Climate alarmists put forth scary scenarios saying that global warming is causing unprecedented sea level rise and the rise is accelerating. Well, don’t sell your beach-front property yet.
Measuring sea level is more complicated than pounding a stake into a beach. There are daily and seasonal variations, storm surges, and effects from periodic oscillations such as El Niño. And there are tectonic events: is the ocean rising or is the land sinking?
Ideally, global sea level would be a rotating oblate ellipsoid of polar radius of 6365.752 km and equatorial radius of 6378.137 km in absence of any other forces. Gravity, distorts this ideal shape to make it lumpy. Or, as the National Geodetic Survey of NOAA puts it, global mean sea level is “The equipotential surface of the Earth’s gravity field which best fits, in a least squares sense” the ideal geoid.
Since the end of the last glacial epoch, sea level has risen 120 meters (393 feet), about one meter per century. The graph above is a reconstruction of sea level rise since the end of the last glacial epoch. (Source: http://tinyurl.com/co4q82 ).
Larsen and Clark (2006) studied the rate of sea level rise for the past 6,000 years, based on geologic evidence and the historic record. The researchers found that there has been no acceleration of sea level rise in response to increased temperature or CO2 levels.
In another study, Holgate (2007), using data from worldwide coastal tidal gauge records, shows that the rate of sea level rise is decreasing. Specifically, the mean rate of global sea level rise was “larger in the early part of the last century (2.03 ± 0.35 mm/yr 1904-1953), in comparison with the latter part (1.45 ± 0.34 mm/yr 1954-2003).” [NOAA puts normal rise at 1 to 3mm per year, about the thickness of one or two pennies.]
The second graph shows satellite measurements of sea level. Notice there has been no acceleration of rise and that the rate levels off beginning in 2006. But the rate of rise is 3.2±0.4 mm/yr. This rate is higher than Holgate’s 1.45 mm/yr and thus gives the impression that the rate is increasing.
However, it depends where you start looking. Holgate’s study shows that the rate of sea level rise is cyclical. See the third graph below.
It just so happens that the satellite measurements were taken during an upswing of the cycle, thereby giving the false impression that the rate was accelerating. (Unfortunately, Holgate’s graph stops at the year 2000.)
Notice that although the cycles have greater amplitude, the general trend of the rate of sea level rise has been decreasing since the 1950s.
Holgate does not address possible causes of the rate cycle. However, Kolker and Hameed2007), report “a major fraction of the variability and the trend in mean sea level at key sites along the Atlantic Ocean are driven by shifts in the position and intensity of the major atmospheric pressure centers that reside over the Atlantic Ocean, the Azores High and the Icelandic Low,” which they refer to as atmospheric centers of action. Apparent sea level is also affected by variability of storms, winds, floods, waves, shifts in major ocean currents, volcanically-induced ocean heat content variations, the El Niño Southern Oscillation, subsidence, uplift, tectonics, and freshwater fluxes.
These data show that the current rate of sea level rise is neither unprecedented, nor is it accelerating.
Note also, that even if all Arctic and Antarctic sea ice melts, it will have no effect on sea level because floating ice displaces an equal weight of water.
Holgate, S.J. 2007. On the decadal rates of sea level change during the twentieth century. Geophysical Research Letters 34: 10.1029/2006GL028492
Kolker, A.S. and Hameed, S. 2007. Meteorologically driven trends in sea level rise. Geophysical Research Letters 34: 10.1029/2007GL031814
Larsen, C.E. and Clark, I. 2006. A search for scale in sea-level studies. Journal of Coastal Research 22: 788-800.
Graph #1, Post-Glacial Sea Level Rise, created by Robert A. Rohde / Global Warming Art