Ancient sea level rises may have been fairly minimal

A study of shorelines in the Bahamas may change estimates of past sea levels: Scientists survey coastal rocks formed there when sea levels were higher. Image: By courtesy of Blake Dyer.

Maybe ancient sea level rises were not so dramatic. But they’d still have been pretty frightening.

LONDON, 12 August, 2021 − Earth scientists have measured the rising tides of a warmer world more than 100 millennia ago and found a glimmer of good news: ancient sea level rises during a warm spell in the last Ice Age were quite possibly only about 1.2 metres higher than they are today.

Since, between 128,000 and 117,000 years ago, the world was perhaps as much as 2°C warmer than it would become for most of human history, this really is encouraging. Right now, climate scientists project a rise of somewhere between 60cm and 1.5m later this century, as global temperature levels rise 2°C or more above those normal before the Industrial Revolution.

But until now, geological orthodoxy has proposed that during the last “interglacial” or sudden warming, sea levels rose by six metres or possibly even nine metres. This could only happen if the Antarctic or Greenland ice sheets had collapsed.

And although these are indeed already losing ice at an accelerating rate, it doesn’t seem possible for such a colossal quantity of ice to melt in only a handful of decades.

Missing factor

So there was a mismatch between the predictions of the world’s scientists and the apparent evidence from the past.

Now a new study in the Proceedings of the National Academy of Sciences offers a solution: calculations about past sea level heights may have been perhaps too gloomy because they did not fully factor in sea level’s other great uncertainty — the movement of the continents lapped by the sea.

This bedevils all predictions about sea level rise. Seas rise and fall with global temperatures, but so do landmasses. Right now, although sea level is creeping up at a rate measured in millimetres per year, the land under a number of great coastal cities is sinking dramatically, as humans build  ever more massive cities and abstract ever more groundwater. So predictions warn that millions could be at risk of coastal flooding.

But there is another, deeper reason for the uncertainty: as rising temperatures remove the massive burden of ice from glaciated land, and wind and rain erode mountains, so the subterranean rocks in the Earth’s mantle, far below the crust, respond by inching upwards. Even the seemingly solid rocks are elastic, subsiding under pressure and rising when the mass is removed.

“Models of ice sheets are still in their toddlerhood”

All this means that, unless researchers can make an accurate estimate of land movement as well, sea level estimates are riven with uncertainties.

So a team from Columbia University in the US has looked at evidence of sea level rise and fall preserved in fossilised reefs and dunes in just one 1200km chain of islands − the Bahamas in the Atlantic − to come up with a new set of projections.

In the next 100 years, sea levels will rise by about 1.2 metres. This could be too modest: sea levels could just possibly rise by perhaps 5.3 metres, but this doesn’t seem likely. And a nine-metre rise is highly improbable.

“To get to nine metres of sea level rise, you’d have to melt large parts of Greenland and Antarctica,” said Blake Dyer, of the university’s Lamont-Doherty Earth Observatory.

Tricky calculation

“This suggests that didn’t happen. So maybe we should feel not so bad about the future. On the other hand, our lower estimate is bad, and our upper one is really bad.”

At the heart of the puzzle is a phenomenon known to geophysicists as isostasy: vast tracts of continental landmass have been heaving up and down, imperceptibly, over periods of tens of thousands of years, in response to ice and erosion.

So calculating sea level rise and fall when the thing on which sea level measurements are recorded − the land − is itself always shifting becomes tricky. That has always been why climate projections of sea levels contain a range of forecasts, rather than a hard number.

The argument is that changes recorded along the north-south lie of the Bahamas would provide a new and more sophisticated way of reconstructing sea heights in the relatively recent past.

Melting not guaranteed

The study doesn’t settle the question: estimates of past sea level change on a dramatic scale come from many parts of the planet, and glaciologists still have to reconstruct the rate at which the northern ice, for instance, may have retreated while the southern ice cap continued to advance during the last interglacial: that too would have limited sea level rise.

“This is still a question. Models of ice sheets are still in their toddlerhood,” said Maureen Raymo, director of the Earth Observatory and a co-author.

Human carbon emissions are now heating the globe far more rapidly and evenly than during the last interglacial, so there is no guarantee of any melting at different rates in two hemispheres

“That makes it more difficult to apply the results to today. The easy thing to say would be, ‘Oh we showed that sea levels were not so bad, and that’s terrific.’  The harder answer, the more honest answer, is that maybe things were different then, and we’re not in the clear.” − Climate News Network