Rising Sea Levels
Sea level have risen in parts of Puget Sound such as Seattle (Figure 4) and are projected to continue to rise (Figure 1). Sea levels in Skagit Bay are not currently being measured and this rpresents a significant data gap.
The implications of rising sea levels in the Skagit region are critical because the Skagit delta is in a low-lying area near sea level. Rising sea levels place pressure on dikes, reduce gravity fed drainage of agricultural lands, increase flood implications for Mt. Vernon and the other small cities, and affect other infrastructure and homes.
The Intergovernmental Panel on Climate Change (IPCC) has published global projections of sea level rise. However translating the global projections of sea levels into local projections for the Puget Sound hinge on multiple local factors, like atmospheric circulation patterns and vertical land movement. For instance in Puget Sound the Olympic Peninsula shorelines are rising whereas the Skagit delta shorelines are sinking.
Converting the IPCC projections for sea level rise by accounting for local factors, estimates for the Puget Sound region range from about 6 inches for the low emissions scenario to about 50 inches for the highest emissions scenario. Due to the fact that the delta is sinking, relative sea level rise may be greater in the Skagit areas than in other parts of Puget Sound.
Sea level rise is relatively greater in the Skagit area than near the Olympic Peninsula because much of Puget Sound is subsiding, or sinking (Figure 3). Scientists believe the Skagit Delta is sinking between 0 to 1 mm per year as a result of deep subsidence, due to shifting in tectonic plates. Since the regional sea level rise estimates assume 1mm/yr of uplift on average across the whole region, a local subsidence rate of 1mm/yr in the Skagit area may shift the range of sea level rise projections upwards by up to 8 inches or more by 2100 for the Skagit area. River deltas with levees also experience shallow subsidence due to soil compaction, decomposition, erosion, and loss of sediment. As these areas sink, they become more vulnerable to flooding from rising groundwater, another consequence of sea level rise.
The science underpinning global sea level rise projections and estimates of local impacts are rapidly evolving, and more recent studies indicate higher rates of global sea level rise than those discussed above.
More information is available in the recent Northwest Science Special Issue articles “Assessing Tidal Marsh Vulnerability to Sea-Level Rise in the Skagit Delta” and “Sensitivity of Circulation in the Skagit River Estuary to Sea Level Rise and Future Flows.”