How Rapidly Can Sea Level Rise?

New evidence from caves in Majorca, Spain indicates that Earth’s natural climate cycles could have produced big, fast changes in sea level within the last 100,000 years.

The findings, which were published in January in Science, contradict the generally accepted recent history of sea level as a gently oscillating curve of highs and lows. The researchers found evidence that, at least once, sea level rose much faster and peaked 15 to 30 meters higher than previously thought.

“Both before 81,000 years ago, and soon after, we had dramatic changes in sea level,” said Bogdan Onac of the University of South Florida, one of the paper’s authors.

The question, say scientists, is whether the caves in Spain tell a global story. If they do, it’s an indication that the Earth’s glaciers can grow and shrink rapidly, causing big changes in sea level in relatively short periods of time. “You could see a change of one meter in fifty or sixty years,” said Onac.

Scientists use evidence of past sea levels and atmospheric temperatures to make predictions about the future. Current projections for the next century, based on available data, range from a rise of 0.2 to 0.6 meters, according to the 2007 report by the United Nations’ Intergovernmental Panel on Climate Change — a mere fraction of the rise 81,000 years ago that Onac found evidence for.

“We have to look to the past to learn more about melting ice sheets and the resulting sea level rise,” explained Andrea Dutton, a geologist at the Australian National University in Canberra who studies past sea levels. If we want to be able to predict how quickly ice sheets will melt as Earth continues to warm over the next century, said Dutton, we have to understand how glaciers have responded to changes in temperature and carbon dioxide in the past.

The caves in Majorca represent one of the few places on Earth where scientists can find direct evidence of past sea levels. Limestone caves form when water moves through rock, dissolving minerals and leaving open space in its wake. The dissolved minerals re-solidify in places where the water drips and evaporates, locking in trace amounts of radioactive material as they harden. Over time icicle-like structures — stalactites and stalagmites — form throughout the cave.

Because they form only in the presence of water, these structures act almost like rings on a bathtub, indicating the highest point of sea level at various times in Earth’s past. And they carry their own timestamp: the radioactive clock of uranium decaying to thorium.

But unlike a porcelain bathtub, the Earth doesn’t sit still as water levels fluctuate. Its surface moves up and down in response to tectonic forces from within, and the heavy load of glaciers weigh Earth’s rocky crust down, causing it to sag into the softer layer underneath. When the ice load is removed, the surface springs back over thousands of years of slow-motion rebound.

All this movement means that different places on Earth sometimes tell different stories about the rise and fall of oceans — and this makes some scientists wary of the new evidence from Spain. “The idea that one site can fundamentally change our ideas about sea-level … I don’t think that’s a very strong scientific argument,” said Mark Siddall, a glaciologist at the University of Bristol in the United Kingdom. Instead, said Siddall, there are other, more robust ways of reconstructing the rise and fall of oceans.

Siddall argues that deep-sea sediment cores offer a more accurate record of sea level. The cores contain oxygen isotopes that can be used to estimate the amount of ice on Earth millions of years ago, and unlike evidence from caves or coral reefs, which provide snapshots from specific times, the marine cores provide continuous records.

What the cores don’t contain, pointed out Dan Muhs of the United States Geological Survey, is evidence of an absolute age — they can’t be dated using radiometric techniques. If the cave evidence provides a time-stamped snapshot of past sea levels, the marine sediments are more like a reel of film on which no one recorded a date.

The marine core records are matched up to periods in Earth’s history based on regular, cyclical changes in Earth’s orbit, and then “tuned” to data from specific locations — but they agree better with evidence from some locations, like the Bahamas and New Guinea, than others. Muhs said other findings from California, Bermuda, and the eastern seaboard of the United States agree more closely with the Majorca study.

Some geologists suggest that the best approach is to incorporate all three factors — marine cores, estimated changes in the Earths’ crust, and evidence from locations — into a model to get an overall picture. These models already exist, said Dutton, and the evidence from Majorca should be analyzed using them. “I’d like to plug this new data point into the model and have a look,” said Dutton.

Muhs says more studies like the one in Majorca are needed in order to learn more and build better predictions for ice sheets in Greenland and Antarctica. The question we should be trying to answer, said Muhs, is, “What can we learn about the past that could tell us something about what’s going to happen in the future?”