330 Years of Sea Level
By Ed Caryl
Sea level data suffer from the same problem as temperature and other climate measures; there isn’t enough of it. Satellite sea level data only goes back twenty years, even less than the satellite temperature data. Fortunately, because the West was civilized by seafaring nations, we have some tide gauge records back into the 19th century for locations in Europe and a few locations around the world. These go back far enough that the chief source of error becomes not measurement accuracy or care in record keeping, but Post-Glacial Rebound (PGR) or Glacial Isostatic Adjustment (GIA).
As described in Rebound, my last article, PGR for the Nordic countries, and other ports around the Baltic Sea, make sea level measurements in those ports a measure of PGR, not sea level. Fortunately, accurate GPS PGR measurements have been made for nearly all the tide gauges in the world, and these measurements can be used to correct the tide gauge sea level measurements.
Fifteen tide gauge records from around the world were downloaded from PSMSL Explorer, and one record from Amsterdam NAP. These were selected based on quality: (no quality red flags), length (over 100 years preferred), good continuity, low PGR, and low tectonic activity in the area. These were normalized to their average level in the period from 1960 to 1980, then corrected for PGR. The result is Figure 1.
Figure 1 is the normalized data from 16 quality long record tide gauges.
Figure 2 below is the average of the tide gauge data in Figure 1.
This is the average of 15 Mean Sea Level (MSL) records since 1958, 11 from 1948 to 1958, at least 10 since 1900, at least 7 from 1885, at least 5 since 1864, 4 since 1849, and 2 for most of the period since 1807.
Figure 3 is a chart from climatedata.info. This is presented as a validation of figure 2.
We see an indication here that sea level has been rising since about 1855 to 1860. Before that, sea level was flat or falling. Here is a chart showing trends in thirty-year intervals.
Figure 4 is chart of sea level trends since 1810 using the averaged data from figure 2.
The sea level trend was nearly flat in the whole of the 1800’s, trending up 0.4 mm/year in the first half of the century, then up by 0.9 mm/year in the last half, as the Little Ice Age ended in mid-century. In the whole of the 20th century the trend was up by less than 1.7 mm/year. All together, there has been a sea level rise of about 25 cm in the last 150 years, or about 10 inches. There the trend slowed during the cool 1950 to 1980 period, then increased to 2.25 mm/year in the warming years late in the century. The satellite record begins in the bottom of the cooling years of the Pinatubo volcanic eruption. That is why that record, combined with the artificial GIA/PGA 0.3 mm/year “correction” is listed in U of Colorado satellite data as having a 3.2 ± 0.4 mm/year trend.
Figure 5a, the sea level trend since 1890. Figure 5b, the sea level rise in the last 33 years.
The recent upward tick in 2012 and 2013 due to rebound from the 2011 La Niña is just two years long, and is not nearly as dramatic as several previous short increases and decreases in the record. These are all due to short periods of warming and cooling associated with El Niños, La Niñas, and volcanic events. The 1983 and 1992 El Chichon and Pinatubo volcanic cooling events are particularly obvious in Figure 5b.
We do have one good sea level record from before 1800. The Dutch have been very concerned about sea level for a very long time, as a third of their country is below it.
Figure 6 is a Huddestenen, named for Amsterdam Mayor Hudde, a marble block set in 1684, 2.67 meters above Amsterdam level.
After a severe flood in 1675, the mayor of Amsterdam decided that the only way to make sure dikes were high enough to prevent flooding was to know precisely where the tops of the dikes were relative to the normal high water mark. To that end, during the year 1683 to 1684, September to September, daily records were kept of the tide, and an average high water mark was calculated. Since that time records have been maintained and an almost continuous sea level record exists from 1700 to 1925. This record is called the “Normaal Amsterdams Peil” (NAP) or the Amsterdam Ordinance Datum sea level record. Here is that record, corrected for PGR (or GIA), normalized to and plotted with the average from Figure 2.
Figure 7 is 330 years of measured sea level data.
The sea level rise over the last 200 years, from 1807, is no more than 27 cm, or about 11 inches. In the 125 years before that, there was no rise at all.
So…what will happen in the future? If the cooling predicted by the adherents to the solar climate driver hypothesis comes to pass, then in the next thirty years we will get a hesitation in sea level rise similar to the 1950 to 1980 period. Much of the easily melted ice stored during the Little Ice Age has melted already, so only the large ice reservoirs, Greenland and Antarctica, can contribute to sea level rise. Most of the rise from pumping aquifers and draining land-locked lakes has reached a limit. Thermosteric rise due to ocean heating has reached a limit due to increased evaporation from tropical seas. If all these are taken into account, sea level rise in the remainder of the the 21st century cannot exceed that in the last century, or about 1.7 mm/year, and perhaps less. This indicates a sea level rise by the year 2100 of less than 15 centimeters, or less than six inches.
In areas with glacial rebound, sea level rise will not be noticed at all. In areas with subsidence, the subsidence should be of concern. In the rare areas where neither is happening, normal dike and seawall maintenance and normal replacement of infrastructure will suffice. Coral Islands can grow upwards at 1 cm/year, so will have no problem with a sea level rise 1/6th of that. If the past is any indication of the future, there is little to fear from sea level rise.