RECOVERING AND REANALYZING 19TH CENTURY SEA-LEVEL AND TIDAL MEASUREMENTS (Invited Presentation)

Tide data are the oldest and longest oceanographic records and therefore comprise one of few tools for understanding, quantifying, and separating century-scale human and climate impacts on the coastal zone. However, though continuous measurements of tides began as early as 1844 in North America and had been collected at more than 25 stations by the year 1900, only a few (New York and San Francisco) are analyzed for sea-level back to the mid-19^th century and most are digitally available only as 20^th century data. In this presentation we describe historical tide measurements in North America, their current status, their possible uses, and ongoing efforts to recover the data from the national archives, focusing in particular on San Diego (1853-1872) and Astoria (1853-1876) data. After digitizing historical tabulations or marigrams (the original, analog tidal trace) we apply ancillary data such as benchmarks, historical leveling, and historical gauge checks to assess quality and estimate sea-level. Thus, recovering the historical data may help address the current controversy about the start of mean-sea-level acceleration, and may provide a rich comparison to sea-level estimates made by bio-geochemical methods. The tide data also show how tidal dynamics have changed, particularly in tidal rivers; for example, the primary M2 constituent has increased by 10% in the Columbia River estuary, and has more than doubled in some upstream environments. The old tide data may therefore help make sea-level reconstruction more accurate by providing a better estimate of tidal statistics such as mean-high water.