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Climate change is causing global sea levels to rise faster than they ever did within the last 4,000 years, a new analysis of minerals from sea caves in Mallorca, Spain, shows.
Published in the journal Science Advances, the new study found that preindustrial sea levels probably remained stable from about 2,840 years ago. Before that, there was an episode lasting about 420 years during which sea levels rose about 0.54 millimeters per year. The modern rate of sea level rise is much faster - between 1993 and 2018, sea levels rose 3.5 millimeters per year.
"It's really just confirming how exceptional human-caused climate change today and the rapid rates of sea-level rise really are," said oceanographer Jennifer Walker of Rutgers University, who was not involved in the study.
Knowing how sea levels varied before climate change is important for understanding how today's sea level rise stacks up against natural ups and downs. Industrialization and its accompanying greenhouse gas emissions began around the same time people started recording sea levels, around 100 to 150 years ago. But certain places preserve natural records of sea levels that can reach back thousands of years.
"Over the past several decades, especially, there have been a lot more of these reconstructions of relative sea level change [in the past]. And they're really useful for understanding kind of the background context for what we're seeing today," said Walker.
For their study, geologist Bogdan Onac of the University of South Florida and his colleagues used a natural record of the sea level preserved in sea caves in Mallorca, Spain. The caves are full of stalactites - finger-like mineral growths that reach down from the roofs of caves - that dip into the seawater below. As the water evaporates, it leaves a thin mineral crust on the stalactites right at the sea surface.
Because the tides move the water level up and down, the mineral crusts build up football-shaped overgrowths on the stalactites. They're thinnest at the low and high tide points, where sea level remains only briefly, and thickest in the middle - the average sea level.
In addition to the unique cave growths, Mallorca is very stable. It almost never experiences earthquakes and other disturbances, which can perturb the local sea level.
"We do have a kind of - I will not say a "paradise," but it's a very important location. ... It's in [the] right place to allow us to do these very precise measurements of the sea level," said Onac.
For the new study, Onac and his team sampled 13 mineral overgrowths collected from Mallorca and used geochemical techniques to date each layer of mineral crust in the thickest layer.
Like tree rings, the ages of the mineral layers show how quickly the overgrowths formed. This, together with the presence of multiple overgrowths on the same stalactite - evidence that sea levels changed in the past - allowed the scientists to reconstruct a timeline of the sea level in Mallorca over the last 4,000 years.
The new data revealed that the sea level appears to have remained stable within 8 centimeters in Mallorca for almost 3,000 years before the industrial revolution began. Before that, it rose for about 400 years at a rate at least six times slower than today's rate.
Previous studies have made similar findings using other natural records, but the record of ancient sea levels beyond 2,500 years ago wasn't very certain.
Onac said he hopes policy makers will take note of his findings.
"I would say they need to be very, very serious about how they plan the future of the coastal communities," he said.
He'd also like to see future work confirming his results with different methods and wants to follow up with studies of overgrowths from caves in Italy, Cuba, Mexico and elsewhere.
For her part, Walker said she'd like to see researchers start incorporating data on ancient sea levels into predictions of future sea level rise, rather than relying exclusively on modern data - which the new study would suggest aren't "normal" for the Earth at all.
"It kind of makes you wonder," said Walker. "With the rates that we're seeing today, which are already much greater, how much have we really disrupted the climate system? And how much of that is kind of locked in at this point? And what can we expect going into the future?"