Timing a Rise in Sea Level

The fast-retreating Sheldon Glacier in Antarctica. A collapse of a polar ice sheet could result in a jump in sea level.

Thirty-five years ago, a scientist named John H. Mercer issued a warning. By then it was already becoming clear that human emissions would warm the earth, and Dr. Mercer had begun thinking deeply about the consequences.

His paper, in the journal Nature, was titled “West Antarctic Ice Sheet and CO2 Greenhouse Effect: A Threat of Disaster.” In it, Dr. Mercer pointed out the unusual topography of the ice sheet sitting over the western part of Antarctica. Much of it is below sea level, in a sort of bowl, and he said that a climatic warming could cause the whole thing to degrade rapidly on a geologic time scale, leading to a possible rise in sea level of 16 feet.

While it is clear by now that we are in the early stages of what is likely to be a substantial rise in sea level, we still do not know if Dr. Mercer was right about a dangerous instability that could cause that rise to happen rapidly, in geologic time. We may be getting closer to figuring that out.

An intriguing new paper comes from Michael J. O’Leary of Curtin University in Australia and five colleagues scattered around the world. Dr. O’Leary has spent more than a decade exploring the remote western coast of Australia, considered one of the best places in the world to study sea levels of the past.

The paper, published July 28 in Nature Geoscience, focuses on a warm period in the earth’s history that preceded the most recent ice age. In that epoch, sometimes called the Eemian, the planetary temperature was similar to levels we may see in coming decades as a result of human emissions, so it is considered a possible indicator of things to come.

Examining elevated fossil beaches and coral reefs along more than a thousand miles of coast, Dr. O’Leary’s group confirmed something we pretty much already knew. In the warmer world of the Eemian, sea level stabilized for several thousand years at about 10 to 12 feet above modern sea level.

The interesting part is what happened after that. Dr. O’Leary’s group found what they consider to be compelling evidence that near the end of the Eemian, sea level jumped by another 17 feet or so, to settle at close to 30 feet above the modern level, before beginning to fall as the ice age set in.

In an interview, Dr. O’Leary told me he was confident that the 17-foot jump happened in less than a thousand years — how much less, he cannot be sure.

This finding is something of a vindication for one member of the team, a North Carolina field geologist, Paul J. Hearty. He had argued for decades that the rock record suggested a jump of this sort, but only recently have measurement and modeling techniques reached the level of precision needed to nail the case.