Introduction

In our effort to understand the implications of the secular variations of relative sea level so clearly recorded on tide gauges from which sufficiently long records are available (see Chapter 3), it is important that we recognize the extraordinary range of physical processes that contribute to such observations. Although it is now understood that the record length must be sufficiently long to allow us to average out the influence of the interannual variability associated with El Nino and other processes, it is not as widely understood that very much longer timescale geological processes may also significantly contaminate such observations. Here the word "contamination" refers to the contribution to the secular rate of change of sea level from any process other than those associated with modern climate drift, whether the latter be of "natural" or "anthropogenic" origin. This chapter focuses on the one specific source of geological timescale contamination that appears to dominate all others from the point of view of its global incidence. This concerns the phenomenon of glacial isostatic adjustment (hereafter GIA), a physical process caused by the intense cycle of glaciation and deglaciation to which the planet has been subjected for the past 900,000 years of the Pleistocene period of Earth history (e.g., see the discussion of the oxygen isotopic ice volume proxy from the ODP677 core in Shackleton et al, 1990). Although other geological processes may also contribute to modern observations of secular rates of sea level change in specific regions, for example, near subduction zones where oceanic lithosphere descends into the mantle and where the descent may be accommodated seismogenically through the occurrence of intense dip-slip earthquakes such as are responsible for the continuing uplift of the coast of the Huon Peninsula of Papua New Guinea, these other processes seem not to be of global import and so will not be considered here.

To set the stage for the discussion to follow, consider the analysis of tide gauge secular sea level trends presented in Fig. 4.1, taken from Peltier and Tushingham (1989). It was first established in this paper that the glacial isostatic adjustment process was a significant global source of "contamination" of such

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Figure 4.1 Results for the analysis of the global rate of RSL rise obtained from the linear regression analyses of Peltier and Tushingham (1989). The dash-dotted curve depicts the decrease in the number of tide gauge stations as the minimum record length employed in the analysis increases. The three individual estimates of the globally averaged rate of RSL rise versus minimum record length are based upon equal area averages of the raw (dashed curve) and GIA-corrected (solid curve) tide gauge data and a site-by-site average of the corrected data (dotted curve). The solid curve should provide the most accurate linear regression estimate of the present-day global rate of relative sea level rise.

MINIMUM RECORD LENGTH (years)

Figure 4.1 Results for the analysis of the global rate of RSL rise obtained from the linear regression analyses of Peltier and Tushingham (1989). The dash-dotted curve depicts the decrease in the number of tide gauge stations as the minimum record length employed in the analysis increases. The three individual estimates of the globally averaged rate of RSL rise versus minimum record length are based upon equal area averages of the raw (dashed curve) and GIA-corrected (solid curve) tide gauge data and a site-by-site average of the corrected data (dotted curve). The solid curve should provide the most accurate linear regression estimate of the present-day global rate of relative sea level rise.

observations. The figure shows (dash-dot line) the way in which the number of tide gauges decreases as a function of the length of the observational record that the individual instruments provide (as of the date of publication of the original paper). The same diagram also shows how the globally averaged rate of secular sea level change varies as a function of the lower timescale cutoff that is applied to determine whether the time series from a given gauge will be allowed to contribute to the determination of the globally averaged rate. The results obtained on the basis of such analyses are shown for two variants of the analysis procedure, one in which the "raw" rates of secular sea level rise are averaged by employing an areal weighting (dashed line) to account for the fact that the tide gauge installations do not sample the ocean surface uniformly, and one in which these areally weighted "raw" rates are "decontaminated" by subtraction of the influence on each gauge that is predicted by a theory of the GIA process (solid curve). In the third of the results (dotted curve) no attempt to weight the data for area coverage has been made; this is the least meaningful of the three estimates. Two points will be noted by inspection of these results. First, when the tide gauge (areally weighted) rates of secular sea level rise are corrected for the influence of glacial isostatic adjustment, the globally averaged rate of relative sea level (RSL) rise is somewhat increased. Second, and more important, however, is the fact that, as the minimum record length of the set of time series that is employed to determine the average is increased, the globally averaged rate of RSL rise also increases initially. This demonstrates clearly that, when short records are included in the set for which the global average is determined, the short timescale incoherence among them leads to an underestimate of the actual global rate of RSL rise that is ongoing in the Earth system. This point is further illustrated by Fig. 3.11 in the previous chapter of this book.

It is also notable, however, that in the analysis of Peltier and Tushingham (1989) the globally averaged rate determined on the basis of the longest records begins to fall rapidly once the minimum record length exceeds approximately 60 years. This fall is not compatible with the result shown in Fig. 3.11 in the preceding chapter. The reason for this has to do with the fact that in constructing Fig. 4.1 those records which were long but contaminated by local anthropogenic effects (ground water extraction, etc.) were not removed from the mix. To optimize our inference of the ongoing globally averaged rate of relative sea level rise, therefore, we will need to pay close attention to the application of three primary criteria. First, the records we employ must be sufficiently long to enable us to accurately average out the influence of the El Nino-related and other interannual variability. Second, we must eliminate all long records known to be strongly contaminated by well-recognized local tectonic or anthropogenic but not climate-related processes. Third, we need to filter these records to remove, as accurately as possible, the contribution to the local rate of secular sea level change at each tide gauge that is due to the influence of GIA.

In what is to follow, I will first review the global theoretical model of the GIA process that my students and I have developed over the past 20 years. Following a brief discussion of the global character of the solutions for the present day rate of RSL rise predicted by this theory, and of the quality of the fits to individual time series in the global data base of Holocene sea level histories that have been achieved with it, I will focus upon the results obtained by employing various versions of the global model to filter the same set of tide gauge data as were employed in Chapter 3 to infer the strength of the global signal that may be related to modern climate change. Finally, I will employ the theory in conjunction with specific observational constraints to show that the contribution to this inferred globally averaged rate of sea level rise that could be ascribed to persistent melting of the Antarctic and Greenland ice sheets since mid-Holocene time is extremely small. These results suffice to place the onus for the explanation of the observed modern rate of RSL rise squarely upon modern climate change.

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