February 9, 2007

Shocking Facts about Sea Level Rise

Filed under: Sea Level Rise

One of the pillars of the global warming scare is that sea level is rising, the rise is accelerating due to ever higher global temperatures, and in the absence of some immediate policy to curb emissions of greenhouse gases, the sea level rise will inundate islands and coastlines throughout the world. Who could ever forget seeing the World Trade Center Memorial under water in Gore’s blockbuster movie? In addition, no fewer than 1.4 million websites are found if you search “Global Warming and Sea Level Rise.” Many would be shocked to find anyone daring to question accelerated sea level rise, and yet, as covered many times before in World Climate Report, the scientific literature is full of surprises when it comes to global warming and sea level rise. How many would believe that global sea level actually dropped for a period in the mid-to-late 1990s?

Well, it is true, and an article in the recent issue of Geophysical Research Letters provides some shocking news for those who never question the accelerated sea level rise axiom.

S.J. Holgate of the Proudman Oceanographic Laboratory in Liverpool, UK correctly notes that “When it comes to calculating long term global sea level means from tide gauge data, there are a number of problems. Firstly there is a bias in the distribution of tide gauges towards certain regions, notably Northern Europe and North America. Secondly there is the problem that not all tide gauge records are of equivalent quality. This can either be due to their location (being for example in an earthquake-prone region or an area of high glacial isostatic adjustment, GIA) or due to the quality of the instrumental record (being perhaps too discontinuous or lacking critical datum information to account for local vertical land movements).”

Taking these concerns into account, Holgate notes that an extremely high quality database has been developed for nine stations around the world including New York (1856–2003), Key West (1913–2003), San Diego (1906–2003), Balboa (1908–1996), Honolulu (1905–2003), Cascais (1882–1993), Newlyn (1915–2004), Trieste (1905–2004), and Auckland (1903–2000). These tide gauge stations (Figure 1) have long term data that have been carefully recorded relative to a consistent reference level on the nearby land. Holgate states “Hence the tide gauge data presented here is of the very highest quality available. All these records are almost continuous and are far away from regions with high rates of vertical land movement due to GIA or tectonics.”

Figure 1. The distribution of the nine tide gauge records used in the Holgate (2007) study.

To begin with the results, Holgate notes that “All the stations in this study show a significant increase in sea level over the period 1904–2003 with an average increase of 174 mm during that time. This mean rate of 1.74 mm/yr is at the upper end of the range of estimates for the 20th century in the Intergovernmental Panel on Climate Change, Third Assessment Report.” Well, it seems sea level is rising at what many would call an alarming rate of 174 mm (6.8 inches) per century, although this hardly seems alarming to us.

But here comes the big surprise. The Figure 2 shows decadal rates of sea level change over the past century, and as noted by Holgate “the two highest decadal rates of change were recorded in the decades centred on 1980 (5.31 mm/yr) and 1939 (4.68 mm/yr) with the most negative decadal rates of change over the past 100 years during the decades centred on 1964 (-1.49 mm/yr) and 1987 (-1.33 mm/yr).” How about that – the greatest global sea level rise occurred around 1980, well before the greenhouse scare got off the ground. Also, it is immediately obvious that the rate of sea level rise during most recent couple of years has been, well, unremarkable, with declining sea levels for a short period in the mid-to-late 1990s.

Figure 2. Comparison of the global mean decadal rates of sea level change based on the nine records with the rates from a 177 stations used in a previous study. All rates are corrected for glacial isostatic adjustment and inverse barometer effects. The shaded region indicates ±1 standard error (from Holgate, 2007).

Holgate then writes “Despite the high decadal rates of change in the latter part of the 20th century, it is found that the first half of the record (1904–1953) has a higher rate of rise overall (2.03 ± 0.35 mm/yr) than the 1954–2003 period which had a rate of 1.45 ± 0.34 mm/yr.” Adding more salt to the wounds of the global warming advocates, Holgate notes “However, a greater rate of rise in the early part of the record is consistent with previous analyses of tide gauge records which suggested a general deceleration in sea level rise during the 20th century.” Finally, we learn “not only is there considerable decadal variability in the individual sea level records, but there is generally little correlation between them. Stations which are in close proximity and which are affected by similar ocean and atmospheric processes show the greatest correlation.”

Here is what the new IPCC Summary for Policymakers has to say about observed sea level rise:

Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 [2.4 to 3.8] mm per year. Whether the faster rate for 1993 to 2003 reflects decadal variability or an increase in the longer-term trend is unclear.

Although the IPCC would have us believe that sea level rise has been accelerating recently, they are not emphatic about it, and leave open the possibility that decadal variations may be responsible for the perceived rate increase. As shown by Holgate’s new research results that possibility looks like the leading contender.

From this article, we learn from the actual data that (a) sea level is generally rising, (b) the rate of rise decelerated during the 20th century, (c) the rate of sea level rise over the past two decades has been both positive and negative, (d) the rate of sea level rise has been quite small over the last few years, and (e) stations can witness an increase or decrease of sea level quite independently of one another.

At the very least, Holgate shows that the global warming – sea level linkage is far more complicated than typically presented to the public. Or the results show that what the public has been told about sea level rise is simply wrong.

You decide, but the facts look rather obvious to us.


Holgate, S. J., 2007. On the decadal rates of sea level change during the twentieth century, Geophysical Research Letters, 34, L01602, doi:10.1029/2006GL028492.

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