But there were no humans around burning fossil fuels in the year 55.6 million before present. Primates had only just evolved. So what could have caused atmospheric carbon and global temperatures to spike so dramatically?
In the journal Science on Thursday, researchers report the discovery of tiny, glass globules in rock core samples as far apart as New Jersey and Bermuda — which they believe may be evidence of an ancient, catastrophic comet impact that set off the period of global warming.
The glass spheres, called microtektites, are the remains of the molten rock that gets blown in the air when a massive object collides with the Earth. They contain “shocked” quartz that appears only in the wake of an impact. In 1980, the discovery of shocked quartz and tektite deposits in the Caribbean from 66 million years ago helped convince Nobel Prize-winning physicist Luis Alvarez that a massive asteroid may have been what killed the dinosaurs; a decade later, geophysicists discovered the Chicxulub crater off the coast of Mexico, where the impact occurred.
The newly described microtektites come from the part of the fossil record where carbon starts to spike — a layer known as the Paleocene-Eocene boundary.
“Those glass impact sperules really point to that there may have in fact been an impact,” said Dennis Kent, who studies Earth magnetism at Rutgers and the Lamont-Doherty Earth Observatory and is a co-author on the paper. “If so, the fact that it occurred just at the time of the PETM is either an amazing coincidence, or it says there may be some close correlation, there may be some causation involved.”
Kent has argued that a comet impact could have triggered the thermal maximum since 2003, when he reported on finding strange magnetic nanoparticles in sediment cores taken off the Atlantic coast. Comets carry large amounts of carbon-12, the isotope that became suddenly abundant during the PETM. Kent believes that an initial, instantaneous release of carbon from the impact could have set off a greenhouse effect that caused frozen methane in the seafloor to melt, releasing more carbon and starting a positive feedback cycle that made the planet hotter and hotter. The impact could also have triggered landslides in the north Atlantic that exposed the methane, and volcanic activity that was happening at the hotspot underneath what is now Iceland would have exacerbated the problem.
The mikrotektites are a “coup de grace,” Kent said, “which really points to an impact.”
They were discovered by Morgan Schaller, a geochemist at Rensselaer Polytechnic Institute and lead author of the paper, and doctoral student Megan Fung. Fung had been examining sediment cores from various spots around the Atlantic looking for fossils of foraminifera, a class of single-celled sea creatures that was one of the main victims of the PETM warming. Months had gone by with no success, so Schaller came over to see what was wrong.
“Morgan came down and kind of dumped the sample on a tray,” Fung recalled, “and in a couple of minutes found one of these sphereules.”
“And then we jumped up and down and got really excited,” Schaller interjected. They both laughed.
Other scientists are less convinced. Geology is a discipline that operates on immense time scales: It takes a while to irrevocably change the Earth, and even longer for that change to be recorded in the rock record. Even the most conservative explanations of the PETM describe a phenomenon that happened far faster than almost anything in Earth's history, so researchers are understandably skeptical of suggestions that it could have started in a day.
James Zachos, a paleo-oceanographer and PETM specialist at the University of California in Santa Cruz, noted that the abundance of microtektites Schaller and Fung found suggests a relatively small impact. A comet of that size wouldn't have contained nearly enough carbon to trigger the massive changes that characterized the PETM.
“These things happen on Earth every million years,” he said — the timing of the impact relative to the thermal maximum is probably just a coincidence.
Unlike Kent, who is convinced that the initial pulse of carbon was released very quickly, Zachos argues that it happened over the course of thousands of years. He thinks it's more likely that volcanic activity at the ocean floor heated up carbon in the crust and sent it pluming into the atmosphere.
If all this seems like an obscure debate about events that occurred millions of years ago, keep in mind that the Paleocene-Eocene Thermal Maximum is the only period in history that comes close to mirroring the climate change that Earth is experiencing today. If scientists want to understand what happens when a certain amount of carbon is added to the atmosphere, or when ocean temperature rise a specific number of degrees, the PETM can tell them.
“All the things you hear about when climatologists talk about what is happening now and will happen more so in the future, that is all based on theory, based on climate models,” Zachos said. “And yet we see exactly that happening 56 million years ago during the PETM, so that is an independent verification of theory.”
“I see that, and I know the models are correct,” he continued, “Because it happened before.”