Is the intensifying wave climate of the U.S. Pacific Northwest of more concern than sea level rise?

Deep water wave-buoy data from the U.S. Pacific Northwest (PNW) document that the annual average significant wave height has increased at a rate of approximately 0.015 m/yr since the mid-1970s, while averages of the five highest significant wave heights per year have increased at the appreciably greater rate of 0.071 m/yr. Histograms of the hourly measured significant wave heights more fully document this shift toward higher values over the decades. While the modes of the histograms have shown little increase, the highest measured waves generated by major storms have increased significantly, enhancing the skewness of the histograms. This developing skewness illustrates the non-stationary nature of the PNW wave climate which translates into substantial increases in extreme-value projections. The buoy data have been analyzed by employing various time-dependent extreme value theory models that directly assess the progressive increases in the 25- to 100-year projections. Model results depend somewhat on the assumptions made in the statistical procedures, on decisions as to the numbers of storm-wave heights included, or the threshold value of the measured significant wave height for inclusion in the analyses, but the results are generally consistent with the linear regressions and shifts in the histograms. A simple total water level model is employed to investigate the relative importance of this intensifying wave climate on the potential for an increased probability of coastal erosion and flooding along the sandy beaches of the PNW. Model results suggest that if the decadal-scale increases in storm intensity continue into the future, this process will have a greater impact on increasing the probability of coastal hazards, via the relationship between wave height and wave runup, than even relatively high estimates of relative sea level rise rates over the next half century. The combined effect of both of these climate controls operating simultaneously is predicted to increase erosion/flood frequency by as much as an order of magnitude for some beach geometries. These results confirm the need to incorporate climate-controlled processes in methodologies designed to assess the risks of enhanced coastal hazards to humans and infrastructure.