It's about time someone took a lesson from history.
Scientists at Scripps Institution of Oceanography in San Diego are trying to predict the impact of current global warming by comparing it with a similar trend from several million years ago.
Flavia Nunes and Richard Norris analyzed deep-sea sediment cores from 14 sites around the world, studying what occurred 55 million years ago, when the Earth experienced a period of warming—analogous to modern-day climate change—known as the Palaeocene/Eocene Thermal Maximum (PETM).
"The PETM is one of the few periods in the geologic record where we observe rapid increase in global temperatures," said Nunes, the lead author of a paper published in this week's Nature.
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Nunes and Norris looked at carbon isotopes from the shells of a single-celled animal called foraminifera, found in deep-sea sediment samples. Analyzing the isotopes allowed them to determine the temperature, salinity and nutrient content of the water, and even the direction of ocean currents.
"Foraminifera are tiny sea creatures that make their shells by taking chemicals from sea water," said Nunes. "The chemistry of their shells therefore reflects the chemistry of the waters in which they lived."
Nunes and Norris found evidence of abrupt shifts in worldwide deep-ocean circulation patterns during the PETM.
"There was a background warming trend at that time," said Nunes. "Our data show that at the same time that there was a big spike in temperatures, there was a reversal in circulation patterns."
In their study, Nunes and Norris say history suggests that changes in ocean current can happen abruptly—but may have lasting effects. In the case of the PETM, it took 100,000 years for oceans to revert. The paper also highlights the correlation between conditions 55 million years ago and conditions today.
"Modern carbon dioxide input to the atmosphere from fossil fuel sources is approaching that estimated for the PETM, raising concerns about future climate and circulation change," the authors write in the paper.
Debbie Thomas, a paleoceanographer at Texas A&M University, said that the events of the PETM are still not well understood; it is important for the current finding to be interpreted carefully. Thomas believes that Nunes and Norris's model uses data from two key sites: one in the Southern Ocean, which is located around Antarctica, and another in the North Atlantic.
Thomas is not yet certain that the rock record from the North Atlantic site, Site 401, properly chronicled everything that happened.
"I am willing to be convinced that 401 is complete," she said, "but the present paper did not convince me."
Still, if Nunes and Norris are correct, our steady warming trend could spike in the relatively near future.
"The lesson we learned from the PETM is that a slow warming trend can trigger a snowball effect, leading to abrupt warming.” Nunes said. “The amount of CO2 that we are putting into the atmosphere is causing a gentle increase in temperatures, but that could lead to an abrupt and more catastrophic change.
"We have to be careful with what we are doing. Our actions could have unpredictable consequences based on what we now know."