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The last time humans came up against rising seas due to major global warming, they tried to protect themselves by putting up physical barriers and possibly appealing to divine powers to hold back the water.

Following the last glacial maximum 21,000 years ago, Earth warmed by about 3 to 5掳C over thousands of years, probably due to a slight change in its orbit that increased sunshine exposure. This melted ice sheets that once covered much of North America and northern Europe and raised global seas by about 120 metres to today鈥檚 levels.

There are no written accounts of this period, as writing was invented only about 5000 years ago. But Patrick Nunn at the University of the Sunshine Coast in Australia has found clues to how our ancestors coped with sea level rise in stories that have been passed down orally for hundreds of generations.

Ancestral stories

In Australia, for example, a quarter of the continent was swallowed by the rising ocean between 18,000 and 8000 years ago. At least 26 Aboriginal groups living in coastal areas have preserved stories of their ancestors鈥� experiences.

In one story told by the Wati Nyiinyii people of southern Australia, their ancestors tried to block the incoming water by stacking bundles of thousands of spears along the coastline. In a story told by the Gunggandji people of north-east Australia, their ancestors heated boulders with fire and rolled them down cliffs into the water.

The fact that so many groups have stories of sea level rise suggests it was transformational enough to warrant being described to hundreds of subsequent generations, says Sean Ulm at James Cook University in Australia. 鈥淎lthough there is scepticism that oral histories record events more than a couple of hundred years old, the evidence for Aboriginal oral histories documenting sea level rise more than 8000 years ago is compelling.鈥�

Efforts to prevent sea level rise weren鈥檛 limited to Australia. Seas continued to rise in the Mediterranean until about 6000 years ago. Divers off the coast of Israel recently discovered a stone wall built about 7000 years ago, seemingly to protect the ancient village of Tel Hreiz from flooding.

In France, the Carnac stones 鈥� parallel rows of stones along the Brittany coast thought to be more than 6000 years old 鈥� may also have been laid down to ward off rising seas, says Nunn.

The stones have previously been interpreted as astronomical symbols or graveyards, but these explanations haven鈥檛 held up to scrutiny, says Nunn. He adds that they were probably symbolic barriers, as they are spaced apart rather than forming continuous walls.

Spiritual offerings may have been another way that people tried to resist sea level rise, says Nunn. He believes neatly arranged collections of stone tools and human remains that have been found along the north-west coast of Europe were placed there to 鈥渢ry to persuade the gods to stop the sea level from continuing to rise鈥�.

Ulm says this is certainly possible, but it is hard to read into the motivations of people in the distant past. 鈥淎rchaeological remains along coasts may well have served a variety of purposes,鈥� he says.

New challenges

Ultimately, these attempts were futile. In Australia, the coastline contracted by about 140 kilometres. In Israel, Tel Hreiz was completely submerged. Legends also tell of cities like Ys in France, Cantre鈥檙 Gwaelod in Wales and Dv膩rak膩 in India that disappeared underwater. Though these may not represent literal cities, they may be based on memories of real places that were drowned by rising seas, says Nunn.

People living at this time presumably survived by moving inland, says Nunn. But it will be harder to do this in response to modern climate change because we have many more people and fewer places to relocate to, he says.

Ulm agrees. 鈥淚n today鈥檚 world with substantially higher population densities focused around coastlines, managing the relocation of people and infrastructure inland will present a massive social, engineering and resourcing challenge,鈥� he says.

However, the main advantage that we have over ancient people is that we know what is causing sea level rise this time and we have the power to minimise it if we drastically reduce our greenhouse emissions, says Nunn. 鈥淲e are again facing a challenge to our adaptability but it doesn鈥檛 have to be the end of the world,鈥� he says.