Sea Level Rise on the West Coast

Sea level rise on the west coast is, deservedly, a topic that is getting a lot of attention these days, especially from those responsible for big coastal cities like Seattle and Vancouver BC. Also not surprisingly there are researchers focused on developing projections of sea level rise on a global scale and regional scale, who publish papers on their results. Then, every few years, some big multi-researcher effort will come along that takes all of the research and all of the papers and attempts to come up with a "we-all-agree-on-this" estimate that can be taken as representing the best available collective science. The Intergovernmental Panel on Climate Change is one of those massive global-scale consolidators of sea-level projections derived from researchers working all over the world. The Fourth Assessment Report, published in 2007, included projections of sea level rise (on the order of UP TO 60 cm by 2100) that, based on research published in the late 2000's, seemed too low. In fact, I draw exactly that conclusion in an assessment currently in development for the Olympic Coast National Marine Sanctuary.

Now, additional evidence to support that conclusion comes with the publication of the National Academy of Sciences report on sea level rise for the coasts of California, Oregon and Washington. In it, the cast of researchers involved (and the list is long), agree that sea level rise by 2100 may be more extreme than previously thought (on the order of UP TO 1.4 m by 2100). Here is Dr. Gary Griggs (who just happens to be my academic adviser from UC Santa Cruz) summarizing the conclusions:

These new projections are invaluable, because they incorporate the research from the end of the 2000's that suggests that the IPCC projections are too low. The one thing that I think is potentially of concern for those of us working in the Pacific Northwest, though, is the focus on deriving relative sea level estimates (they call them "regional" sea level rise projections in the report). Relative sea level takes the change in sea level due to ocean temperature changes, expansion of sea water, and added ocean water volume due to ice melt, and incorporates "vertical land movement" to come up with an estimate of sea level change relative to the elevation of the land. This is important, since "relative" sea level is what matters for communities planning for a changing sea level. For example, even if "eustatic" sea level (the "true" change in the level of the sea away from land) rises 1.0 m in 100 years, the relative sea level will be much greater in a place like New Orleans, where the land is subsiding, than in a place like Kodiak, Alaska, where the land is rising.

The projections presented by Gary in the summary, which are also featured in the executive summary and elsewhere in the report, are relative and attempt to take vertical land movement into account. This is conceptually okay, but there is an assumption in Dr. Griggs' summary that the entire coast north of Cape Mendicino is uplifting at about the same rate. The evidence suggests, though, that there is quite a bit of variability in rates of vertical land movement around the Pacific Northwest (see also this reference on the topic). In other words, the relative sea level projections offered by the NAS should be used cautiously in the Pacific Northwest since, for a particular community, they may really underestimate the likely sea level over the next 100 years. Instead, we need to take their updated projections of eustatic sea level rise and combine them with community-scale estimates of vertical land movement to really come up with useful projections for cities and communities to work with.

Regardless of that small oversight on the part of the NAS, Gary's final conclusion is dead-on: "Coastal communities need to begin to understand these processes and what sea-level rise indicates for them, and plan accordingly"