Comparison of 20th Century and Late Holocene Rates of Relative Sea Level Rise for the North American Atlantic and Gulf Coasts: The Importance of Glacio-Isostatic Adjustment

Analysis of instrumental and geologic rates of relative sea level (RSL) rise for three geographic areas on the North American continent, demonstrates that striking differences exist in the ratio of present-day rates of RSL rise to the “background rates” of the late Holocene. The three areas used for this analysis (Nova Scotia, Connecticut and the north-central Gulf Coast) have some of the best-resolved late Holocene RSL records anywhere in the world, along with lengthy (>85 years) and continuous tide gauge records. The difference between instrumental and geologic rates of RSL rise is strikingly similar for each study area (1.6 to 2.1 mm/yr); these values correspond closely to the globally averaged rate of eustatic sea-level rise over the last century. Our study proves to be valuable in three ways: (1) it provides additional verification of the occurrence of an acceleration in the rate of RSL rise compared to previous millennia; (2) it demonstrates that the dominant process controlling the late Holocene RSL history for most of eastern North America is glacio-isostatic adjustment (GIA), even for the US Gulf Coast that is relatively distant (intermediate-field) from the centers of glaciation; and (3) it validates the rate of GIA as predicted by geophysical models for the north-central Gulf Coast. Of importance to coastal communities along the north-central Gulf Coast, where background rates of RSL rise are low (0.56 mm/yr), is the observation that the 20th century acceleration in the rate of RSL represents a four-fold increase compared to previous millennia. For Nova Scotia and Connecticut, where the background rates are higher (1.73 and 1.00 mm/yr, respectively), this acceleration amounts to a two- to three-fold increase.