A new study of ancient ocean temperatures, published today in Science, shows that the abyssal Atlantic Ocean was once a truly balmy 20 °C – warmer even than the surface of the modern Mediterranean Sea.
Scientists, including Dr Philip Sexton of the OU, say the new data, spanning the last 60 million years, show the huge impact of higher CO2 levels in the geological past, and stress the need to avoid continued CO2 rise in the future.
The international team, including scientists from the OU, The University of Bergen, Utrecht University, University of St. Andrews, University of California Los Angeles and ETH Zürich, used the chemical fingerprints of tiny microfossil shells from deep sea mud cores to reconstruct ancient ocean temperatures.
Using state of the art new laboratory measurements, they were able to obtain the most accurate temperature estimates to date, showing that, when atmospheric CO2 was last as high as it will likely be by the end of this century, temperatures were much warmer – and more variable – than previously thought.
The very warm temperatures of 20 °C for the abyssal Atlantic come from a time called the Eocene, about 15 million years after the demise of the dinosaurs, when atmospheric CO2 was around 3 times higher than today.
Previous fossil indicators have shown this time was marked by super warm surface temperatures, with palm trees and crocodiles inhabiting the Arctic.
Dr Sexton onboard the drilling ship
Dr Sexton, Senior Lecturer in Earth Sciences and co-author of the study, said:
“These new data allow us to test the long-held assumptions and uncertainties inherent in existing records of past ocean temperatures. We now show that this extreme surface warmth was also felt all the way down in the ocean’s abyssal depths”.
Prof Nele Meckler, of the University of Bergen who led the study, explains:
“We looked at the way that different atoms of carbon and oxygen were clumped together in these ancient microfossils, which turns out to be a really accurate method of working out the temperature at the time the microfossils were formed”.
Microfossils under the microscope (credit Nele Meckler)
Dr Sexton added:
“Because mud and microfossils continually pile up on the seabed, a long sequence of this mud – up to 3 km long – is like a time capsule”,
“As you go deeper down the core, the older the microfossils become, and by measuring their chemistry we get a long continuous record of past climate change”.
“Today the deep ocean is filled with icy cold waters”, said Dr James Rae, from the University of St Andrews, who co-authored the study. “But 50 million years ago, it was as warm as the surface Mediterranean is today”.
“These ancient super-warm climates serve as benchmarks to allow crucial tests of the computer climate models that are being used to predict our future”, said Dr Rae. He continued “If the models can replicate the features of these past warm climates we will have much more confidence in their predictions of our future. These new estimates of extreme abyssal ocean warmth now serve as an important new target for these models to reproduce”.
“This much warmer Atlantic Ocean abyss raises the need to reassess the history of ocean temperatures”, Dr Sexton emphasised.
“This finding could also have implications for our understanding of the sensitivity of global climate to future rises in atmospheric CO2 concentrations, because our new data from past warm climates suggests this climate sensitivity is much higher than previously thought”.
