Sea Level Trends and Accelerations Based on U.S. and Global Tide Gauges

Rising sea level represents a persistent coastal hazard due to the concentration of infrastructure and population along the World's shorelines. Methods of shoreline stabilization are limited and if sea level accelerates in the early 21^st Century as many suggest, strategies will need to be developed and implemented on an expedited time frame. Here, to lay a foundation for improved understanding of sea level change, we address the question of whether sea level has accelerated over the approximate 20^th Century. Two data groups from 57 U. S. tide gauges with monthly records were selected for analysis by two methods resulting in four combinations of data groups and analysis methods. The criterion for inclusion of individual tide records was a maximum of 5% missing data. One of the data groups extended from 1930 to 2010 and the other from 1910 to 2010. Analysis Method 1 (AM1) comprised a monthly running analysis of the average record of a data group to determine the sea level trend and acceleration, thereby allowing the effect of record length to be examined. AM2 conducted the same analysis on individual records followed by averaging the trends and accelerations. The sea level trend stabilized after approximately 55 years; however, the acceleration continued to oscillate over a small range of the full period analyzed. Overall, the accelerations appear to be very small and negative such that their contribution to sea level rise over a century would be about 1 cm. The results obtained from the two analysis methods and two data groups were consistent, thereby lending credence to the results. Finally, a recent study by Houston and Dean (2011) of global accelerations over the 20^th Century found similar results.