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Historical sea level CHANGES

Long term

Introduction

A very brief history of sea level:

  • Over the last 140,000 years sea level has varied over a range of more than 120 metres. The most recent large change was an increase of more than 120 metres as the last ice age ended
  • Sea level stabilised over the last few thousand years, and there was little change between about 1AD and 1800AD
  • Sea level began to rise again in the 19th century and accelerated again in the early 20th century
  • Satellite altimeter measurements show a rate of sea-level rise of about 3 mm/year since the early 1990s - a further increase in the rate

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The last 140,000 years

Sea level varied by over 100 metres during glacial-interglacial cycles as the major ice sheets waxed and waned as a result of changes in summer solar radiation in high northern hemisphere latitudes. Paleo data from corals indicate that sea level was 4 to 6 m (or more) above present day sea levels during the last interglacial period, about 125 000 years ago. Climate and ice-sheet model simulations indicate that Greenland was about 3C warmer than today and that the Northern Hemisphere ice sheets contributed 2.2 to 3.4 metres to the higher sea level, with the majority of the rise coming from the partial melting of the Greenland ice sheet.

During the last ice age, sea level fell to more than 120 metres below present day sea level as water was stored in ice sheets in North America (Laurentian, Cordilleran), Greenland, northern Europe (Fennoscandia and the Barents region) and Antarctica. As the ice melted, starting around 20 000 years ago, sea level rose rapidly at average rates of about 10 mm per year (1 m per century), and with peak rates of the order of 40 mm per year (4 m per century), until about 6000 years ago.

Sea level over the last 140,000 years

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The last few thousand years

Sea level rose much more slowly over the past 7,000 years. The sea level 2,000 years ago can be deduced by (for example) examining fish tanks built by the ancient Romans. Because the tanks had to be at sea level for the sluice gates to function, we can precisely estimate sea level during the period of their use. Comparison of this level with historical records indicates that there has been little net change in sea level from 2000 years ago until the start of the 19th century.

A number of other data types are used to estimate sea level over preceding ages. Some of them are:

  • Geological - e.g. raised beaches, wave-cut shelves, transgressive sequences
  • Biological - e.g. shells, tree stumps, corals, salt marshes
  • Man-made - e.g. Ancient Roman fish tanks, Crusader wells in the Palestine, middens

Plot of global sea level over the last 3,000 years

Ancient Roman fish tanks

Ancient Roman fish tanks ("Piscinae") give an accurate indication of sea level in this area around 2,000 years ago. From Kurt Lambeck, Australian National University

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Website owner: Neil White | Last modified 11/03/09


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