Salish Sea Ecosystem Conference

A multiple-methods vertical land movement analysis and its integration into probabilistic sea level rise projections for coastal Washington

In order to support climate change planning and adaptation at the community scale, climate projections should ideally be down-scaled, and also provide meaningful representations of uncertainty. As part of the NOAA-funded Washington Coastal Resilience Project, our team developed new sea level projections for Washington State that feature two innovations. First, the projections include an assessment of the likelihood of occurrence of different sea level magnitudes at decadal intervals through 2150. Next, sea level is projected in a relative framework using vertical land movement information. This presentation will describe the development of the most comprehensive database of vertical land movement observations, along with their uncertainties, ever assembled for coastal Washington State. Vertical land movement observations are derived from multiple sources, including 6 different continuous GPS databases, a single-differencing approach using tide-gauge data, and repeat leveling of survey control monuments near highways. The observations were coupled with a tectonic deformation model of the Cascadia Subduction Zone to develop a best-fit surface for all of coastal Washington, along with its associated uncertainty. The best fit surface and its uncertainty was principally guided by the observations, but in locations with sparse data tectonic deformation model dominated the fit of the surface. The results suggest considerable variability in coastal vertical land movement in coastal Washington State. Rates can vary by > 3 mm/yr over spatial scales of only 10s of kms. Uncertainties also vary, ranging from less then 0.5 mm/yr in places with dense observational data, to >2 mm/yr along parts of the coast of Washington and parts of northern Puget Sound. Using a Monte Carlo approach, vertical land movement estimates and their uncertainties are integrated into probabilistic absolute sea level projections. Then, relative sea level projections are derived at high resolution along Washington's coast. These relative projections are also presented in a probabilistic format, and take into account the uncertainty in sea level projections and the uncertainty in the vertical land movement estimates. The variability in vertical land movement translates into spatial differences in projected relative sea level change of ~0.3 m by 2100. Coastal locations in Washington State with the highest rates of uplift are assessed to have, as a best estimate (i.e. median projection), a relative sea level by 2100 of ~0.4 m relative to contemporary sea level. By contrast the best estimate of relative sea level 2100 at locations with land subsidence may exceed ~0.7 m relative to contemporary sea level.