There is strong evidence that global sea level is now rising at an increased rate and will continue to rise during this century.
While studies show that sea levels changed little from AD 0 until 1900, sea levels began to climb in the 20th century.
The two major causes of global sea-level rise are thermal expansion caused by the warming of the oceans (since water expands as it warms) and the loss of land-based ice (such as glaciers and polar ice caps) due to increased melting.
Records and research show that sea level has been steadily rising at a rate of 1 to 2.5 millimeters (0.04 to 0.1 inches) per year since 1900.
This rate may be increasing. Since 1992, new methods of satellite altimetry (the measurement of elevation or altitude) indicate a rate of rise of 3 millimeters (0.12 inches) per year.
This is a significantly larger rate than the sea-level rise averaged over the last several thousand years.
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 Goreâ€™s blockbuster movie? 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?
S.J. Holgate of the Proudman Oceanographic Laboratory in the UK did a study correcting for possible vertical changes in measuring stations and used a study of high quality data points including New York (1856â€“2003), Key West (1913â€“2003), San Diego (1906â€“2003), Balboa (1908â€“1996), Honolulu (1905â€“2003), Cascais (1882â€“1993), Newlyn (1915â€“2004), Trieste (1905â€“2004), and Auckland (1903â€“2000). These tide gauge stations have long term data that have been carefully recorded relative to a consistent reference level on the nearby land. Holgate states â€œHence 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.â€
To begin with the results, Holgate notes that â€œAll the stations in this study show a significant increase in sea level over the period 1904â€“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.â€ 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 â€œthe 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).â€ How about that â€“ 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.
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 Holgateâ€™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.