Dramatic Sea-Level Changes

In the past, the oceans have swelled and contracted dramatically on shorter time scales than previously known, according to a new study that may help researchers envision sea-level changes in the next few decades and beyond.

Sea surface heights varied by as much as 98 feet (30 meters) within just a few thousand years, scientists report in the April 15 issue of the journal Science. Above, sea cliffs exposing the last interglacial coral deposits are visible at the northern tip of Barbados.

Researchers already had solid evidence for changes of up to 328 feet (100 meters) over 100,000-year glacial cycles, oscillations driven mostly by fluctuations in Earth's orbit. When more of the planet's water is frozen, it remains locked near the poles and sealevels are lower.

The shorter-term variations have been harder to pin down.

To gain the new insight, scientists looked at corals, which prefer shallow water. As the sea rises, corals grow atop one another, creating a terrace. A new method allowed for more precision in dating the layers in these terraces.

The terraces reveal sealevel fluctuations equal to the height of a six- to 10-story building that repeated every 3,000 to 9,000 years. That's too frequent to be explained by changes in the planet's orbit. The fluctuations took place between 130,000 and 90,000 years ago.

The research was done by William Thompson and Steven Goldstein of the Lamont-Doherty Earth Observatory.

Though scientists admit they don't know for sure what could cause such significant short-term changes, they think it is not just a question of how much water is in the oceans. Water temperature plays a big role. Water expands as it warms, and other scientists have suggested that the present warming of Earth's climate will cause the seas to expand. Because ocean temperatures change relatively slowly, the expansion -- which means seas will rise -- will play out over a few centuries.

"Most of the ... sea-level rise expected by 2100 will be due to heating of the oceans," says Gideon Henderson, a University of Oxford researcher who wrote an analysis of the new study for Science. "On millennial timescales, temperature change extends to the deep ocean and might cause several meters of sea-level change."

-- Robert Roy Britt

Image Credit: William G. Thompson