Unlocking the mystery of rising seas

Climate Central

The crime under investigation happened more than 14,000 years ago – a cold case in more ways than one. It was a time when the great Laurentide ice sheets that covered vast stretches of North America had begun to melt, as the most recent Ice Age gave way to the current, relatively warm planet we now live on. At the glaciers' greatest extent, maybe 4,000 years earlier, so much water was locked in the ice that sea level was more than 300 feet lower than it is now. As the glaciers retreated, the ocean rose at a mostly stately pace.

But then, abruptly, the sea started rising a lot faster, in an explosive episode of ice loss known as MWP-1A (‘meltwater pulse 1A'). Then, after less than a thousand years, it stopped.

The whole thing would have no more than historical, or prehistorical, interest, except for one thing: the sea is rising again, and nobody is quite sure how fast or how high it will go. Thanks to heat-trapping, human-generated greenhouse gases, oceans have already gone up an average of 8 inches over the past century or so worldwide. In the US alone, that translates into a threat for millions of Americans. Sea level is bound to go higher, partly because water expands as it warms, but also because the ice that remains in Greenland and Antarctica is both melting and sliding into the ocean as it warms.

Scientists understand the expansion, but the melting and sliding is still poorly understood. As recently as 2007, the Intergovernmental Panel on Climate Change pretty much threw up its hands, acknowledging that “ice sheet dynamics,” or the movement of ice sheets on Greenland, mostly, but also on Antarctica, might respond to warming in unpredictable ways. Since then, new research has pegged the likely sea level rise by 2100 at another 2-to-3 feet – a pretty drastic amount – but it could be less than that. Or it could be significantly more.

That's why it's so important to understand how ice responded to warming in the past, and why a new study in Nature is potentially so important. The MWP-1A incident, it turns out, is like the crime that launches an episode of CSI, albeit with generally less glamorous characters and fewer special effects. Since the mid-1990s, there have been two theories of the crime. The first and most reasonable is that MWP-1A involved a sudden ramp-up in the disintegration of the Laurentide ice sheet. It was melting already; it would be almost entirely gone a few thousand years later. It was the prime suspect.

But under scientific interrogation, there turned out to be inconsistencies in the story. “If you delve into the records,” said Rutgers earth scientist Robert Kopp, who wrote a Nature commentary on the new study, in a phone interview, “and look at the state of the Laurentide ice sheet at the time, it's not clear that it could have been the source of the big burst.”

In the mid-90s, some sea level experts came up with an alternate theory: MWP-1 could have come in part from Antarctica, not the Northern ice sheets. And they proposed a crucial and ingenious test. Large masses of ice sitting on land exert a gravitational pull on the nearby sea, pulling the water closer and piling it up into a local sea level that's artificially high. When the ice disappears, that effect is gone, and sea level actually drops in the surrounding area, while rising further away. A great melting in the northern ice should leave a different pattern of sea level rise across the globe than a melting in the south.

In the new Nature paper, author Pierre Deschamps of the Université Aix-Marseille, in France, along with several colleagues, say they've found such a pattern: fossil corals newly excavated from Tahiti suggest that MWP-1 added 12-22 metres there, while corals from Barbados (where the record is less uncertain) show about a 20-metre rise. That pattern is consistent with Antarctic melting; if the great melting had come from the northern hemisphere, Tahiti should have seen significantly more sea level rise than Barbados. Antarctica– specifically West Antarctica, which has much less ice than the bigger East Antarctic ice – is the culprit. Case closed!

If the case were really closed, it might be cause for worry. Most ice-sheet experts think Greenland is going to be the greatest contributor to sea level rise over the next century. If West Antarctica is unstable enough to go into some sort of overdrive, that could add a wild card to the mix, meaning that current projections could be far too rosy. “We can't say this is going to happen again,” Kopp said, “but it's a risk we should keep in mind.”

It's a risk, though not a prediction, for a couple of reasons. One, says Kopp, is that if this much ice vanished from Antarctica abruptly 14,000 years or so ago, “you should be able to see where it happened, and nobody has found evidence of this large a retreat.” Second, even if Antarctica did spawn the great meltwater pulse, it happened at a time when the planet was in a different state than it is today – it was a lot colder, to start with. So what happened back then might not happen again.

But it might.

This article was originally published on Climate Central – www.climatecentral.org. Reproduced with permission.