Ancient CO2 Surge Triggered Widespread Forest Fires & Erosion

The climate warmed up almost as quickly 56 million years ago as it is doing now. When a huge amount of CO2 entered the atmosphere in a short period of time, it led to large-scale forest fires and erosion. Mei Nelissen, Ph.D. candidate at NIOZ and UU, and her colleagues were able to see this very clearly in the layers of sediment drilled off the Norwegian coast. The research was published in Proceedings of the National Academy of Sciences on January 19. 56 million years ago, Earth was already warm. "As a result, there was a lot of vegetation, even at high latitudes. That means that a lot of carbon was stored in, for example, vast coniferous forests," says Nelissen. She analyzed pollen and spores in clearly layered sediment that her supervisors had drilled from the seabed in the Norwegian Sea in 2021. This revealed unique information in great detail—even per season—about what happened when Earth warmed by five degrees in a short period of time 56 million years ago. Layers in drill cores Nelissen says, "We could see that within a maximum of three hundred years from the start of the explosive increase in CO2, the conifer-dominated vegetation disappeared at the studied site and many ferns appeared. The ecosystems on land were disrupted for thousands of years; an increase in charcoal indicates that there were more forest fires. An increase in clay minerals in the sea sediment also indicates that entire sections of land washed into the sea due to erosion." Thanks to the exceptionally well-defined layers in the sediment—even per season—researchers were able to demonstrate for the first time how quickly trees and plants respond to disruption. More was already known about the major impact on the sea, says Nelissen. "In drill cores from the deep sea, for example, we see that there is suddenly no more calcium carbonate, because the seawater rapidly acidified due to all the CO2 it absorbed. This made the water too acidic for organisms to form calcium carbonate skeletons or shells." Even faster warming now than then What was going on? The period around 56 million years ago is known as the PETM: Paleocene-Eocene Thermal Maximum. It was already warm and 'suddenly' it became even warmer. Nelissen says, "The cause is unknown; it is probably a combination of factors. Methane hydrates in the seabed became unstable due to the heat, which led to methane emissions. There was also a lot of volcanic activity during that period." Nowadays, climate change is mainly due to the burning of fossil fuels. "Today, CO2 emissions are about two to ten times faster than in the PETM, but the rate at which CO2 concentrations in the atmosphere increased at that time is closest to the increase caused by human emissions. In geological terms, such a rate is unprecedented." The disruption amplified the warming It is important to know what consequences the disruption of the carbon cycle and warming had at that time, because we can deduce what lies ahead if the rapid warming of today continues, the researchers write. We are already seeing more forest fires, but we also expect more extreme weather with more intense rainfall, flooding and drought. Nelissen explains, "We must take this seriously. Our results are consistent with findings from other researchers in other areas. We now know that terrestrial ecosystems can respond quickly and dramatically to climate change. The carbon released into the atmosphere by the terrestrial disturbances, including fires and soil erosion, can further exacerbate global warming." Nelissen's supervisors, Joost Frieling (University of Oxford and Ghent University) and Henk Brinkhuis (NIOZ and Utrecht University), went on a sea expedition with the International Ocean Discovery Program in 2021 to take sediment samples. The drill cores turned out to be particularly clearly 'laminated': they showed very distinct layers, even per season. When they found the microfossils of the algae Apectodinium augustum, they happily posed for a photo together. Nelissen says, "That's when my Ph.D. position came about. This microfossil was proof that this beautifully preserved sediment comes from the PETM period, the period that researchers are keen to learn more about."