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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.

Sea-level rise (SLR) presents risks to communities and ecosystems because of hazards like coastal erosion. In order to adapt, planners and the public seek estimates of shoreline change with high confidence and accuracy. The complexity of shorelines produces considerable uncertainty in the timing, location and magnitude of change. We present and discuss a probabilistic shoreline model for SLR planning. Using the coast of Maui as an illustrative case, we compare this model to a common deterministic model. We discuss the advantages of a probability-based model for SLR adaptation, including for prioritizing actions, phasing, visualizing risk and uncertainty, and improving adaptive management.

Waikiki is an internationally recognized tourist destination and a major source of economic activity in Hawaii. Erosion is a constant threat to Waikiki's beaches and coastal properties. This applied study presents an assessment and comparison of three erosion management approaches – beach nourishment, armoring and managed retreat – in the context of Waikiki. A geographic information system (GIS) is used to project erosion rates to 2050 and 2100, while approximating effects of sea level rise (SLR). The spatial extent of erosion and costs of each management approach are estimated. Suitability of each approach is evaluated based on management impacts to economy, environment, recreation, storm protection, and resources. This framework provides a comparative analysis that can be replicated in similar settings. The findings indicate that nourishment may be a beneficial and economical approach to manage erosion in this densely developed, economically significant beach setting. Nourishment costs are on the same order of magnitude as armoring, while retreat costs are an order of magnitude higher. The larger indirect impacts and economic losses associated with armoring (beach loss) and retreat (property loss) make nourishment the least impactful option for addressing erosion in Waikiki. As the projections and understanding of climate change impacts improve, solutions may evolve. Nourishment may provide a no-regrets, suitable approach for managing erosion in Waikiki until other solutions emerge. This paper presents opportunities and implications of nourishment for planning and future research in Waikiki.

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.

Landscape change associated with urbanization poses major challenges to aquatic ecosystems. Extensive studies have shown that the composition of land cover within a watershed can account for much of the variability in water quality and stream ecological conditions. While several studies have addressed the relationship between watershed urbanization and biotic integrity in streams, few have directly addressed the question of how urban patterns influence ecological conditions. These studies typically correlate changes in ecological conditions with simple aggregated measures of urbanization (e.g., human population density or percent impervious surface). We develop an empirical study of the impact of urban development patterns on stream ecological conditions in 42 sub-basins in the Puget Sound lowland region on a gradient of urbanization. We hypothesize that ecological conditions in urbanizing landscapes are influenced through biophysical changes by four urban pattern variables: land use intensity, land cover composition, landscape configuration, and connectivity of the impervious area. Using community measures of benthic macroinvertebrates as indicators of in-stream biotic integrity we examined the relationships between urban development patterns and ecological conditions in these basins. Significant statistical relationships were found between landscape patterns—both amount and configuration of impervious area and forest land—and biotic integrity of streams suggesting that patterns of urban development matter to aquatic ecosystems.