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Tide gauges   
HISTORICAL TIDE GAUGE MEASUREMENTS
Traditionally, global sea level change has been estimated from tide gauge measurements collected over the last century. Tide gauges, usually placed on piers, measure the sea level relative to a nearby geodetic benchmark. Shown in Figure 1 is the most commonly used tide gaugeTide Gauge Measurements measurement system, a float operating in a stilling well. Surveys of the tide gauge site are performed regularly to account for any settling of the site. Tide gauges may also move vertically with the region as a result of post-glacial rebound, tectonic uplift or crustal subsidence. This greatly complicates the problem of determining global sea level change from tide gauge data. Differences in global sea level estimates from tide gauge data usually reflect the investigator's approach in considering these vertical crustal movements. Tide gauges also monitor meteorological factors that affect sea levels, suchBench Mark as barometric pressure and windspeed, so that these variable factors can be eliminated from long-term assessments of sea level change. Although the global network of tide gauges comprises of a poorly distributed sea level measurement system, it offers the only source of historical, precise, long-term sea level data. Major conclusions from tide gauge data have been that global sea level has risen approximately 10-25 cm during the past century.


TIDE GAUGE ESTIMATES OF GLOBAL SEA LEVEL RISE

Estimates of global sea level rise which were derived from tide gauge records are found in Table 1. Most of the investigators reported that the estimated values were sensitive to the choice of record length and the tide gauges selected. This sensitivity coupled with different computationa techniques and modeling would certainly explain some of the differences shown below.

Table 1: Estimates of Global Sea Level Rise from Tide Gauge Records
Sea Level
Rise (mm/yr)		 Error
(mm/yr)		 Data Used
(years)		 # of Tide Gauge Stations References
      1.43     ±0.14 1881-1980 152 Barnett, 1984
      2.27     ±0.23 1930-1980 152 Barnett, 1984
      1.2     ±0.3 1880-1982 130 Gornitz & Lebedeff, 1987
      2.4     ±0.9 1920-1970 40 Peltier & Tushingham, 1989
      1.75     ±0.13 1900-1979 84 Trupin & Wahr, 1990
      1.7     ±0.5 N/A N/A Nakiboglu & Lambeck, 1991
      1.8     ±0.1 1880-1980 21 Douglas, 1991
      1.62     ±0.38 1807-1988 213 Unal & Ghil, 1995