While the public gets the impression that there is a simple relationship between global warming and sea level rise, often underwritten by geoscientists with inadequate knowledge, it can be argued that sea level changes are a result of many factors. This makes the prediction of future trends difficult and unreliable.
According to IPCC (Intergovernmental Panel on Climate Change), Al Gore and the media, global sea level is rising ever faster, and in a very frightening way. Furthermore, sea level will rise even more rapidly in the future, due to global warming, linked to increasing CO2content in the atmosphere.
But is this really state of the art knowledge on sea level rise?
There is no doubt that sea level is currently rising on a global scale. However, in 2001, IPCC concluded that «no significant acceleration in the rate of sea level rise during the 20th century has been detected.» In 2007, IPCC noted that «global mean sea level rose at an average rate of 1.8 mm per year from 1961 to 2003. The rate was faster from 1993 to 2003: about 3.1 mm per year. Whether the faster rate for 1993 to 2003 reflects decadal variability or an increase in the longer-term trend is unclear.»
So, a key question is not whether sea level is actually rising, but rather, has there been any acceleration in its rise during recent decades?
Based on global tide gauge data, Holgate (Proudman Oceanographic Laboratory, Liverpool) concludes in a recent paper: «it is found that the first half of the record (1904-1953) has a higher rate of rise overall (2.03 mm/yr) than the 1954-2003 period which had a rate of 1.45 mm/yr.» Similar numbers are being found from other studies, including those based on meteorologically driven trends and GPS-data. The result from these studies is a sea level rise of around 1 mm/yr, but IPCC claims 3 mm/yr, based on satellite altimetry. However, «Sea level rise estimates from satellite altimetry exceed those from tide gauges; it is unclear whether this represents an increase over the last decades, variability, or problems with satellite calibration,» according to IPCC.
Our first conclusion it therefore that as yet we have not reached a consensus on the rate at which sea level rises.
The driving mechanisms?
It is generally assumed that sea level rise is caused by the melting of the glaciers in Antarctica and Greenland, and the expansion of sea water caused by temperature increase.
But is this what we actually observe?
With respect to the melting of continental glaciers, a 2007 paper in the well respected magazine Science reviews what is known about sea-level contributions arising from wastage of the Antarctic and Greenland Ice Sheets, focusing on the results of 14 different satellite-based estimates of the imbalances of the polar ice sheets that have been derived since 1998. The conclusion is that the current «best estimate» of the contribution of polar ice wastage to global sea level change is a rise of 0.35 mm/yr.
With respect to sea water expansion, another 2007 paper entitled «Is the World Ocean Warming? Upper-Ocean Temperature Trends: 1950-2000» concludes that «the ocean neither cooled nor warmed systematically over the large parts of the ocean for the entire analysis period.» If there are no changes in the ocean temperature, then there can be no thermal expansion.
The second conclusion is therefore that we have not yet been able to explain the ongoing sea level changes.
Global sea level changes – eustasy
Based on these conclusions, it is now pertinent to investigate in more detail the principles behind sea level changes. As discussed below, this is certainly more complicated than the public is aware.
Sea level changes are termed eustasy (Greek, ev=good, stasis=position). The word «eustasy» was first proposed by Eduard Suess in 1888. His studies of Tertiary stratigraphy indicated that transgressions took place simultaneously all over Europe. Marine sediments found above present sea level were explained by movements of the ocean level. However, it was proposed that the sea level record was influenced, to a certain extent, by local tectonics, but Suess found no sign of large scale vertical movements of the solid Earth. He assumed that changes in sea level in general were caused by climatic variations, tectonic movements, volcanism, and, the most important factor, sedimentation.
Eustasy is commonly defined as globally uniform sea level changes. Great effort has been spent on constructing global eustatic curves for the Quaternary as well as for the entire Phanerozoic. The most well known curves for long term changes are the «Exxon curves».
Eustasy can be described as being one of three types:
1) Glacial-eustasy is controlled by changes in the ocean water volume caused by glaciations and deglaciations. These variations result in rapid sea level fluctuations and would account for the oscillations of sea level inferred for the Oligocene/Miocene and onwards, as well as for the Permian and the Ordovician, as shown by the «Exxon curves».
2) Tectono-eustasy is controlled by variations in the ocean basin volume caused by sedimentation, changes in the volume of ocean ridge systems and hydro-isostasy (see below).
3) Geoidal-eustasy is caused by variations in the earth’s gravity field. Geoidal-eustasy causes changes in the distribution of ocean water. It affects the ocean level globally, but the direction and magnitude of changes differs over the globe (see box).
From this definition, eustasy is clearly complex, and is certainly not globally uniform.
The geoid and the spheroid
It was already well known in the 19th century that the mass of mountains causes considerable deflections of the gravity. It was claimed that this would cause a rise in sea level in the vicinity of the mountains, and the irregular ocean level was later named «the geoid». The geoid is an equipotential surface of the earth’s gravity field corresponding to sea level. The latter can be measured only over oceans, but the geoid is a complete, closed surface.
Where there are local variations in gravity, due to internal density anomalies of the crust and the mantle, the geoid is distorted.
A mathematical figure representing the sea level surface with all irregularities removed is named the spheroid. The spheroid would be the sea level surface of an earth with no lateral variations in density. The difference in elevation between the measured geoid and the spheroid is called the geoid anomaly. Some of the major geoid anomalies, like the geoid high over New Guinea (+70 m) or the geoid low over India (-100 m) are probably related to mantle convection.
Any sea level change causes deflection of the ocean floor, hydro-isostasy, to attain isostatic equilibrium. The hydro-isostasy is approximately 1/3 of the sea level change. Simultaneously the continents are deflected, with a mean magnitude over the continents twice the deflection of the ocean floor. This is due to the fact that the oceanic area is double the land area. An interesting implication of hydro-isostasy is the fact that the sea level history will differ between oceanic islands and continental margins. An island moving with the sea floor will record the full sea level change, while points near the continents record quite different sea level changes. Thus hydro-isostasy is an important factor in determining relative sea level fluctuations.
Relative sea level changes
Observed sea level data, as well as predictions of sea level changes at certain locations, deal with relative sea level, because they include the combined movement of both water and land. In 1921, the Norwegian scientist and explorer Fridjof Nansen introduced a model of glacial isostasy based on the changes caused by the loading and unloading of glaciers on the continents. He is also often given the honour of being the originator of the term hydro-isostasy.
Glacial isostatic rebound is still going on in Scandinavia, caused by the melting of ice after the last glaciation, which ended 8,000 years ago. Hydro-isostasy is also active; the ocean floor is still subsiding as a consequence of the influx of meltwater after the last glaciation. Maximum glacial isostatic uplift of the central part of Scandinavia is about 8 mm/yr, while the hydro-isostatic subsidence is around 1.5 mm/yr.
We can therefore conclude that relative sea level changes are not the same as eustatic changes. With this in mind, it is difficult to imagine where a eustatic change can be measured realistically.
What about the future?
Predictions of future sea level changes for specific locations are compelled to account for glacial-isostasy and hydro-isostasy, as well as other physical processes. A model recently published in Science presents a correlation between global temperature and global sea level rise, predicting that sea level will rise by approximately 1m along the Norwegian coast over the next 100 years. Following the arguments presented here, it is obvious that a model projecting sea level changes simply by correlating with temperature is too simple.
It must be fully understood that projections of future relative sea level changes at specific locations presuppose an understanding of the physical processes that operate. While hydro-isostasy operates globally, glacial-isostasy is important in Scandinavia and North America, including geoidal changes. The crustal response to the glacial and sea water load depends on the rheological properties. Thus sea level projections have to take into account regional as well as local variations in crustal properties around the globe.
I strongly appeal for caution regarding prognoses for future sea level changes at specific locations, unless all the physical processes which may be involved have been accounted for.