Today, I present a simple primer on world sea-level change through geological time [for the full copyright details of the graphs reproduced or hyper-linked, see the Footnote*]. How we need such a perspective:

(a) Sea-level is never stable, globally, regionally, or locally. It is always rising or falling, sometimes both, with movements in either direction changing ecologies and, during human times, economies. The causes of sea-level change are highly complex and multifarious, and include a wide range of geological factors, the thermal expansion and contraction of the oceans, and the altering of the mass balance of land ice;

(b) Over the last 500 million years, during what we call the Phanerozoic, sea-levels have varied by over 400 m, with present-day sea-levels remaining lower than at any time since the Triassic Period (251 to 199 Ma), around 240 million years ago [see graph here]. World sea-levels were at their peak during the Ordovician Period (488 to 443 Ma), especially in what is known as the Tremadocian (488.3 ± 1.7 to c. 478.6 ± 1.7 Ma), when marine transgressions were the greatest for which there is evidence preserved in rocks;

(c) The graph above (top) presents sea-level change over the last 22,000 years, since the peak of the most recent glacial episode [the vertical axis is in (m)]. Since the ‘Last Glacial Maximum’, sea-level has risen by over 120 m, with a significant meltwater pulse (known as ‘Meltwater Pulse 1A’) from deglaciation at 14.7 - 14.2 thousand years ago [to see a larger version of this graph with full axes, go here];

(d) The following graph, below, shows sea-level change at a more detailed level over the last 9,000 years, during what we call the Holocene:

Sea-level continued to rise rapidly until around 7,000 years ago, when the rate of change significantly slowed. Overall, sea-level during the Holocene has risen by more than 14 m, a fifth of the change taking place more erratically, and more slowly, during the last 7,000 years;

(e) This slow, uneven sea-level rise has continued during the last 150 years, as can be seen here, which has witnessed a rise of around 20 cm. This probably represents a very tiny spurt following the end (c.1880) of the period known as the ‘Little Ice Age’;

(f) The change in global mean sea-level predicted by a number of ‘global warming’ models, following ‘business-as-usual’ emissions scenarios and acknowledging the full uncertainty bar, ranges from 10 cm to 80 cm during the next 100 years (see here).

Thus to summarize: seal-level always changes; in geological terms, sea-level remains at its lowest for the last 240 million years, despite a rise during the last 22,000 years of around 120 m; current sea-level is rising in cms per hundred years, and this trend is likely to continue, with or without ‘global warming’, although it may slow if we enter a new cooling phase; any bigger changes will be in millennia; regional and local effects of sea-level change will remain complex, varying as described in a previous posting: ‘Ups and Downs of Sea-Levels’ (March 7).

I believe no further comment is necessary.

There is nothing like a bit of perspective, is there?

[*Footnote re graphs: the beautiful graphs presented here (as well as those for which only links are provided) are all from the excellent ‘Global Warming Art’ Website, and were made by Robert A. Rohde, using published data. The graphs are reproduced under the GNU Free Documentation License Version 1.2.]