11.3 Past Sea Level Changes
11.3.1 Global Average Sea Level over the Last 6,000 Years
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.
