Sea-level rise

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

The possibility of large sea-level rises as a result of global warming is featured prominently in presentations of those, such as former U.S. Vice President Al Gore, who call for urgent action to “stop” global warming. In this section we examine historical trends in sea level to see if there is any indication of an increase in the mean rate-of-rise of the global ocean surface in response to the supposedly unprecedented warming of the planet over the course of the twentieth century. We then examine closely the various scenarios proposed whereby melting ice would cause sea levels to rise.


Mean Global Sea Levels

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 twentieth 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.”

Mörner (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; Mörner, 1973, 2000).” This stasis, in his words, “lasted, at least, up to the mid-60s.” Thereafter, with the advent of the TOPEX/Poseidon mission, Mörner 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, Mörner 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 twentieth 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 twentieth century, citing the work of Woodworth (1990) and Douglas (1992).

Cazenave and Nerem (2004) seemed to dismiss the caveats expressed in Cazenave et al. (2003) when they claimed 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, they then admit “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 twentieth 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) compared estimates of coastal and global averaged sea level for 1950 to 2000. Their results confirmed the earlier findings of “no significant increase in the rate of sea-level rise during this 51-year period.”

Lombard et al. (2005) investigated the thermosteric or temperature-induced sea-level change of the past 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). 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 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. conclude that “we simply cannot extrapolate sea level into the past or the future using satellite altimetry alone.” 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 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 or from decadal changes in thermosteric and halosteric effects.” 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.” 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 twentieth 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.

The figure clearly shows no acceleration of sea-level rise since the end of the Little Ice Age. Jevrejeva et al. say “global sea-level rise is irregular and varies greatly over time,” but “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,” their results pretty much answer the question in the negative.

The observations described above make us wonder why late twentieth century global warming—if it were as extreme as the IPCC claims it has been—cannot be detected in global sea-level data. The effects of the warming that led to the demise of the Little Ice Age—which the IPCC contends should have been considerably less dramatic than the warming of the late twentieth century—are readily apparent to the right of the vertical red line in the figure. Likewise, although the atmospheric CO2 concentration 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.

In concluding our examination of the peer-reviewed sea-level science, 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. 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 datasets, Holgate found their mean rates-of-rise were indeed similar over the second half of the twentieth century; 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 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).”

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 twentieth 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, and has probably fallen.

Konikow (2011) states that "if the removal of groundwater from storage in the continental subsurface is sufficiently large and persistent, it can represent a substantial transfer of water mass from the land to the oceans, and thereby represent a measureable contributor to long-term sea level rise." Thus, Konikow proceeded to assess the magnitude of global long-term groundwater depletion from 1900 and 2008 by developing the first comprehensive aquifer-based estimate of changes in groundwater storage using direct volumetric accounting. He then compared these results with the results of concomitant sea level rise observations.

Konikow, a U.S. Geological Survey researcher, found that groundwater depletion over the period 1900-2008 was about 4,500 km3, which is equivalent to a global sea-level rise of 12.6 mm, or just over 6% of the total observed rise. He notes that the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000-2008), when it averaged 145 km3/year. The average rate of seal-level rise over the 20th century was 1.7 ± 0.5 mm/year, a rate that may have accelerated in recent years. However, all things considered, Konikow states that a "better understanding and quantification of the contribution of groundwater depletion to sea-level rise should facilitate an improved understanding of 20th century sea-level rise."

In 1990 the Intergovernmental Panel on Climate Change (IPCC) suggested that global sea level could rise by 8-29 cm by 2030 and 31-110 cm by 2100. The IPCC projections set the framework for the coastal policy response to sea-level rise in England and Wales, which was developed by the Ministry of Agriculture, Fisheries and Food (MAFF, 1991). Expected relative sea-level rise would result in an increase in wave energy at the base of coastal cliffs that would lead to accelerated cliff recession, risking damage to properties behind actively retreating cliff-lines. Bray and Hooke (1997) suggested "a 22-133% increase in cliff recession rates on the south coast of England by 2050."

As a result of these projections, Lee (2011) decided to analyze the most recent 50-year recession records of the United Kingdom's Holderness Cliffs. He found that "relative sea level has risen over the second half of the 20th century," as have Holderness cliff recession rates, “from around 1.2 m/year in the early 1950s to around 1.5 m/year by 2000." However, there has been no significant acceleration in the rate of global sea-level rise since 1990 and no rapid increase in the recession rate. Thus, predictions of 20-year recession distances made in the early 1990s are likely to have overestimated the risk to cliff-top property and the benefits of coast protection.

According to Church et al. (2011), "in the last two Intergovernmental Panel on Climate Change assessments, the sum of observed contributions to sea-level rise has consistently been less than the observed rise over multi-decadal periods, thus reducing confidence in the sea-level projections." Noting that Earth's sea-level and energy budgets are closely related and must be solved in a consistent manner, the authors considered them together, tracking changes from 1972 to 2008. They found that “the observed sea-level rise (1.8 ± 0.2 mm/year from tide gauges alone and 2.1 ± 0.2 mm/year from a combination of tide gauges and altimeter observations) agrees well with the sum of energy budget contributions (1.8 ± 0.4 mm/year) in magnitude and with both having similar increases in the rate of rise during the period.” It appears that the mean rate-of-rise of Earth's oceans has remained remarkably constant ever since 1972.


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