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The Carbon Time Bomb Beneath the Arctic

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Among the many aspects of global warming expected to accelerate this century—melting sea ice, shrinking glaciers, and the decline of coastal tourism industries—is the thawing of permafrost. Found beneath approximately 15% of the Northern Hemisphere, permafrost is a layer of frozen soil and organic material, rich in carbon accumulated over millennia from decayed plant and animal matter. Locked in a frozen state, this biomass has never had the chance to decompose fully and release its carbon into the atmosphere. However, as the Earth’s surface warms due to human-induced climate change, the fate of permafrost has become a critical concern for scientists and policymakers alike.

The primary questions are: How much of the permafrost will thaw as temperatures rise, and how much carbon will be released into the atmosphere as a result? These questions are complex, influenced by various interconnected processes within the global carbon cycle. To address them, scientists have turned to advanced biogeochemical models that incorporate observational data to simulate the dynamics of permafrost thaw. A recent study by an international team of researchers sheds new light on this issue, providing both alarming and cautiously optimistic insights into the future of permafrost and its role in global warming.

Permafrost forms in regions where the average annual temperature remains below freezing. In areas with average temperatures lower than -5°C, permafrost can remain permanently frozen under the current climate. Historically, during the Last Glacial Maximum, permafrost covered a much larger area than it does today. However, global warming is not evenly distributed across the planet. Due to a phenomenon known as polar amplification, the Arctic has warmed nearly four times faster than the global average since 1979, making it a hotspot for permafrost thaw.

Thawing permafrost poses a significant challenge because it creates a positive feedback loop in the climate system. As permafrost thaws, it releases greenhouse gases like carbon dioxide (CO₂) and methane (CH₄) into the atmosphere. These emissions further enhance the greenhouse effect, accelerating global warming and, in turn, leading to more permafrost thaw. Scientists estimate that approximately 1 trillion tons of carbon stored in permafrost is vulnerable to release due to climate change, but modeling the exact dynamics of this process is an intricate task.

The uncertainties surrounding permafrost thaw stem from a range of factors. These include regional variations in thawing rates, a lack of observational data from remote Arctic areas, and the influence of other environmental processes such as vegetation changes, extreme weather events, and wildfires. Additionally, the interactions between the atmosphere, soil, plants, microbes, and frozen layers add further complexity to the system. Researchers also highlight the role of socioeconomic factors, as the trajectory of permafrost emissions will largely depend on future human actions and climate policies.

To better understand the potential impact of permafrost thaw, the study led by Lei Liu of Zhengzhou University and collaborators employed a refined model incorporating new physical processes. This included analyzing soil carbon exposure and decomposition in deeper layers, up to six meters below the surface, which is double the depth considered in earlier studies. By integrating detailed soil organic carbon profiles from observational datasets, the model provides a more comprehensive view of permafrost dynamics in the Northern Hemisphere.

The model was applied to two widely recognized future climate scenarios known as Shared Socioeconomic Pathways (SSPs). The first scenario, SSP126 (aligned with RCP2.6), represents an optimistic future where global warming is limited to 2.0°C through aggressive climate action. The second, SSP585 (aligned with RCP8.5), assumes a business-as-usual approach with continued reliance on fossil fuels and minimal mitigation efforts. The results highlight stark differences between these scenarios in terms of permafrost degradation and carbon emissions.

Under the SSP126 scenario, the model estimates that 119 gigatons (Gt) of carbon will become available for decomposition by 2100, leading to a net loss of 3.4 Gt of carbon from permafrost ecosystems. In the more extreme SSP585 scenario, 252 Gt of carbon would become available, with a net loss of 15 Gt of carbon. However, only a fraction of this newly thawed carbon—between 4% and 8%—is expected to be released into the atmosphere by the end of the century. This translates to a maximum emission of 10 Gt of carbon under SSP126 and 20 Gt under SSP585.

To put these numbers into perspective, human activities in 2023 emitted approximately 11.3 Gt of carbon, half of which remains in the atmosphere for extended periods. The atmosphere currently contains around 880 Gt of carbon, of which 300 Gt has been added by humans since the Industrial Revolution. While the emissions from thawing permafrost may seem relatively modest compared to annual human emissions, they represent a persistent and growing challenge for climate change mitigation.

The study also highlights secondary effects of permafrost thaw, such as changes in soil nitrogen availability. As organic matter in thawed permafrost decomposes, it releases nitrogen in forms that plants can absorb. This increased nitrogen availability can enhance plant growth and alter ecosystem dynamics, creating a small negative feedback to global warming. In the model, permafrost thaw increased nitrogen stocks in vegetation by 10 million tons under SSP126 and 26 million tons under SSP585. Correspondingly, carbon stocks in vegetation increased by 0.4 Gt and 1.6 Gt, respectively. Although this additional carbon uptake by plants is beneficial, it does not fully offset the carbon losses from permafrost degradation.

Moreover, the degradation of permafrost has broader implications beyond carbon emissions. It can destabilize landscapes, leading to infrastructure damage in Arctic communities, and disrupt ecosystems by altering species composition and biodiversity. Abrupt thaw events, such as the formation of thermokarst lakes, can release large quantities of methane—a potent greenhouse gas—further complicating the climate feedback mechanisms.

The future of permafrost and its role in the global carbon cycle ultimately depends on human actions. To halt warming and stabilize the climate, global carbon emissions must reach net zero. Simply leveling off emissions at current levels will not suffice, as warming will continue as long as greenhouse gases accumulate in the atmosphere. If emissions are not curtailed, the 22nd century could see even more significant permafrost degradation, amplifying the challenges of climate change.

High latitudes and altitudes remain regions of significant uncertainty in climate projections. Processes such as abrupt thaw, root deepening, and microbial activity in deep soils may accelerate the decomposition of organic carbon, adding further complexities to permafrost dynamics. The study underscores the need for continued research and better observational data to refine models and reduce uncertainties.

Their findings were published in the journal Earth’s Future.