Increasing geologic evidence suggests that the Arctic was once a lot warmer than it is today, which means big trouble as we continue to pump greenhouse gases into the atmosphere with abandon. The latest development is a study published Wednesday in Science Advances, which uses geologic cave deposits to deduce that the region was at least 6.3 degrees Fahrenheit (3.5 degrees Celsius) warmer than today just a little more than 500,000 years ago.
To try and get a grasp on what the Arctic’s past may have looked like, researchers analyzed a 4.73-inch-thick (12-centimeter-thick) calcite mineral deposit that they found in a cave in Northeast Greenland. Even though things the Arctic is rapidly heating up, it’s still largely cold and covered in ice. Calcite needs much warmer and wetter conditions to form than exist in the area today. Researchers dated the sample using uranium-series dating and analyzed the oxygen composition of the formation, which was formed by flowing water.
“We aimed to produce a paleoclimate record from a time period in the past when the Arctic was warmer and wetter than today as such knowledge will hopefully allow us to improve predictions for the future,” Gina Moseley, a researcher at the University of Innsbruck in Austria and the lead author of the paper, told Earther in an email.
While it’s tough to precisely date an old piece of rock, the uranium dating allowed researchers to pinpoint the deposit’s origin to a period between 588,000 and 549,000 years ago. The sample’s carbon-13 isotope profile and oxygen composition also pointed to signs of a warmer, wetter climate. This, Moseley said, marked the first time that cave deposits have been used to give us a paleoclimate record for Greenland and reveals some important information about what the Arctic may have looked like back then.
Our previous knowledge of Greenland’s historic climate has come from samples taken from the ice sheet that covers the island. This gives us great information on what Greenland was like when that ice sheet formed, but it isn’t much help for trying to understand what came before. The ice cores scientists have are limited to an interglacial warm period around 130,000 years ago. (Earlier this month, researchers said that sediment found in the dredges of an ice core had plant fossils that suggested that Greenland was once warm enough to be ice-free.)
“The Greenland ice core records are...biased towards cold climates and possibilities to extend further back in time to warm periods are limited because the ice sheet does not tend to survive warm periods,” Moseley explained. “The new record from the caves has allowed us to tap into a past warm period beyond the limit of the Greenland ice cores.”
Bogdan Onac, a geologist at the University of South Florida who was not involved in the study, calls the findings “a great achievement” and “solidly, carefully-crafted research.” He cautions that more research and samples need to be taken to fully flesh out the climate profile begun by this work.
“This research highlights that you could have time over the Earth’s history where temperatures were higher than present, and that was a natural trend,” he said. “Having those high temperatures means more melting is likely in the central part of Greenland where you have the ice sheet. More melting means more water into the ocean.”
Figuring out as much as possible about the Arctic’s past has huge importance for predicting what its increasingly imperiled future may look like. Some estimates project that Greenland’s ice sheet could raise sea levels by 20 feet (6.1 meters) if it fully melted, which, in addition to plaguing coastal cities worldwide, would also wreak havoc on ocean currents by injecting enormous amounts of freshwater into the sea. Knowing that Greenland was once so much warmer than it was today naturally is worrisome considering how we’re supercharging climate change today with carbon dioxide (the ice sheet is now melting six times faster than it was in the 1980s).
“We know what is going on now in Greenland,” Onac said. “Imagine temperatures that were three or four degrees warmer than today, how much more ice would be melting. This study points out that the [levels of] greenhouse gases at that time were very low. Today, we have greenhouse [gases] bumped up a lot. What’s going to happen in a couple centuries or millennia?”