Climate change stands poised to melt the planet’s ice caps and raise sea levels worldwide, with dramatic effects for human society. But these effects won’t be felt all at once, a new study suggests.

The research, led by the University of Exeter in partnership with Cornwall Archaeological Unit, Cardiff University and 14 other institutes, focused on the Isles of Scilly, a group of islands of the UK’s south-west coast. The 140 islands of today are the remnants of a single large island which was gobbled up by the seas less than 1,000 years ago, the team reports. But the changes in land area and the shifts in human cultures associated with them took place at variable rates, they add, and were often ‘out of step’ with the average rate of sea-level rise.

Such findings showcase that the effects of rising seas are more complex and unpredictable than we assume, and will be much further reaching than simply forcing coastal communities to relocate.

Rising seas flood all boats

“When we’re thinking about future sea-level rise, we need to consider the complexity of the systems involved, in terms of both the physical geography and the human response” said lead author Dr. Robert Barnett, of the University of Exeter. “The speed at which land disappears is not only a function of sea-level rise, it depends on specific local geography, landforms, and geology.”

“Human responses are likely to be equally localized. For example, communities may have powerful reasons for refusing to abandon a particular place.”

The team studied the process through which the old island turned into the current cluster of 140 islands, which overall lasted some 12,000 years. They first developed a sea-level curve for the Isles over this time (a chart that shows sea level over time). Then, the team analyzed how these changes influenced the landscape, vegetation, and human populations from archaeological evidence as well as samples of pollen and charcoal collected by the Lyonesse Project (2009 to 2013).

The findings suggest that between 4,000 and 5,000 years ago, the island was rapidly becoming submerged. The inhabitants were seemingly trying to adapt to the changes in their landscape rather than abandoning the area altogether. Around 4,400 years ago, during the Bronze Age, the island had a permanent population that showed “a significant acceleration of activity”.

Land losses during this time were quite quick despite sea levels rising quite slowly, because a large part of Scilly at this point was relatively flat and close to sea level. According to the team, land here was being lost at a rate of around 10,000 sq. meters per year (a 100 by 100 meter square), roughly equivalent to a large rugby stadium. Exactly why these early inhabitants weren’t scared by higher seas is unclear; however, the team believes that they created more opportunities for fishing, the collection of shellfish, or the hunting of marine bird species. It’s possible that much of this lost land developed into intertidal habitats (exposed at low tide and submerged at high tide), which were still useful and traversable to coastal communities.

After around 4,000 years ago, the island was progressively submerged, even during times with lower rates of sea-level rise (around 1 mm per year).

“It is clear that rapid coastal change can happen even during relatively small and gradual sea-level rise,” said Dr. Barnett.”The current rate of mean global sea-level rise (around 3.6 mm per year) is already far greater than the local rate at the Isles of Scilly (1 to 2 mm per year) that caused widespread coastal reorganization between 5,000 and 4,000 years ago.”

“It is even more important to consider the human responses to these physical changes, which may be unpredictable. As can be seen today across island nations, cultural practices define the response of coastal communities, which can result in polarised agenda, such as the planned relocation programs in Fiji versus the climate-migration resistance seen in Tuvalu.”

Sea level rise led to new marine resources becoming available for communities on the island of Scilly, but the team believes it is “unlikely” that this mechanism will prove enough to support today’s communities as they become affected or displaced by rising sea levels.

“More certain though, is that societal and cultural perspectives from coastal populations will be critical for responding successfully to future climate change,” says Dr. Barnett.

The study “Nonlinear landscape and cultural response to sea-level rise,” has been published in the journal Science Advances.