This one study deals with changes in permafrost depending on carbon dioxide removal options (zero or negative emissions) and its role in climate change. An Earth model is used to simulate emissions under zero and negative emissions after a period of positive anthropogenic CO2 emissions. The goal is to understand when and how much carbon will be emitted from permafrost, what the ratio of CO2 to CH4 will be, and whether there is a permafrost hysteresis at zero and negative emissions and how it will affect the ecosystem carbon balance.

Permafrost – affected soils in the northern circumpolar permafrost region store approximately 1460 to 1600 PgC. Global warming leads to the decomposition of organic carbon in these soils, which releases greenhouse gases such as CO2 and methane (CH4) into the atmosphere, potentially amplifying human-induced global warming. Plants should grow faster and absorb more carbon from the atmosphere, which could offset some of the carbon losses from the soil. Changes in the net carbon balance in permafrost in response to zero and negative emissions are complex, with major implications for future responses of the carbon-climate system to anthropogenic emissions.

Key questions and methodology

1. How will the extent of permafrost and the thickness of the active layer (ALT) change under zero and negative emissions?
2. How much carbon from the permafrost ecosystem will be emitted or absorbed along the emission reduction pathways?
3. What are the main processes responsible for these changes?
4. Is there a permafrost hysteresis at zero and negative emissions, and if so, how will this affect the net ecosystem carbon balance?

To answer these questions, simulations were performed using the Community Earth System Model version 2 (CESM2) coupled with the Community Land Model version 5 (CLM5). The simulations are based on two idealized and continuous emission pathways involving zero (Exp_zero) or negative emissions (Exp_neg) following positive anthropogenic CO2 emissions.

Findings

• Changes in the extent of near-surface permafrost: The extent of near-surface permafrost is highly dependent on near-surface air temperature (SAT). In Exp_zero, a pattern of thawing and refreezing occurs following a warming and cooling cycle. In Exp_neg, temperature fluctuations (warming, cooling, rewarming) lead to a cycle of thawing, refreezing, and rethawing.
• Development of the net ecosystem carbon balance: During the positive emissions phase, the vegetation carbon pool continues to increase, while the sum of litter and soil carbon pools decreases. From 2060, litter and soil carbon begin to decline sharply, leading to a net loss of ecosystem carbon in the permafrost region. After the end of positive emissions in Exp_zero, net carbon uptake by vegetation begins to decline, but vegetation carbon remains stable. Due to the continued loss of litter and soil carbon caused by high temperature, total ecosystem carbon gradually decreases, leading to a total ecosystem carbon loss of ~14 PgC by the end of the simulation.
• Irreversible changes in CH4 emissions: As the climate warms, land ice melts, and surface water deposition increases, the flooded fraction increases primarily in grid cells that are already partially saturated, which overlaps significantly with the area dominated by organic soils. As a result, the increase in CH4 emissions is concentrated in areas with significant increases in flooded areas and high SOM content, i.e., areas where CH4 emissions are initially high. The increased flooded fractions, especially in organic soils, do not return to their previous state after the onset of zero and negative emissions, demonstrating irreversibility.
• Hysteresis of near-surface permafrost and its carbon cycle: The area of near-surface permafrost exhibits hysteresis in relation to CO2 concentration and SAT. As CO2 concentration decreases, the area of permafrost recovers, but it is smaller at the same CO2 concentration and temperature compared to the period of increasing CO2.

After the end of positive emissions, the extent of permafrost and ALT begin to recover, but the recovery is slow compared to the rate of temperature decline due to the thermal inertia of the soil, especially in areas with high SOM content. The total ecosystem carbon anomaly in the permafrost region is positive during the positive emissions phase due to increased carbon uptake by vegetation, which exceeds soil respiration. However, after the onset of zero and negative emissions, vegetation carbon remains stable or decreases, while respiratory carbon loss from litter and soil carbon pools gradually increases, leading to a net loss of ecosystem carbon in the permafrost region.

Uncertainties

Projections of carbon emissions from permafrost ecosystems under zero and negative emissions contain uncertainties related to changes in permafrost and Arctic ecosystems and climate change driven by AMOC. The amount of carbon emissions from soil is uncertain due to a lack of understanding of the dynamics of decomposition in deep soils and soil carbon content, as well as incomplete representation of permafrost processes in the model. CH4 emissions and their sensitivity to climate change are still uncertain due to the complexity of processes related to CH4 biogeochemistry and interaction with soil moisture. Many processes related to Arctic ecosystems are still poorly represented: simple plant communities and static vegetation composition. Spring

Glossary of key terms

• Permafrost: Permafrost is soil that remains below freezing for at least two consecutive years.
• Active layer (ALT): The top layer of soil that thaws and freezes annually.
• Ecosystem carbon: The sum of carbon stored in vegetation, litter, and soil.
• Heterotrophic respiration: The decomposition of organic matter by microorganisms, which releases CO2.
• Atlantic Meridional Overturning Circulation (AMOC): A system of ocean currents in the Atlantic Ocean that carries heat from the tropics to the north.
• CO2 fertilization effect: Increased plant growth due to higher concentrations of carbon dioxide (CO2) in the atmosphere.
• Net Primary Production (NPP): The rate at which plants fix atmospheric carbon through photosynthesis.
• Hysteresis: The dependence of the state of a system on its past. In the context of permafrost, it means the delay or inability of the system to fully recover even after conditions return to their original state.
• Cryoturbation: The process of soil mixing caused by repeated freezing and thawing.
• Like this: An area of unfrozen ground in a permanently frozen area.
• PgC (Petagram of Carbon): A unit of measurement of carbon, with 1 PgC = 10^15 grams of carbon.
• CO2 equivalent: A measure for comparing the radiative forcing (global warming) of different greenhouse gases.
• Net-Zero emissions: A state where anthropogenic CO2 emissions are balanced by anthropogenic CO2 removals over a period of time.
• Negative emissions: Removing carbon dioxide from the atmosphere at a greater rate than carbon dioxide emissions into the atmosphere.