Impacts of Rising Seas on the British Columbia Coast in the 21st Century

Changes in the level of the sea relative to the land on the northwest coast of North America occur on timescales differing by ten orders of magnitude, from a few minutes to thousands of years. Daily and seasonal tidal changes override all other factors; the tidal range in this region is 2-7 m. Non-tidal fluctuations, however, are also important and include: 1) subsidence and uplift of the coastline during large earthquakes, 2) ephemeral changes in sea level associated with storm surges, 3) seasonal and decadal variations in sea level related to El Niño-Southern Oscillation (ENSO) events originating in the equatorial Pacific and to fluctuations in atmospheric pressure, 4) global (eustatic) sea-level changes related to glacier volume changes, 5) changes in ocean water temperature and salinity (steric effects), 6) variations in the storage of water in reservoirs on land, and 7) long-term, slow changes in relative sea level associated with tectonism, sedimentation, and residual isostatic adjustment of Earth’s crust following the retreat of the last continental ice sheet in northwest North America. The most important non-tidal factors, from a societal perspective, are those linked to climate. Sea level on the west coast of North America is expected to rise between about 60 cm and 1.4 m by the end of the century due to warming of upper ocean waters and additions of meltwater from alpine and arctic glaciers, Antarctica, and Greenland. Towards the end of the century, the main sources of meltwater will be melting and calving of the Antarctic and Greenland ice sheets. The magnitude of sea-level rise will differ geographically along the coast. However, in the likelihood that eustatic sea level rise increases from its current rate of a 2-3 mm yr^-1 to about 1 cm yr^-1 by the end of the century, no part of the Pacific coast will be unaffected. Erosion and flooding of low-lying coastal areas can be expected during rare extreme storms, especially when wind-driven ocean surges coincide with high tides or with strong El Niño events. Flooding and erosion may be exacerbated by changes in storminess in the warmer climate that is anticipated later in the century. Unfortunately, sections of the coast of northwest North America that are most vulnerable to flooding and erosion are those that support the largest human populations and infrastructure, specifically the southern and central Strait of Georgia, Juan de Fuca Strait, and the Pacific coast of Washington. In contrast, sparsely populated, steep rocky shorelines that constitute most of the British Columbia coast will be much less affected.