidence for 467 pathways by which human health, water, food, economy, infrastructure and security have been recently impacted by climate hazards such as warming, heatwaves, precipitation, drought, floods, fires, storms, sea-level rise and changes in natural land cover and ocean chemistry. By 2100, the world’s population will be exposed concurrently to the equivalent of the largest magnitude in one of these hazards if emmisions are aggressively reduced, or three if they are not, with some tropical coastal areas facing up to six simultaneous hazards. These findings highlight the fact that GHG emissions pose a broad threat to humanity by intensifying multiple hazards to which humanity is vulnerable.

Groundwater elevations in coastal cities will be affected by climate-change-induced sea level rise (SLR) and wastewater collection systems will experience increased groundwater infiltration (GWI) due to greater submergence of sewer pipes. Commercial sewer hydraulics models consider GWI to be a constant quantity estimated via a low-flow monitoring campaign and are incapable of predicting future flows due to changes in GW elevations. A global sensitivity analyses conducted for a two-dimensional GWI pipe flow model found the most important input parameters are groundwater head and surrounding soil hydraulic conductivity. Two case studies were conducted considering a range of pipe defect severity to estimate increases in GWI associated with predictions of future SLR. The findings are that SLR will begin to have noticeable impacts in terms of increased average dry weather flow (ADWF) as soon as 2030 (3%&ndash, 10%) and will increase dramatically in the future (10%&ndash, 29% by 2050, and 50% or more by 2100). Daily and seasonal tide ranges affect the normal diurnal flow variations by between 3% and 10%. The estimation methodology and case studies described here illustrate the coming future importance of SLR effects on GWI in coastal collection systems that should be included in facilities planning and design.