Cazenave et al. (2003) studied climate-related processes that cause variations in mean global sea level on interannual to decadal time scales, focusing on thermal expansion of the oceans and continental water mass balance. In doing so, they determined that the rate of thermal-induced sea level rise over the past 40 years was about 0.5 mm/year. From early 1993 to the end of the 20th century, however, analyses of TOPEX-Poseidon altimetry data and the global ocean temperature data of Levitus et al. (2000) yielded rates-of-rise that were approximately six times greater than the mean four-decade rate, which suggested to them that "an acceleration took place in the recent past, likely related to warming of the world ocean." However, as they alternatively note, "the recent rise may just correspond to the rising branch of a decadal oscillation." In addition, they say that "satellite altimetry and in situ temperature data have their own uncertainties and it is still difficult to affirm with certainty that sea level rise is indeed accelerating." In fact, they cite the work of Nerem and Mitchum (2001) as indicating that "about 20 years of satellite altimetry data would be necessary to detect, with these data alone, any acceleration in sea level rise."

Morner (2004) provided a more expansive setting for his analysis of the subject by noting that "prior to 5000-6000 years before present, all sea level curves are dominated by a general rise in sea level in true glacial eustatic response to the melting of continental ice caps," but that "sea level records are now dominated by the irregular redistribution of water masses over the globe ... primarily driven by variations in ocean current intensity and in the atmospheric circulation system and maybe even in some deformation of the gravitational potential surface." With respect to the last 150 years, he reports that "the mean eustatic rise in sea level for the period 1850-1930 was [on] the order of 1.0-1.1 mm/year," but that "after 1930-40, this rise seems to have stopped (Pirazzoli et al., 1989; Morner, 1973, 2000)." This stasis, in his words, "lasted, at least, up to the mid-60s." Thereafter, with the advent of the TOPEX/Poseidon mission, Morner notes that "the record can be divided into three parts: (1) 1993-1996 with a clear trend of stability, (2) 1997-1998 with a high-amplitude rise and fall recording the ENSO event of these years and (3) 1998-2000 with an irregular record of no clear tendency." Most important of all, in his words, Morner states "there is a total absence of any recent 'acceleration in sea level rise' as often claimed by IPCC and related groups," and, therefore, "there is no fear of any massive future flooding as claimed in most global warming scenarios."

Church et al. (2004) used TOPEX/Poseidon satellite altimeter data to estimate global empirical orthogonal functions, which they combined with historical tide gauge data, to estimate monthly distributions of large-scale sea level variability and change over the period 1950-2000. Their resultant "best estimate" of the rate of globally-averaged sea level rise over the last half of the 20th century was 1.8 ± 0.3 mm/year. In addition, they noted that "decadal variability in sea level is observed, but to date there is no detectable secular increase in the rate of sea level rise over the period 1950-2000." What is more, they reported that no increase in the rate of sea level rise has been detected for the entire 20th century, citing the work of Woodworth (1990) and Douglas (1992).

Cazenave and Nerem (2004) summarized what was known shortly after the turn of the century about past and then-current rates of sea level rise because, in their words, "determining the rate of sea level change over the last century is critically important in order to determine if the present-day rate of sea level change has changed appreciably," which, as we have noted, is something one would expect to have occurred if 20th-century global warming was truly as unprecedented as climate alarmists say it was, bringing the planet to a level of warmth they claim was unprecedented over the past two millennia and within less than a degree of the all-time high temperature of the last million or more years. What they learned in this endeavor was that "the geocentric rate of global mean sea level rise over the last decade (1993-2003) is now known to be very accurate, +2.8 ± 0.4 mm/year, as determined from TOPEX/Poseidon and Jason altimeter measurements," and that "this rate is significantly larger than the historical rate of sea level change measured by tide gauges during the past decades (in the range of 1-2 mm/year)."

However, as Cazenave and Nerem continue, "the altimetric rate could still be influenced by decadal variations of sea level unrelated to long-term climate change, such as the Pacific Decadal Oscillation, and thus a longer time series is needed to rule this out." They also noted that satellite altimetry had revealed a "non-uniform geographical distribution of sea level change, with some regions exhibiting trends about 10 times the global mean." In addition, they note that "for the past 50 years, sea level trends caused by change in ocean heat storage also show high regional variability," which fact "has led to questions about whether the rate of 20th-century sea level rise, based on poorly distributed historical tide gauges, is really representative of the true global mean." Consequently, and in spite of the many new instruments and techniques that are being used to search for a global warming signal in global sea level data, Cazenave and Nerem report that "these tools seem to have raised more questions than they have answered."

Noting that global climate models "show an increase in the rate of global average sea level rise during the 20th century," but that several prior studies (Douglas, 1991, 1992; Maul and Martin, 1993; Church et al., 2004; Holgate and Woodworth, 2004) had shown the measured rate of global sea level rise to have been rather stable over the past hundred years, White et al. (2005) conducted yet another analysis of the available data in an attempt to find the elusive predicted increase in the global sea level's rate of rise, comparing estimates of coastal and global averaged sea level for 1950 to 2000. When all was said and done, their results confirmed the earlier findings of "no significant increase in the rate of sea level rise during this 51-year period," i.e., over the last half of the 20th century, including its last two decades, which are especially demonized by climate alarmists for their supposedly unprecedented rate of temperature increase.

Lombard et al. (2005) investigated the thermosteric or temperature-induced sea-level change of the last 50 years using the global ocean temperature data of Levitus et al. (2000) and Ishii et al. (2003). This work revealed that thermosteric sea level variations are dominated by decadal oscillations of the planet's chief ocean-atmosphere climatic perturbations (El Niño-Southern Oscillation, Pacific Decadal Oscillation and North Atlantic Oscillation); and in terms of the global mean, as they describe it, thermosteric trends computed over 10-year windows "show large fluctuations in time, with positive values (in the range 1 to 1.5 mm/year for the decade centered on 1970) and negative values (-1 to -1.5 mm/year for the decade centered on 1980)." In the mean, however, and over the full half-century period Lombard et al. investigated, there was a net rise in sea level due to the thermal expansion of sea water, but only because the record began at the bottom of a trough and ended at the top of a peak. In between these two points, there were both higher and lower values, so that one cannot be sure what would be implied if earlier data were available or what will be implied as more data are acquired. Noting that sea level trends derived from TOPEX/Poseidon altimetry over 1993-2003 are "mainly caused by thermal expansion" and are thus "very likely a non-permanent feature," Lombard et al. thus concluded that "we simply cannot extrapolate sea level into the past or the future using satellite altimetry alone." Consequently, even the 50 years of global ocean temperature data we possess are insufficient to tell us much about the degree of global warming that may have occurred over the past half-century, as any long-term increase in global sea level that may have been caused by the temperature increase is absolutely dwarfed by decadal-scale variability.

Carton et al. (2005) introduced their study of the subject by noting that "recent altimeter observations indicate an increase in the rate of sea level rise during the past decade to 3.2 mm/year, well above the centennial estimate of 1.5-2 mm/year," noting further that "this apparent increase could have resulted from enhanced melting of continental ice," as climate alarmists often claim, "or from decadal changes in thermosteric and halosteric effects." Hence, they explored these opposing options "using the new eddy-permitting Simple Ocean Data Assimilation version 1.2 reanalysis of global temperature, salinity, and sea level spanning the period 1968-2001," and in doing so they determined that "the effect on global sea level rise of changing salinity is small except in subpolar regions." However, they found that warming-induced steric effects "are enough to explain much of the observed rate of increase in the rate of sea level rise in the last decade of the 20th century without need to invoke acceleration of melting of continental ice." And as determined by Lombard et al., as described in the preceding paragraph, the high thermosteric-induced rate-of-rise of global sea level over the past decade is likely "a non-permanent feature" of the global ocean's transient thermal behavior. Consequently, and in harmony with the findings of Levitus et al. (2005) and Volkov and van Aken (2005), Carton et al. found no need to invoke the melting of land-based glacial ice to explain the observed increase in global sea-level rise of the past decade.

Even more revealing was the globally-distributed sea level time series study of Jevrejeva et al. (2006), who analyzed information contained in the Permanent Service for Mean Sea Level database using a method based on Monte Carlo Singular Spectrum Analysis and removed 2- to 30-year quasi-periodic oscillations to derive nonlinear long-term trends for 12 large ocean regions, which they combined to produce the mean global sea level (gsl) and gsl rate-of-rise (gsl rate) curves depicted in the figure below.

Mean global sea level (top), with shaded 95% confidence interval, and mean gsl rate-of-rise (bottom), with shaded standard error interval, adapted from Jevrejeva et al. (2006).

In discussing their findings, Jevrejeva et al. say they show that "global sea level rise is irregular and varies greatly over time," noting that "it is apparent that rates in the 1920-1945 period are likely to be as large as today's." In addition, they report that their "global sea level trend estimate of 2.4 ± 1.0 mm/year for the period from 1993 to 2000 matches the 2.6 ± 0.7 mm/year sea level rise found from TOPEX/Poseidon altimeter data." With respect to what the four researchers describe as "the discussion on whether sea level rise is accelerating," therefore, their results pretty much answer the question in the negative.

The observations described above make us wonder why late 20th-century global warming - which climate alarmists describe as having been unprecedented over the past two millennia - cannot be detected in global sea level data. We are even more intrigued about the matter in light of the fact that the effects of the warming that led to the demise of the Little Ice Age - which by climate-alarmist contention should have been considerably less dramatic than the warming of the late 20th century - are readily apparent to the right of the vertical red line in the above figure. Likewise, we are perplexed by the demonstrable fact that although the rising atmospheric CO2 concentration - which climate alarmists claim is primarily responsible for the supposedly unprecedented global warming of the late 20th century - experienced a dramatic increase in its rate-of-rise just after 1950 (shifting from a 1900-1950 mean rate-of-rise of 0.33 ppm/year to a 1950-2000 mean rate-of-rise of 1.17 ppm/year), the mean global sea level rate-of-rise did not trend upwards after 1950, nor has it subsequently exceeded its 1950 rate-of-rise. These observations clearly indicate that something is drastically wrong with the warped world-view of the world's climate alarmists.

In concluding this summary, we report the findings of the most recent study of Holgate (2007). In a previous paper, Holgate and Woodworth (2004) derived a mean global sea level history from 177 coastal tide gauge records that spanned the period 1955-1998; and in an attempt to extend that record back in time another half-century, Holgate chose nine much longer high-quality records from around the world (New York, Key West, San Diego, Balboa, Honolulu, Cascais, Newlyn, Trieste and Auckland) to see if their combined mean progression over the 1955-1998 period was similar enough to the concomitant mean sea level history of the 177 stations to employ the mean nine-station record as a reasonable representation of mean global sea level history for the much longer period stretching from 1904 to 2003.

In comparing the sea level histories derived from the two data sets, Holgate found that their mean rates-of-rise were indeed similar over the second half of the 20th century; and this observation thus implied, in Holgate's words, that "a few high quality records from around the world can be used to examine large spatial-scale decadal variability as well as many gauges from each region are able to [do]."

As a result of this finding, Holgate constructed the nine-station-derived wavering black line in the figure below as a reasonable best representation of the 1904-2003 mean global sea level history of the world, and based on that history calculated that the mean rate of global sea level rise was "larger in the early part of the last century (2.03 ± 0.35 mm/year 1904-1953), in comparison with the latter part (1.45 ± 0.34 mm/year 1954-2003)."

Another way of thinking about the century-long sea level history portrayed in the figure below is suggested by the blue curve we have fit to it, which indicates that mean global sea level may have been rising, in the mean, ever more slowly with the passage of time throughout the entire last hundred years, with a possible acceleration of that trend over the last few decades.

Cumulative increase in mean global sea level (1904-2003) derived from nine high-quality tide gauge records from around the world. Adapted from Holgate (2007).

In any event, and whichever way one looks at the findings of Holgate - either as two successive linear trends (representative of the mean rates-of-rise of the first and last halves of the 20th century) or as one longer continuous curve (such as we have drawn) - the nine select tide gauge records indicate that the mean rate of global sea level rise has not accelerated over the recent past (if anything, in fact, it's done just the opposite), when climate alarmists have incessantly claimed that (1) the earth warmed to a degree that is unprecedented over many millennia, (2) the warming resulted in a net accelerated melting of the vast majority of the world's mountain glaciers and polar ice caps, and (3) global sea level rose at an ever increasing rate. The real-world data-based results of Holgate, as well as those of all of the other studies we have noted in this brief summary, clearly suggest that all of these claims are likely to be false.

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