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A ground-breaking study shows the likely impact of our carbon levels. By

Global study on SA coast warns of monster oceans

T he last time atmospheric CO2 matched present-day levels, Earth’s oceans were remarkably high. That revelation is a warning, according to ground-breaking international research on the Cape coasts.

Welcome to the Pliocene — an ancient proxy for a future world in which “big carbon” can rewrite global temperatures and expanding oceans can swallow liveable land.

To catch a glimpse of how such a hostile planet could become a reality for modern humans, a new study by the American Geophysical Union suggests we travel back three to five million years into the past.

Back then, the atmosphere’s carbon dioxide needle hovered around 400 parts per million, roughly the same as today – what’s more, these conditions churned up sea levels many metres higher than modern oceans, the paper shows.

Supported by a $4.25-million grant from the US National Science Foundation to cover a research period of five years, the analysis set out to see just how high sea levels could soar in a 400ppm world. The study was written by a group of researchers from US, German, Australian and South African institutions.

The scientists’ research zone was a 700km stretch of South African coast — between Hondeklip Bay in the west and Mossel Bay in the east — and their work involved hunting for features such as tide-sculpted rock outcrops and quarries holding fossil shells and marine invertebrates. Even whale bones were fair game.

Tracing the ghostly edges of the Pliocene ocean by car “or for hundreds of metres on foot”, the team stumbled upon impressive coastal plains, sometimes stretching inland for “10 or 15km”, according to lead author Professor Paul Hearty.

“So we’re screaming down the road at 100km/h — and all of a sudden I’m lying on the brakes,” said Hearty of the moment they hit paydirt near Bredasdorp in Western Cape.

“And, about 15m off the road, there’s this shell bed,” he smiled, “and the shells are still in their living position.”

According to the sea-level sleuths’ calculations, several storeys’ worth of water submerged the South African coastline — a region chosen by this study for its tectonically stable features.

A noteworthy Western Cape site at the mouth of the Olifants River in Papendorp delivered paleo-shorelines as high as 30m. These features are estimated to be 4.6 million years old.

This site “shows exactly how the sea level comes up”, explained Hearty, a marine geology and sea-level specialist with the Jackson School of Geosciences at the University of Texas in Austin.

At a younger age of three million years, discoveries at De Hoop/Bredasdorp yielded up to 28m heights; while Hondeklip Bay in the Northern Cape revealed a 33m-high fossil shoreline.

Rolling in at one million years, fossil shores at Olifants River, De Hoop/ Bredasdorp and Hondeklip Bay ranged between 15m and 20m. These date to the subsequent Pleistocene age.

What Hearty and his collaborators’ prodigious research into South African and other stable coasts such as the southeast US and Australia reveals, is that CO2 is a proxy for temperature — and, ultimately, sea-level rise.

To explain primordial oceans thundering into the South African interior, “you have to melt all of Greenland and about 25% to 30% of Antarctica”, said Hearty. Such a phenomenon could only be explained by rising temperatures melting ice caps, a process fuelled by CO2, of which we have plenty in Earth 2020’s atmosphere.

In June 2020, for instance, the needle at Hawaii’s Mauna Loa Observatory struck a record 418ppm — a waypoint not seen since the Pliocene.

The result of geological processes such as volcanism, weathered rock and buried organic matter pumping out high amounts of CO2, the Pliocene’s mercury was about 2°C to 3°C hotter than now, said coauthor Professor Maureen Raymo.

Yet humans, she warned, were also “geological agents” … adding CO2 to the atmosphere at “ten times the natural rate”.

Recently appointed as interim head of Columbia University’s Lamont-Doherty Earth Observatory — which popularised the term “global warming” — Raymo is a leading paleoclimate authority. She is widely considered one of the most influential scientists of her time.

The world has warmed by at least 1°C since 1880, say records by NASA’s Goddard Institute for Space Studies, previously headed for decades by the climatologist Professor

James Hansen, who has co-authored several climate papers with Hearty. And – without limiting global temperatures to well below 2°C above pre-industrial levels — sea levels could rise by about a metre by 2100, warns the UN Intergovernmental Panel on Climate Change. By 2400, business-as-usual would raise the level to more than 5m, and so on, placing at risk coastal communities across the globe.

What scientists could not necessarily predict, said Hearty, was how soon human activities would raise modern oceans.

However, the South African coastline study does serve as a robust example of why a version of the Pliocene could well lie in our future.

“Ice sheets are,” as the study puts it, “likely to experience dynamic changes when subject to climatic parameters of today”.

Raymo describes the current coronavirus pandemic as a practice run for the climate crisis.

“We see that here in the US,” — that is, compared with “countries that looked at the data, understood what it meant for the future and didn’t pretend it wasn’t going to happen,” Raymo said.

“Those are the countries and communities now doing quite well. So yeah, these are global problems that need global solutions based on the best science.” DM168

The team stumbled upon impressive coastal plains, sometimes stretching inland for 10 or 15km