Department of Defense (DoD) installations on Pacific islands face multiple climate-related threats, including tropical cyclones, droughts, floods, and increased rates of sea-level rise. Understanding and preparing for how future climate change may contribute to changes in the likelihood of extreme events, loss of coastal land including protective landforms, and changes in freshwater resources are significant challenges facing DoD planners. With this work the project team provides probabilistic information on potential climate-related threats that might arise from changes in water availability, sea-level rise, and changes in tropical cyclone activity for DoD bases across the Pacific over the 21st century.The objective of this project is to provide probabilistic information on potential climate-related threats for DoD installations across the Pacific over the next century, including those that might arise from hydrological changes, sea-level change, and changes in tropical cyclone activity.

The project involves a hierarchical framework of modeling whereby each successive scale will provide constraints for the next; i.e., from state-of-the-art global climate models to regional tropical cyclone downscaling to surge/wave/morphodynamic models. Field programs allowed the project team to examine the geological and geomorphic setting of sites targeted for morphodynamic modeling and in the case of reef islands in the Marshall Islands to reconstruct the evolution of these systems. Quantitative future projections of climate impacts in the Pacific sector (e.g., sea-level rise, freshwater availability, extratropical storminess, etc.) were developed using a suite of fully-coupled global climate models (GCMs) and Earth System Models (ESMs) for emission scenarios representative concentration pathway (RCP) 4.5 and RCP 8.5 from the Coupled Model Intercomparison Project, phase 5 (CMIP5). The Project team used a subset of these simulations to model future tropical cyclone scenarios using the downscaling approach of Emanuel et al. [2006]. The combined results allowed them to examine potential changes in precipitation, ocean circulation, wave climate, and extreme tropical cyclone-induced events using a hierarchy of hydrodynamic models.

In order to assess the potential impacts of these changes on the coastal landforms of particularly vulnerable DoD assets, the project team used the output from the hydrodynamic simulations to drive a series of geomorphic models for the Ronald Reagan Ballistic Missile Defense Test Site, Kwajalein Atoll, and Marine Corp Base Hawaii, Barking Sands, Kaua’i. Geomorphic modeling provides information on changes in sediment transport that will point to which areas might experience net erosion or accretion in response to future changes in wind-driven wave climates.

The analysis reveals that under the influence of the anthropogenic rise in greenhouse gases, tropical cyclone activity exhibit robust increasing trends over much of the North Pacific, especially over the central subtropical Pacific region. Increases in tropical cyclone activity under global warming in the simulations is mostly attributable to the reduced vertical wind shear and greater potential intensity (primarily due to higher sea surface temperature). Significant regional differences are projected in tropical cyclone activity, with the Hawaiian Islands of Oahu and Kaua’i projected to experience the greatest rise in activity. Conversely, Kwajalein is projected to experience more modest increases in activity by the end of the century. Hydrodynamic modeling of tropical cyclone-induced inundation at Kwajalein from downscaled results from three climate models yields varying return interval estimates ranging from modest reductions in the return period for the 100-year flood to a 53% increase. However, dramatic increases in the probability of tropical cyclone induced flooding are expected as a result of sea-level rise. For example, increases in sea-level over the next century will cause the return period of 100-year events to increase significantly, even in the scenarios where models project modest reductions in the 100-year flood based solely on changes in tropical cyclone climate.

Relatively modest changes in non-tropical cyclone wind fields and waves are expected over the coming century as a response to anthropogenic climate change. For example, at Kwajalein GCM ensemble forecasts of wave climate predict a 15% reduction in wave energy by 2100 CE relative to 1976-2005, corresponding to an 8% reduction in average significant wave height. The direction and magnitude of shoreline response to sea-level rise and wave climate changes remain poorly constrained, and depend on adjustments to local sediment budgets and the frequency of extreme wave events. Increasing cyclone frequency would increase beach mobilization even more. Increased frequency of cyclone landfalls on Kwajalein Atoll's islands would increase geomorphological activity, and could result in extreme erosion or reworking of unprotected shorelines, especially as Kwajalein Atoll islands lack the protective conglomerate platforms and rubble platforms found on atolls in regions with greater tropical cyclone activity.

The results indicate that the mean wave climate is the predominant driver of reef island evolution and when coupled with predicted rates of sea-level rise, can help predict future responses of reef island to climate change. Results from the Reef Island Overwash Model the project team developed indicate that as sea-level rise rates increase, reef islands will tend to narrow, which enhances sediment transport from the ocean to the lagoon side via storm overwash. This process results in a net island lagoonward migration, which, as model results demonstrate, can be largely enhanced by a reduction in offshore sediment supply to the island. As islands approach the reef/lagoon edge, a larger amount of sediment could be potentially lost into the lagoon, enhancing the rate of island degradation. The project team concludes that passive flooding alone underestimates the rate at which reef islands will deteriorate in future decades.

The analysis of potential changes in freshwater stress indicates that significant changes are not projected for a number of islands with substantial DoD assets (Kaua’i, Oahu, Kwajalein, Guam, Okinawa) by 2050. However, by the end of the 21st century aridification is projected in Okinawa and a modest increase in water availability is projected for Guam. Little change in water availability is projected in Kaua’i, Oahu, and Kwajalein.

The results provide detailed projections on potential future climate related stresses at DoD installations across the Pacific. These results can be used for planning adaptation strategies to make DoD assets more resilient to these threats and provide motivation for developing alternatives if vulnerable assets are likely to lack resilience.