Global warming and CO2 emissions 56 million years ago resulted in massive forest fires and soil erosion

56 million years ago, the 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.’ Biologist Mei Nelissen is conducting PhD research at NIOZ and Utrecht University. She analysed 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 the Earth warmed by five degrees in a short period of time those 56 million years ago.

Layers in drill cores

Nelissen: '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 and 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: '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: '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.'

Milestone in the research

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: ‘That's when my PhD 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.’

More information

• Article: Widespread terrestrial ecosystem disruption at the onset of the Paleocene-Eocene Thermal Maximum

Text: Royal Netherlands Institute for Sea Research (NIOZ)
Images: NIOZ; Utrecht University