Atlantis and Paleogeography
Global Sea-Level Change

This diagram shows two different but similar curves of sea-level change. The are both derived from dated samples of coral, and what they show is really the relative sea-level change on one location. This is an effect of the combination of, mainly, eustatic effects and isostatic effects.

Eustacy and Isostacy

Eustatic effects are those caused by global sea-level change. If the sea level rises and nothing else changes, there will be transgression, and if it falls, there will be regression.

Isostatic effects are those caused by changes in elevation of the land itself. If the land level rises and nothing else changes, there will be regression, and if it falls, there will be transgression. The isostacy can be caused by the accumulation or melting of glaciers, by bending of the earth's crust beyond land ice sheets, by changes in sea level, and by global adjustments to these changes. These effects affect the details of the sea-level changes in many parts of the world.

Transgression and Regression

Transgression is when land is lost to the sea, and regression when land is gained from sea floor, whether the cause is isostatic, eustatic, or a combination.

A global sea-level rise (eustatic rise) does not necessarily lead to a global transgression. For instance, many coasts in Scandinavia are still rising quickly, up to 9 mm/yr, due to lingering isostatic effects from the last ice age. This is known as "landhöjning" ('land-raising') in Swedish; when the regression was detected in the 18th century it was thought that the water in the oceans was dissappearing, and the phenomenon was initially labeled "vattuminskning" ('water-diminishing').

Coastal Geomorphology

When there is transgression or regression in an area, the net effect is not simply that the shoreline moves. The geomorphological processes are quite strong just at the shoreline. Waves contain a large amount of energy, and it is expended in a narrow zone at the shoreline. In cold climates also drift ice and perhaps even permafrost come into play, and in warm climates bioherms (e.g., coral reefs) and chemically precipitated sand, ooids, play a role. At river mouths the deposition pattern changes with changes in the baseline.

All these factors contribute to the effect that the global eustatic effects have in a particular area. Some of them alleviate the effect, others make it worse.

It must be said, though, that if infrastructure is built on a beach, then one must expect that the waves wash over. Beaches are formed by waves, so waves on a beach are as normal as snow on Greenland. In a transgression a natural beach can move and adjust with the transgression. The problem only arises if the natural response is prevented from taking place. The problem is then not the transgression, but man's interference with the natural state of affairs.

A point in case are the so-called outer banks on the US east coast. They are not islands with a beach; they are beaches. Just like all of floodplains and deltas sooner or later get flooded, all of beaches sooner or later get flushed over by waves. Incidentally, Lindorm is developing an erosion mitigation device in the form of a wave damper as an environmental-friendly way to alleviate the problem.

Future Change

It has been predicted that the sea level will rise globally due to global warming and climate change. The rise has been predicted to be in the order of millimeters per year. Earth scientists who have studied global sea level change have pointed out though, that it would take about half a century to even detect such a change with traditional methods, since the signal-to-noise ratio is very small.

In the past the world experienced much larger rates of change (it's worth considering Atlantis seriously in this context; see this hypothesis). There were, hypothetically, very real catastrophes. But let us keep things in perspective; while a large change will be disastrous, a small will not. A change that can not even be detected until in 50 years does not even begin to compare with the events that caused the hypothetical disaster(s) that appear in Plato's tale.

Eustatic global sea level change

Two sea-level change curves from 20 kBP to present. Both are derived from coral samples from locations in the Caribbean area thought to be tectonically rather stable. Fairbank's curve has been recalibrated from 14C to calendar years for this graph. The maximal regression during the last ice age, estimated at -125 m, occurred about 25,000 years ago during the Late Glacial Maximum (LGM). The first sudden transgression is called Melt-Water Pulse 1a (mwp-1a) and it is universally accepted. The last rise is known to have taken place but its size and rate can be debated. The middle rise, at the ending of the ice age 11,600 years ago, is perhaps the least supported by other data. The latter two sudden rises appear prominently in this hypothesis for an historic background to Plato's Atlantis tale.
References
Blanchon, P., & Shaw, J., 1995: Reef drowning during the last glaciation: evidence for catastrophic sea-level rise and ice sheet collapse. Geology, 23:4-8.
Fairbanks, R.G., 1989: A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature, 342:637-642.

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