Current concerns regarding global sea-level rise associated with anthropogenic warming of the atmosphere and oceans and its societal and physical impacts on coasts have resulted in increased interest in past sea-level changes. In fact, more than 1.2 billion people worldwide live in coastal areas now deemed to be at risk from rising sea levels. The mean global rate of sea-level rise has been estimated at ~1.8 mm yr-1 for the last century.
Since 1993, satellite altimetry data provide estimates of ~3 mm yr-1. Twentieth and 21st century rates of sea-level rise rates are as much as two to three times higher than those reconstructed for the past millennia. However, these predictions assume a uniform global eustatic rise and do not include any detail of future regional and sub-regional changes in sea levels. Future combinations of gravitational and steric effects on the ocean surface and regional changes in surface wind patterns and ocean currents mean that local rates of sea-level rise will not conform to the global average. These, as yet not fully quantified, variables have widespread implications in terms of acceptable risk and projected cost for those charged with planning and implementing local mitigation strategies.
In addition, the causes of ongoing rapid sea-level rise have not been precisely determined. Observations suggest that the average global rate of sea-level rise during the 20th century is ~1.8 mm yr-1, but the best independent estimates of the individual contributions to sea-level change total only 0.7±1.5 mm yr-1. Resolving the current source of sea-level rise is of critical importance for understanding and quantifying the natural and anthropogenic contributions to ongoing climatic change.
Several studies based on analyses of long tide-gauge records have concluded that the eustatic contribution of the Greenland Ice Sheet has been the dominant control on 20th century sea-level rise in the North Atlantic Ocean.
However, these studies have been criticised for three principal reasons:
1) they are based on tide-gauge records that are too short and geographically too restricted to give reliable results;
2) they do not take into account steric change (thermal expansion); and
3) corrections for vertical land movements produced by Glacial Isostatic Adjustment (GIA) effects are model dependent and, therefore, potentially imprecise.
On the other hand, recent results indicate that mass loss from continental ice reservoirs has likely been underestimated in previous studies and so is a good candidate for explaining the discrepancy. It has been suggested that the melting of polar and glacier ice provides a detectable global sea-level signal which depends on the source location and geometry: sea level falls near the melt source and rises in the far field.
The pattern of sea-level change thus offers a fingerprint from which the sources of ice melt can be determined. With regard to recent (20th Century) sea-level rise, this principle has been applied to predict the sea-level fingerprints associated with Antarctic, Greenland and mountain glacier melting. However, there is insufficient data coverage to uniquely determine the melt contributions of these sources.
In addition to the source of the current sea-level acceleration, there has been considerable debate about the question when modern rates of sea-level rise commenced and whether any long-term accelerations of sea-level rise occurred in the 19th and 20th centuries. Different authors estimate that sea level began to rise at modern rates approximately at the middle of the 19th century, around the end of the 19th century, during the 1920s, or 1935. Some studies have even proposed decelerations within the 20th century.
On one issue all studies agree: instrumental records of sea-level change are generally too short and provide insufficient spatial coverage to give definitive answers in this debate. Recent geological research offers great potential to supplement the temporal and spatial global database of instrumental (tide-gauge) observations of sea-level change.
Proxy records from salt marshes from the North Atlantic Ocean have provided the first indications that modern rates of sea-level rise (last ~100 years) may be more rapid than the rate of rise in preceding centuries, and that the timing of this acceleration may be indicative of a link with human-induced climate change. Thus far, these high resolution sea-level reconstructions have only come from the western and northern margins of the North Atlantic and a first study is now available from New Zealand. Because (multi-)decadal patterns of sea-level change are spatially highly variable across the globe6, it is important to verify these results and obtain comparable sea-level reconstructions from other regions. The sea-level research community aims to obtain the high-quality sea-level data from selected areas to examine the issue of recent changes in the rate of global sea-level rise.