The threat of sea-level rise remains one of the greatest global concerns about climate change, and scientists are still improving their predictions of how much — and how quickly — the world's oceans may rise. To help answer those questions about the future, some researchers are looking into the past.

New research has provided one of the most detailed looks yet into the patterns of sea-level rise that occurred during the world's last major warming period, more than 10,000 years ago. The study, published yesterday in Nature Communications, suggests that during this time water rose rapidly, in punctuated bursts, rather than gradually over time. It was likely driven by uneven pulses of meltwater from the world's collapsing glaciers.

The researchers suggest these past events could be viewed as a kind of “analog” for the future — a warning of the events that could yet come under future climate change.

“It's not exactly the same situation,” acknowledged André Droxler, a professor of marine geology at Rice University and one of the study's authors, in an interview with E&E News. Present-day warming is being driven not by natural processes, but by carbon emissions from large-scale burning of fossil fuels, an unprecedented event in the Earth's history.

But the researchers suggest there may be similarities between the collapse of ice sheets thousands of years ago and the destabilization of the world's ice sheets in the future.

“We still have plenty of ice volume to be melted,” Droxler said. “We know that the Greenland ice sheet is melting, the western Antarctic ice sheet is melting. And so I think our study, during this time of well-established global warming, could become a great analog for where we are living and where we will be living the next few centuries.”

In order to look back so many thousands of years into the past, the researchers turned to a surprising source of information: fossilized coral reefs, located just off the coast of Texas.

The area is known for its beautiful living reefs, including the Flower Garden Banks National Marine Sanctuary. But the corals that interested the researchers have been dead for more than 10,000 years, drowned and now submerged nearly 200 feet below the surface of the water.

Fossilized corals can contain all kinds of useful scientific information about the ancient world. Sampling their preserved bodies can yield data on past temperatures and ocean chemistry — and because many coral species can only grow at certain depths, typically close to the surface, the location of their remains can tell scientists what the water levels there used to be like.

With this in mind, Droxler and a group of colleagues, including lead study author Pankaj Khanna, a Ph.D. candidate at Rice University, set off on the Schmidt Ocean Institute's research ship, the Falkor, to investigate the site. They used a special sonar system to create high-resolution 3-D maps of the dead reefs on the seafloor — and in the process, they discovered an intriguing pattern.

The fossil reefs were arranged along the seafloor in a series of six stairlike shelves, or terraces — a classic signal of past sea-level rise. As water levels rise, corals must scramble backward toward the retreating shoreline in order to stay close enough to the surface to survive. In the process, they produce a kind of skeletal, vertical shelf and then grow to fill in the space closer to the shoreline behind it. That forms a coral terrace. There they remain until the next period of rapid sea-level rise, when they produce another shelf and move backward again.

The next step was to figure out how old the terraces are. To do so, researchers matched the terrace depths to a global sea-level curve, a kind of general estimate of sea-level changes throughout geological history.

The results suggested the terraces had arisen during a period between 12,000 and 14,000 years ago, before the rate of sea-level rise finally overcame them and they died. This time coincides with a warming period following the end of the last ice age, when massive amounts of ice from the world's glaciers melted and poured into the sea.

Scientists already knew this was a time marked by significant sea-level rise. But the existence of the terraces suggests, for the first time, that the process was not gradual, but rather occurred in sharp, sudden bursts. In fact, the terraces suggest several meters of sea-level rise may have occurred on the scale of just decades during this time.

“The study is important because people had not thought of this before, that these kinds of small-scale events would be so common during a warming world,” said Khanna, the study's lead author.

The researchers suspected these spurts of sea-level rise were driven by pulses of meltwater from destabilizing glaciers. So they compared their estimates to an existing record of the ancient climate in Greenland, which was created using ice core samples from the ice sheet.

Even while the ice age was ending and the planet was heating up, the ancient climate was still experiencing fluctuations in temperature — and the researchers found that all but one terrace corresponded with warm periods, which were likely accompanied by sudden influxes of meltwater into the sea.

“This one exception is probably because our timing is not perfect,” Droxler acknowledged. In the future, the scientists hope they may be able to conduct another expedition to drill physical samples from the dead corals, which will help them come up with more precise dates for each of the terraces.

In the meantime, the conclusions represent “a kind of interesting and new idea,” according to Andrea Dutton, a geology professor and paleoclimate expert at the University of Florida who was not involved with the new research.

“We've known for quite some time that there was at least one very rapid pulse of sea-level rise during the deglaciation between the last ice age and the present — we've known for a very long time that sea level can rise with a sudden jump related to the dynamics of the ice sheet,” she said. “What's new here is you can have a bunch of kind of smaller pulses that are spaced really closely together, and that's kind of the new idea that they're putting forward.”

And it's an idea that could hold dramatic implications for the planet's warming future. Scientists are still investigating the physical processes affecting the melting of the world's current ice sheets, and there's still a great deal of uncertainty about in what ways, and how quickly, they might react to future warming. But the idea that many rapid bursts of sea-level rise have occurred in the past, and could occur again, could be critical information for coastal communities as they plan for the future.

For now, scientists are placing increasing emphasis on the study of both the Greenland and Antarctic ice sheets to better understand the physical processes affecting the melting of glaciers. As Dutton pointed out, a great deal of the ice that contributed to sea-level rise at the end of the last ice age — much of which existed in the midlatitudes — has now completely melted away, and it's unclear whether the world's remaining ice sheets will respond to human-caused climate change in the same ways.

“While we're not sure, it kind of opens up the door to this possibility that as ice sheets retreat quickly, maybe they do so in a stepwise fashion,” she said. “But it's not clear that the mechanisms will be exactly the same between those two ice sheets, and that's something to be determined.”
