Figure 11.7: Time-series of relative sea level for the past 300 years from Northern Europe: Amsterdam, Netherlands; Brest, France; Sheerness, UK; Stockholm, Sweden (detrended over the period 1774 to 1873 to remove to first order the contribution of postglacial rebound); Swinoujscie, Poland (formerly Swinemunde, Germany); and Liverpool, UK. Data for the latter are of “Adjusted Mean High Water” rather than Mean Sea Level and include a nodal (18.6 year) term. The scale bar indicates ±100 mm. (Adapted from Woodworth, 1999a.)

The geological evidence for the past 10,000 to 20,000 years indicates that major temporal and spatial variation occurs in relative sea level change (e.g., Pirazzoli, 1991) on time-scales of the order of a few thousand years (Figure 11.5). The change observed at locations near the former centres of glaciation is primarily the result of the glacio-isostatic effect, whereas the change observed at tectonically stable localities far from the former ice sheets approximate the global average sea level change (for geologically recent times this is primarily eustatic change relating to changes in land-based ice volume). Glacio-hydro-isostatic effects (the Earth’s response to the past changes in ice and water loads) remain important and result in a spatial variability in sea level over the past 6,000 years for localities far from the former ice margins. Analysis of data from such sites indicate that the ocean volume may have increased to add 2.5 to 3.5 m to global average sea level over the past 6,000 years (e.g., Fleming et al., 1998), with a decreasing contribution in the last few thousand years. If this occurred uniformly over the past 6,000 years it would raise sea level by 0.4 to 0.6 mm/year. However, a few high resolution sea level records from the French Mediterranean coast indicate that much of this increase occurred between about 6,000 and 3,000 years ago and that the rate over the past 3,000 years was only about 0.1 to 0.2 mm/yr (Lambeck and Bard, 2000). These inferences do not constrain the source of the added water but likely sources are the Antarctic and Greenland ice sheets with possible contributions from glaciers and thermal expansion.

In these analyses of Late Holocene observations, the relative sea level change is attributed to both a contribution from any change in ocean volume and a contribution from the glacio-hydro-isostatic effect, where the former is a function of time only and the latter is a function of both time and position. It is possible to use the record of sea level changes to estimate parameters for a model of isostatic rebound. In doing this, the spatial variability of sea level change determines the mantle rheology, whereas the time dependence determines any correction that may be required to the assumed history of volume change. Solutions from different geographic regions may lead to variations in the rheology due to lateral variations in mantle temperature, for example, but the eustatic term should be the same, within observational and model uncertainties, in each case (Nakada and Lambeck, 1988). If it is assumed that no eustatic change has occurred in the past 6,000 years or so, but in fact eustatic change actually has occurred, the solution for Earth-model parameters will require a somewhat stiffer mantle than a solution in which eustatic change is included. The two solutions may, however, be equally satisfactory for interpolating between observations. For example, both approaches lead to mid-Holocene highstands at island and continental margin sites far from the former ice sheets of amplitudes 1 to 3 m. The occurrence of such sea level maxima places a upper limit on the magnitude of glacial melt in recent millennia (e.g., Peltier, 2000), but it would be inconsistent to combine estimates of ongoing glacial melt with results of calculations of isostatic rebound in which the rheological parameters have been inferred assuming there is no ongoing melt.

The geological indicators of past sea level are usually not sufficiently precise to enable fluctuations of sub-metre amplitude to be observed. In some circumstances high quality records do exist. These are from tectonically stable areas where the tidal range is small and has remained little changed through time, where no barriers or other shoreline features formed to change the local conditions, and where there are biological indicators that bear a precise and consistent relationship to sea level. Such areas include the micro-atoll coral formations of Queensland, Australia (Chappell, 1982; Woodroffe and McLean, 1990); the coralline algae and gastropod vermetid data of the Mediterranean (Laborel et al., 1994; Morhange et al., 1996), and the fresh-to-marine transitions in the Baltic Sea (Eronen et al., 1995; Hyvarinen, 1999). These results all indicate that for the past 3,000 to 5,000 years oscillations in global sea level on time-scales of 100 to 1,000 years are unlikely to have exceeded 0.3 to 0.5 m. Archaeological evidence for this interval places similar constraints on sea level oscillations (Flemming and Webb, 1986). Some detailed local studies have indicated that fluctuations of the order of 1 m can occur (e.g., Van de Plassche et al., 1998) but no globally consistent pattern has yet emerged, suggesting that these may be local rather than global variations.

Estimates of current ice sheet mass balance (Section 11.2.3.1) have improved since the SAR. However, these results indicate only that the ice sheets are not far from balance. Earth rotational constraints (Section 11.2.4.2) and ice sheet altimetry (Section 11.2.3.2) offer the prospect of resolving the ice sheet mass balance in the future, but at present the most accurate estimates of the long-term imbalance (period of several hundred years) follows from the comparison of the geological sea level data with the ice sheet modelling results (Section 11.2.3.3). The above geological estimates of the recent sea level rates may include a component from thermal expansion and glacier mass changes which, from the long-term temperature record in Chapter 2 (Section 2.3.2), could contribute to a sea level lowering by as much as 0.1 mm/yr. These results suggest that the combined long-term ice sheet imbalance lies within the range 0.1 to 0.3 mm/yr. Results from ice sheet models for the last 500 years indicate an ongoing adjustment to the glacial-interglacial transition of Greenland and Antarctica together of 0.0 to 0.5 mm/yr. These ranges are consistent. We therefore take the ongoing contribution of the ice sheets to sea level rise in the 20th and 21st centuries in response to earlier climate change as 0.0 to 0.5 mm/yr. This is additional to the effect of 20th century and future climate change.

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