A new study of past sea level changes spanning 35,000 years shows that during the period from 6,000 years ago until the mid-19th century there is no evidence of sea level changes over periods of longer than 200 years and no evidence of changes that exceeded 20cm.
For comparison, sea levels have risen by around 20cm since the start of the 20th century. This is due both to fresh water run-off from the land, manly glacial melt water, and also to the thermal expansion of the oceans both of which are associated with global warming.
The new research is published in the prestigious US journal the Proceedings of the National Academy of Sciences (PNAS). Around 1,000 measurements of ancient sea levels derived from sediment samples were used by the researchers to build a history of sea level change during the past 35,000 years. These measurements were taken away from ice margins.
The research team led by Kurt Lambeck of the Australian National University found that sea levels fell rapidly by around 40m within 2,000 years at the start of the last period of glaciation (ice age).
Their research paper, entitled “Sea level and global ice volumes from the Last Glacial Maximum to the Holocene”, reported that as the ice age ended and the great ice sheets melted, sea levels rose at a steady rate of around 12m per 1,000 years, punctuated by bursts of more rapid rise.
However, the researchers also found that sea levels have remained constant from around 2,500 years ago until around 150 years ago when the current rise began. And there is “no evidence” of oscillations exceeding 15–20 cm in time intervals of less than 200 year from 6,000 years ago until 150 years ago, they report in their paper.
This would make the current rate of sea level rise as unprecedented in the last 6,000 years.
PNAS describes the significance of this research as: Several areas of earth science require knowledge of the fluctuations in sea level and ice volume through glacial cycles. These include understanding past ice sheets and providing boundary conditions for paleoclimate models, calibrating marine-sediment isotopic records, and providing the background signal for evaluating anthropogenic contributions to sea level. From ∼1,000 observations of sea level, allowing for isostatic and tectonic contributions, we have quantified the rise and fall in global ocean and ice volumes for the past 35,000 years. Of particular note is that during the ∼6,000 y up to the start of the recent rise ∼100−150 y ago, there is no evidence for global oscillations in sea level on time scales exceeding ∼200 y duration or 15−20 cm amplitude.
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