A New Zealand-led study published in Nature today traces thousands of years of sea level rise and suggests that if countries miss their Paris targets, up to a third of the Antarctic ice sheet could melt, eventually. The findings rely on an astonishing series of geological strokes of luck, which, over millions of years, combined to preserve layers of marine mud and sand in what, today, is the Whanganui basin.

Around five million years ago, an almost unbelievably fortunate string of events started to unfold off the coast of New Zealand, somewhere between what is now Whanganui and modern-day Taihape.

Back then, the area wasn’t covered in farmland and cities – it was under the sea, and it was near a tectonic plate boundary, just as it is today.

The Whanganui basin, as the area is known, began sinking when the tectonic plate it was on moved slowly downwards. It happened to subside at just the right rate to preserve all kinds of information -- information that would turn out to be crucial in the 21st Century, for all the wrong reasons.

As the basin was sinking, sediment was eroding from the mountains and land above it and accumulating on the sea floor. Normally, these layers of mud and sand would have been eroded by waves at the surface, but the seafloor happened to sink at the same pace it was being replenished, keeping the new layers of sediment safe beneath the ocean. There, particles were moved around, as waves from the Tasman Sea crashed onto the west coast of New Zealand.

The flow of new sediment arrived just fast enough to keep the seafloor in a sweet spot – had it been deeper, the sediment would have accumulated too far below the surface to be affected by waves. As it was, however, the sinking and the sediment accrual remained in balance for millions of years.

In a final boon to future scientists, though not to the creatures alive at the time, a series of earthquakes then lifted a 5km-thick chunk of the seafloor out of the ocean and up to become what is now part of the western North Island, from the Ruahine ranges in the east to the coast near Whanganui.  

Millions of years later, paleontologists and climatologists would be stunned to find that -- sitting under farmland, or jutting up spectacularly in cliffs along the Rangitikei river – there was a beautifully-preserved sediment record from the warm part of the Pliocene epoch, around three million years ago. They didn’t even need to drill, or charter boats, to find it.

This ancient epoch is suddenly, urgently interesting to climate researchers, because temperatures were about 2C to 3C hotter than they are today, and carbon dioxide concentrations were similar to today’s levels.

The latest clue to what these conditions might mean for sea levels has been revealed in a study published in the journal Nature today, by scientists from Wellington, Hamilton, the Netherlands, the United States and Chile.

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For the study, GNS Science’s Georgia Grant traced the rise and fall of past sea levels through the layers of ancient North Island rock. She found that sea levels regularly fluctuated by on average 13m, and up to 25m, during the warm parts of the Pliocene epoch.

The magnitude of the biggest changes shows that up to a third of Antarctica’s ice sheets must be capable of melting at temperatures not much above the Paris Agreement’s goal of 2C warming, the study's authors concluded. During the highest sea levels, not only was the ice gone from the more vulnerable, West Antarctica -- the parts of East Antarctica that sit below sea level had also succumbed to melting.

Tim Naish of Victoria University’s Antarctic Research Centre, who was involved in the study, says the sediment record suggests that a “near miss” of the Paris target could unlock widespread melting that would unfold over thousands of years.

The study doesn’t alter the IPCC’s more immediate sea level rise projections of around 1m by 2100. (Or less than 1m, if greenhouse emissions are cut quickly, or possibly more if polar melting outpaces projections).

But once the widespread melting was unleashed, it might not be stoppable even if emissions were halted.

Grant says, once temperatures cross a threshold, melting of the marine portions of the ice sheet could carry on regardless of emissions. “Our new study supports the idea that a tipping point may be crossed, if global temperatures are allowed to rise more than two degrees, which could result in large parts of the Antarctic ice sheet being committed to melt-down over the coming centuries,” she said in a statement when the study came out.

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Already, the ocean has absorbed 90 percent of the heat humans have added to the climate system, with a disproportionate amount of the warming going into the Southern Ocean. The Southern Ocean is the body of water surrounding the Antarctic ice. The vulnerable portion of the ice – the part that sits below sea level -- holds the equivalent of about 20m global sea level rise.

Naish says other physical records, independent of the Whanganui basin one, had already concluded seas could rise by 20 or so metres, after widespread melting of Antarctica. Drilling through rock in Antarctica had shown that some of East Antarctica could melt in certain conditions, as well as West Antarctica. (One bright spot is that all the research so far suggests we will not lose the whole of Antarctica, which holds 60m of sea level rise.)

Grant’s work adds another piece of independent evidence to the annals, and shows how widely sea levels varied during the Pliocene.

She made her findings by studying the varying sizes of sediment particles in the ancient layers.

Because it takes stronger wave action (from waves pushing against a shallow sea floor) to move bigger particles of sand, Grant tracked the long-term shifts in sea levels by studying differences in the particles. When seas were deeper, the resulting weaker wave action dropped finer sediment on the sea-bed, while, when seas were shallower, stronger waves carried and dropped bigger particles.

To some people the records might look like just a massive slab of mud, she says, but “when you start to measure the changes in grain size, you can start to see these high-resolution cycles.”

“You can tell what was happening above – was it shallow, active, churning water depositing big grains or was it calmer, deeper water, depositing small grains,” she says.

Aside from confirming how high seas have gone, Naish says the findings also have implications for researchers who are using computer models to make future sea level projections.

Ice sheet modelers test their virtual models against physical records of past sea levels, to see how accurately they can simulate real changes that have happened.

One modeler, Victoria University’s Nick Golledge, who was not involved in the study, says the findings are in line with existing model results showing long-term retreat of the ice sheets at current carbon dioxide levels. Golledge also said the rate of sea level change that Grant observed matched models’ projections of 1m sea level rise by 2100.

The study may not reveal much new about the next 100 years on Earth. But Naish says the findings suggest what may happen over thousands of years, because of today's decisions. “It is more about stewardship of the planet,” he says. “If we achieve Paris, not only do we avoid nasty effects in short term, we don’t end up with tens of metres of sea level rise (in the long-term).”

As well as an Antarctic researcher, you might say Naish was a Whanganui basin enthusiast.

As he describes it, the findings are also a reminder of the extraordinary, lucky series of happenings that gave New Zealand its unusual, easy-to-access record of ancient seas.

“It’s one of the highest-resolution records we have of the past 5 million years,” says Grant. “As you walk down a river, you can walk down 5 million years of history.”

“I tell people it’s world famous in New Zealand," says Naish, "but it’s actually world famous in the world!”