A1 - Carbon Dynamics in the Arctic

Chairs: Victor Brovkin (MPI-M), Lars Kutzbach (UHH), Dirk Notz (UHH)

CLICCS-funded Scientists: Tamara Emmerichs (MPI-M) Niels Fuchs (UHH), Oliver Kaufmann (UHH), Markus Ritschel (UHH), Joseph Tamale (UHH), Evan James Wilcox (UHH)

Team (in alphabetical order): Christian Beer (UHH), Claudia Fiencke (UHH), Leonardo Galera (UHH), David Holl (UHH), Thomas Kleinen (MPI-M), Christian Knoblauch (UHH), Minjung Kwon (UHH), David Nielsen (UHH), Philipp Porada (UHH), Xavier Rodriguez (UHH), Melanie Thurner (UHH), Phillipp de Vrese (MPI-M)

What we do

We investigate the Arctic contribution to the global carbon cycle across a wide range of temporal and spatial scales.

Why we look at this question

The Arctic plays a key role in the long-term uptake, storage and possible release of carbon buffered through the formation and thaw of permafrost. In addition, the Arctic carbon stock is crucial for determining the remaining global CO₂ budget for a specific temperature target. Despite this importance, our understanding of the key processes driving the Arctic carbon cycle is still limited.

How we work

We combine field measurements, laboratory studies, conceptual models, process models and Earth-System Models (in particular MPI-ESM and ICON-ESM), aiming for a traceable line of understanding of the Arctic carbon cycle from the millimeter scale to the global scale.

What we’ve found

Over the past years, we have jointly filled some key gaps in our understanding of the Arctic carbon cycle across a wide range of scales:

Millimeter to meter scale: Carbon release from thawing Pleistocene permafrost

One focus of our work in CliCCS has been to better understand the processes that control the release of CO₂ and methane from thawing permafrost on the millimeter to meter scale that governs a substantial part of the inhomogeneity in permafrost soils. For example, we contributed through modeling and field flux measurements of CO₂ and CH₄ to a better understanding of carbon release from thawing Pleistocene permafrost, including a detailed analysis of the relative contribution of CH₄ and CO₂ to total fluxes (Knoblauch et al., 2021) and of organic versus inorganic carbon to CO₂ fluxes (Melchert et al., 2021). We were able to identify a significant fraction (about a quarter) of CO₂ release from inorganic matter, which is often not accounted for in related modeling studies. The improved understanding of the intricate interplay between the fraction of organic and inorganic carbon in permafrost soil, its microbial activity and the resulting acidity provides crucial information for the development of more general models that can be upscaled to larger scales.

Meter scale: New measurements of methane release from waterbodies

On the meter scale, a particular focus has been on the combination of field measurements and conceptual modeling to better understand the methane release from waterbodies in the Arctic tundra (Rehder et al., 2021). Focusing on the smaller ponds that release a disproportionally large fraction of methane, we collected an extensive data set of methane concentrations during field work in the Lena River Delta in Siberia. Based on this data, we were able to identify the key processes that determine methane concentration in the various pond types. Among others, we found that the fraction of the pond that is covered by plants plays a key role for the methane budget, as the plants provide a substrate to the methane-producing microbes.

Hundreds of meters scale: The importance of thermokarst ponds

Based on our fieldwork in the Russian Arctic, we have been able to show how important thermokarst ponds are for the overall carbon uptake by Arctic tundra landscapes on scales of tens to hundreds of meters (Beckebanze et al., 2022). Our eddy covariance measurements showed that these ponds act as an important carbon source that reduce the carbon dioxide sink strength of permafrost landscapes significantly. Based on these measurements and the resulting understanding, we can now aim at developing process-based statistical parameterisations that allow for a better description of these mechanisms and their possible changes in a future warmer Arctic in larger-scale models.

Kilometer scale: The impact of coastal erosion

We developed a semi-empirical model that allows us for the first time to parameterize and estimate the change in coastal erosion in the Arctic resulting from the retreat of sea ice, the accompanying increase in wave height, and the overall thawing of the ground (Nielsen et al, 2022). By calibrating the model using pan-Arctic observations, we were able to show that the Arctic-mean erosion rate will very likely exceed its historical range of variability for most future emission scenarios. We further found that the sensitivity of the coastal erosion and the related carbon release to a given temperature change will increase further and further with the ongoing warming, reaching 0.4-0.8 m yr-1/°C and 2.3-4.2 TgC yr-1/°C, respectively, by the end of the century. (from Nielsen et al., 2022: https://www.nature.com/articles/s41558-022-01281-0)

Pan-Arctic scale: The impact of temperature overshoots

The Arctic reacts very sensitively even to a small amount of global warming. This is in particular the case if these warming levels are only achieved after an initial temperature overshoot, as we were able to show on the panarctic scale for the permafrost carbon cycle by using the MPI Earth System Model (de Vrese and Brovkin, 2021). Based on these simulations, we were able to show that it takes several centuries until high-latitude ecosystems and the underlying permafrost have adjusted to the new climate conditions that go along, for example, with a 1.5 °C warmer world. More importantly, we were able to show that very different steady states can exist for the same long-term equilibrium temperature, depending on the pathway of the warming. For example, the soil organic matter content can be reduced substantially during the period of a possible temperature overshoot; once lost, this organic matter remains lost even as the cooler long-term temperature target is finally reached. These insights provide important policy implications related to the long-term trajectory on a way to a carbon-neutral society.

Pan-Arctic scale: The future of subsea permafrost

Subsea permafrost stores substantial amounts of organic carbon accumulated during glacial periods whose decomposition leads to CO₂ and methane emissions. To better constrain the possible impact of the thawing of the subsea permafrost on the carbon cycle, we implemented for the first time a subsea permafrost module into an Earth System Model (Wilkenskjeld et al., 2022). We find only little difference between the examined scenarios throughout the 21st century, but in the 22nd century, the SSP5-8.5 scenario results in a hugely amplified ice melt below the seafloor. With a 15fold increase in permafrost thawing relative to the pre-industrial period, this thaw far exceeds those of more moderate scenarios (SSP2-4.5, SSP1-2.6), where the increase always remains less than a factor of four.

Summary

Over the past years, we have made some key contributions to our understanding of the Arctic carbon cycle and its possible implications for the future evolution of the climate system of our planet. In particular, we have learned how strong the linkages are from the smallest to the largest temporal and spatial scales of the Arctic Carbon Cycle. Investigating the key processes over such large range of scales has only been possible through the research enabled by CliCCS.