If there was one small crumb of comfort we could take from our ongoing climate catastrophe, it’s that 100 million years ago, sea levels were a good 15 degrees higher than they are today. That’s a small comfort, but comforting nonetheless: if life was able to develop in those conditions, then our rapidly heating oceans aren’t a worrying anomaly, but following historical precedent.
One problem: new research suggests we were way off with our historical measurements. Rather than being 15 degrees warmer, historically, it looks like ocean temperatures have actually been relatively stable. That means our current warming waters may indeed be without precedent, which in turn means that our slightly gloomy outlook on the fate of the planet may actually have been a “glass half full” appraisal.
Yikes.
Mistaken assumptions
To understand how the researchers came to this depressing conclusion, you need to first hear about how we measure temperatures of water at a time when there were no humans around to do so, let alone any thermometers. While current measurements use a variety of sophisticated tools, historical data is estimated based on analysis of foraminifera – fossils of long-dead marine organisms found in sediment cores on the ocean floor.
These foraminifera form calcareous shells, and scientists can tell the temperature of the water they lived in based on their oxygen-18 content. By analysing this over time, our best minds came to the conclusion that ocean temperatures have fallen by around 15 degrees Celsius over the last 100 million years.
That appears to have been based on a false assumption – that oxygen-18 content remains constant for fossils lodged in sediment. French and Swiss researchers tested the organisms in artificial seawater, containing just oxygen-18 isotopes. They then increased the temperature to simulate the heat they generate when piled beneath sediment and used a nanoscale secondary ion mass spectrometre to analyse the oxygen-18 levels. Sure enough, the levels changed without leaving a visible trace. In other words, our assumption of these being reliable thermometers is completely off.
“What appeared to be perfectly preserved fossils are in fact not. This means that the paleotemperature estimates made up to now are incorrect,” explains Sylvain Bernard, from the French National Centre for Scientific Research (CNRS) and the study’s lead author.
The numbers which we assumed reliably indicated a gradual drop in sea temperatures over the last 100 million years actually show no such thing. They merely show a change in oxygen-18 levels in the fossilised foraminifera shells, likely as part of the sedimentation process where temperatures rise by 20 to 30°C, re-equilibrating with the surrounding water. This, as you might imagine, has a major impact on our historical estimates – especially in areas of colder water. The researchers’ computer simulations indicate that these have been consistently overestimated.
“If we are right, our study challenges decades of paleoclimate research,” says Anders Meibom, from the Swiss Federal Institute of Technology in Lausanne. “Oceans cover 70% of our planet. They play a key role in the earth’s climate. Knowing the extent to which their temperatures have varied over geological time is crucial if we are to gain a fuller understanding of how they behave and to predict the consequences of current climate change more accurately.”
Next steps
The research suggests we’re off, but it doesn’t say how far off. “To revisit the ocean’s paleotemperatures now, we need to carefully quantify this re-equilibration, which has been overlooked for too long,” says Meibom. “For that, we have to work on other types of marine organisms so that we clearly understand what took place in the sediment over geological time.”
They’re working on this as I write, but for now, one thing is clear: the small crumb of comfort we had about our current ocean temperature trajectory has been taken away. Our warming waters – which have gained nearly a degree over the past century – may be completely without precedent.
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