Title: Reduced Arctic Ocean CO2 Uptake Due to Coastal Permafrost Erosion
Authors: David M. Nielsen, Fatemeh Chegini, Joeran Maerz, Sebastian Brune, Moritz Mathis, Mikhail Dobrynin, Johanna Baehr, Victor Brovkin, and Tatiana Ilyina
Journal: Nature Climate Change
Year: 2024
The artic is warming four times faster than the rest of the globe due to of human-induced climate change. This polar region is home to deep layers of perpetually frozen soil and sediment known as permafrost, and its waters contain vast amounts of sea ice. Turning up the temperature in this icy environment can cause permafrost and sea ice to thaw and melt which can result in several different climate feedbacks. One such feedback is the erosion of coastal permafrost by the ocean.
When ocean waves collide with coastal permafrost, the organic carbon contained within the permafrost enters the ocean. There are then a range of mechanisms by which the newly introduced organic matter can alter the marine environment which have implications for the earth’s climate. Firstly, the organic carbon can be broken down and converted into CO2, which can ultimately find its way into the atmosphere and contribute to warming. Alternatively, the additional organic matter and minerals provided by the eroded permafrost may help to feed photosynthetic organisms in the ocean, and therefore reduce atmospheric CO2. Finally, since the organic matter is introduced to the ocean in the form of sediments, some portion will settle to the sea floor, where conversion to CO2 is slowed. The relative importance of each of these pathways relies on a complex interplay between the chemical properties of the organic carbon released into the ocean, as well as physical processes such as the resuspension of ocean sediments.
Earth-system models are used to represent complex earth processes on climate-relevant timescales, but the process of coastal permafrost erosion is not currently included in the latest earth-system models. Earlier this year, researchers from the Max Planck Institute for Meteorology, Nielsen et al., published their work representing this process in the Max Planck Institute Earth system model (MPI-ESM) in order to assess the impact of coastal permafrost erosion on the global climate.

Adapted from Nielson et al., and is used under a Creative Commons Attribution 4.0 International License.
The researchers ran a series of simulations to test the sensitivity of the climate response to variations in the properties of the organic matter added to the ocean. In all cases, the addition of coastal permafrost erosion resulted in a reduction in atmospheric CO2 uptake by the ocean. The researchers noted that “between about one-fourth and up to all of the OC [Organic Carbon] from erosion would escape to the atmosphere as CO2, depending on permafrost OM [Organic Matter] characteristics”. Ocean pH plays a key role in this reduced absorption of atmospheric CO2. As sediment enters the ocean, the ocean is slightly acidified by the enhanced CO2 concentrations coming from organic carbon being broken down, as well as by the increased formation of CaCO3 by ocean organisms. Increased ocean acidity reduces the amount of CO2 that the ocean can absorb, producing the effect observed in the model simulations. While there are competing processes that counteract the pH effects, they are generally smaller in magnitude, making the net change in atmospheric CO2 absorption negative.
The effect of adding erosion to the model was found to be stronger when sea ice cover was reduced (for example during the summer months or in future warming scenarios) for two reasons. Firstly, increased sea-ice acts as a barrier and reduces the force of waves reaching land, which slows erosion. Secondly, increased sea-ice impedes gas exchange between the ocean and atmosphere by covering the surface of the water. This presents a positive feedback process where reduced sea ice results in more coastal permafrost erosion, which then reduces the ocean uptake of CO2, resulting in more warming and sea ice melt.
On a global scale the magnitude of this effect is small, but noticeable. The researchers state that “the yearly increase in atmospheric CO2 due to erosion by 2100 is equivalent to about half of what was emitted by fossil fuel combustion by cars in Germany in 2021”. By including coastal permafrost erosion, and the associated climate feedback processes, in earth system models we are better able to predict future climatic changes and make informed climate policy decisions.
Featured Image by stein egil liland on Pexels.