Ancient Rome's fish pens confirm sea-level fears

09:30 16 August 04

Coastal fish pens built by the Romans have unexpectedly provided the
most accurate record so far of changes in sea level over the past
2000 years. It appears that nearly all the rise in sea level since
Roman times has happened in the past 100 years, and is most likely
the result of human activity.

Sea-level change is a measure of the relative movement between land
and sea surfaces. Tide-gauge records show that the sea level has
been rising 1 to 2 millimetres a year since widespread measurements
began around 1900, but do not pinpoint when the trend started.

Earlier sea levels can be estimated from geological data, but the
accuracy is limited to about half a metre, which is not enough to
precisely chart the history of sea-level rise.

So Kurt Lambeck of the Australian National University in Canberra
turned to fish pens on the Tyrrhenian coast of Italy for a more
accurate record of ancient sea level.


Ice age rebound


The Romans dug these fish pens into bedrock, and the water line in
these well-preserved structures shows that the sea level along the
Italian coast 2000 years ago was 1.35 metres below today's
levels. "They were used for only a very short time, so they make
rather nice markers," says Lambeck

He then analysed how land elevations changed along the Italian coast
due to both plate tectonics and the after-effects of the last ice
age. In a paper to appear in Earth and Planetary Science Letters, he
concludes that geological processes affecting land levels over the
past two millennia accounted for 1.22 metres of the change, which
means that the global sea level rose by 13 centimetres.

That is only about 100 years' worth of rise at the present rate of
around 1 to 2 millimetres per year, implying that nearly all of it
has occurred since 1900. While there is no proof that human activity
is to blame, "I can't think of a natural process that would have
started in 1900," he says.

The result "is a significant one", says Jonathan Gregory, who
studies global changes in sea level at the University of Reading,
UK. The finding supports the idea, based on the few tide-gauge
records that extend back two centuries, that the rise in sea level
did indeed accelerate about a century ago.

While Gregory cautions that this does not prove that global warming
is responsible, both he and Lambeck agree that the results fit the
rise in ocean volume expected from global warming melting glaciers
in the industrial age.


Jeff Hecht