APPARENT SEA LEVEL RISE SLOWS WITH AGE OF TIDE GAUGE

The 128 tide gauge records analyzed by Zervas (2009) in his Table 4 suggest that, as the tide gauge ages, the rate of apparent sea level rise decreases. The independent variable is the age of the tide gauge. The dependent variable is the rate of apparent sea level change. The inverse relation between them becomes evident from Table 4 of Zervas (2009). There are 51 gauges in Table 4 going Eastport, Maine, to St Petersburg, Florida, down the US East Coast.

The 51 gauges include three segments, each with exactly 17 gauges. Segment I goes Eastport, Maine, to Atlantic City, New Jersey. Segment II goes Cape May, New Jersey, to Portsmouth, Virginia; Segment III goes Chesapeake Bay Bridge Tunnel to St. Petersburg, Florida. From each Segment identify the two oldest gauges and the two youngest gauges. This selects four from the 17. Match these four with their sea level rises.

In Segment I, the oldest gauges are 151 and 96 years, at sea level trends of 2.77 and 3.99 mm/yr. (The Atlantic City value of 3.99 mm/yr is the subject of a separate paper.) The youngest gauges are 36 and 42 years, at sea level trends of 2.44 and 2.95mm/yr. In Segment II, the oldest gauges are 107 and 105 years, at 2.79 and 3.08mm/yr. In Segment II, the youngest gauges are two gauges of 32 years and one of 33 years, at 5.48 and 4.78mm/yr and 4.97mm/yr. In Segment III, the oldest gauges are 110 and 94yrs at 2.02mm/yr and 2.24mm/yr. In Segment III, the youngest gauges are 32 and 30yrs at 6.05mm/yr and 2.82mm/yr.

A reasonable explanation of the inverse relation between age of the gauge and apparent sea level rise is this: the loaded pile-supported tide gauge is driven more easily into the sand in the years immediately after the gauge is set up, but the rate that the gauge can be driven decreases as sand and sediment consolidates around the piles supporting the tide gauges. I am aware that there will be claims that such events would be uncovered in periodic survey checks. I doubt that this happens.

The Zervas paper is very good. I have never met Mr Zervas. Notice his use of the term ‘linear’ in describing the sea level trends. The results do appear empirically to be linear. For reasons described by Houston and Dean (2011), linear sea level change is not compatible with accelerating sea level rise. For reasons described by Houston and Dean (2011), linear sea level change is not compatible with accelerating sea level rise.