Some 40 million years ago, the world experienced an extreme spike in global warming. The heat was so intense that deep sea temperatures rose by about 4 degrees Celsius. This enigmatic sultry period, known as the Middle Eocene Climatic Optimum (MECO), marked a 400,000-year-long heat wave in the midst of a long era of global cooling.
Now research published Nov. 5 in the journal Science suggests the rise in surface sea temperature occurred during a time when atmospheric carbon dioxide (CO2) levels were particularly high, according to a research team from Utrecht University and the Royal Netherlands Institute for Sea Research.
First reported by U.S. scientists in 2003, the MECO warming period has been documented by data from a smattering of sites around the world. “Our paper is among the first to show that CO2 concentrations and the temperature varied hand in hand in that time,” says Peter Bijl, a paleoclimatologist at the Netherlands’ Utretcht University and one of the paper’s lead authors.
The study may help put to rest some of the doubts expressed about today’s climate models because it describes an actual warming event rather than one predicted by a model, according to Jeff Kiehl, head of the Climate Change Research Section at the National Center for Atmospheric Research in Boulder, CO.
“This paper shows indeed that the planet is sensitive to CO2 and that it corresponds with a rise in sea surface temperatures,” Kiehl said. “This is something that is independent of a model—this is real data” that can be used to “test today’s models against to see if they agree.”
Richard Norris, a paleobiologist at the Scripps Institution of Oceanography, University of California, San Diego, said the research is “highly relevant.”
“One of the reasons you want to look in the ancient past like the authors have done is that you see complete events. Now that the population is expanding and energy use is shooting up….The question is, what is going to happen?” Norris said. “We can forecast it, but if you are a climate skeptic, you can say, ‘I don’t believe the models.’ Fortunately there is the geological record, and it shows exactly what happened” in the past when increased atmospheric CO2 may have had an impact on the Earth’s systems.
Kiehl said another intriguing find from the study is that 40 million years ago water temperatures reached as high as 25 degrees Celsius in areas where today the water is close to freezing. “That is a completely different world than what it is today,” he said. “That is very intriguing from an Earth system perspective.”
Cause of Increased CO2 Levels in Eocene Remains Unknown
The team analyzed core sediment samples collected in 2000 from an area in the Southern Ocean known as the East Tasman Plateau at Ocean Drilling Program (ODP) Site 1172. Researchers who set out to re-create the opening between the subcontinents of Antarctica and Australia originally took the samples, Bijl says. Some 40 million years ago, Australia was still connected to Antarctica.
The group reconstructed the ancient CO2 concentrations by examining the fossil molecules of algae. The carbon isotopic composition of these algae is strongly controlled by the atmospheric CO2 concentration during growth, or during photosynthesis. The group also studied the changes in the abundance of different groups of fossil plankton in the sediment to help refine their CO2 estimate, Bijl said.
In a mostly positive commentary accompanying the Science study, Paul Pearson, of the School of Earth and Ocean Sciences at the UK’s Cardiff University, cautioned that the actual level of atmospheric CO2 rise and the sea surface temperatures should be considered best estimates because they rely on a series of assumptions made by the researchers. Nonetheless, “what the study did show was that the CO2 was certainly higher during this period, and it at least doubled during the MECO,” he wrote.
What caused the rise in atmosphereric CO2 levels remains unknown. Scientists have proposed a number of potential causes, including the disappearance of an ocean between India and Asia that occured as the Himalayas rose, significant volcanic activity or the recycling of carbon from carbonate sediments in the dying ocean through “extensive metamorphic decarbonation reactions,” Pearson wrote.
“To put it briefly, the change in CO2 40 million years ago was too large to have been the result of temperature change and associated feedbacks,” Bijl said. “Such a large change in CO2 certainly provides a plausible explanation for the changes in Earth’s temperature.”