The Future of Permafrost: How Much Will Melt and Where Will the Carbon Go?
Introduction: Permafrost and Its Role in the Global Carbon Cycle
One of the numerous effects of global warming this century will be the thawing of permafrost, along with sea ice, glaciers, and the tourism sectors in coastal towns. Covering 15% of the northern hemisphere, permafrost is a repository of accumulated organic matter that remains frozen, preventing the release of its carbon.
The Impact of Global Warming on Permafrost Thaw
Warming Effects on Permafrost
The warming to the surface and lower atmosphere due to human-driven greenhouse effects raises pressing questions about the scale of permafrost thaw and the magnitude of carbon emissions into the atmosphere.
Challenges in Understanding the Carbon Cycle
The complexity of the carbon cycle adds layers of difficulty to answering this question. A new Study, leveraging a process-based biogeochemical model integrated with observational data, indicates that much of the thawed permafrost carbon is likely to remain trapped in formerly frozen layers. However, if thawing accelerates, it could pose a major obstacle to future climate change mitigation.
Research Findings: Permafrost Thaw and Carbon Emissions
New Research on Permafrost Thawing
This research, conducted by a team of four scientists from China and one from Purdue University in the United States, has been published in the journal Earth's Future.
Permafrost Formation and Its Vulnerability to Warming
Permafrost predominantly develops in regions where the annual average temperature remains below the freezing point of water. At average temperatures below -5°C, freezing becomes permanent under current climatic conditions, though it was significantly more widespread during the Last Glacial Maximum.
Polar Warming Amplification
This issue is exacerbated by polar warming amplification, where global warming intensifies with latitude rather than being uniformly distributed. For instance, since 1979, the Arctic has warmed nearly four times faster than the global average.
The Feedback Loop: Thawing Permafrost and Global Warming
Carbon Dioxide Emissions from Thawing Permafrost
Thawing permafrost contributes to a positive feedback loop, intensifying global warming through carbon dioxide emissions. The extent of this impact depends on the level of anthropogenic climate forcing. With nearly 1 trillion tons of permafrost susceptible to climate change, projecting its future presents significant modeling challenges.
Uncertainties in Permafrost Thawing Projections
Researchers have focused on minimizing uncertainties in the process, which arise from regional variations in thawing—potentially destabilizing buildings and communities—a lack of observational data in remote areas, shifts in vegetation that might sequester some carbon emissions, extreme weather events and wildfires, and what the authors term "the complex and unique interplay of water, energy, carbon, and nutrients among the atmosphere, plants, soil, frozen layers and microbes."
The volume of carbon released from thawing permafrost is primarily determined by the socioeconomic trajectory humanity chooses moving forward. Consequently, model outcomes are projections based on assumed parameters, not precise predictions.
Future Scenarios: Shared Socioeconomic Pathways (SSPs)
Two Key Scenarios for Permafrost Thawing
The team examined two well-established future scenarios, known as Shared Socioeconomic Pathways (SSPs). One, SSP126 (formerly RCP2.6), represents an optimistic scenario that limits global warming to 2.0 °C, while the other, SSP85 (RCP8.5), is a more extreme scenario in which fossil fuel use continues at current rates, dominating future energy consumption.
This research, led by Lei Liu from Zhengzhou University in China, advances previous models by integrating new physical processes, including the exposure and decomposition of soil carbon due to permafrost thaw at depths of up to 6 meters, double the depth considered in earlier studies.
The study also integrated soil organic carbon profiles derived from observational data sets. After validating the model, it was applied to assess permafrost thaw across the Nothern Hemisphere for the remainder of the century.
Projected Carbon Emissions Under Future Scenarios
The newly developed model estimates that between 2010 and 2015, the permafrost area in the Northern Hemisphere covered 14.4 million square kilometers, with 563 Gigatons (Gt) of carbon stored by 2015. Under the SSP126 scenario, which limits warming to 2.0°C, the model predicts that by 2100, 119 Gt of carbon will be released from permanently frozen soil as permafrost degrades, reducing the carbon in permafrost ecosystems by 3.4 Gt. In the extreme SSP585 scenario, 252 Gt of carbon would be released, depleting the same ecosystem by 15 Gt of carbon.
Long-term Effects of Permafrost Thaw on Carbon and Nitrogen Cycles
The Impact of Thawing on Carbon Release
The model estimates that only about 4% to 8% of the newly thawed carbon will be released into the atmosphere by 2100, aligning with expert projections made in 2015. This would result in a maximum of 10 Gt of carbon in the least impactful scenario and up to 20 Gt in the most extreme scenario.
In 2013, human activities, including burning fossil fuels, land-use changes, and raising cattle, released 11.3 Gt of carbon into the atmosphere, with approximately half remaining for years. Currently, there are 880 Gt of carbon in the atmosphere, 300 Gt of which is human-generated.
Thawing Permafrost and Nitrogen Availability
In this model, thawing permafrost does not seem to pose a major threat this century. However, as permafrost degrades, it enhances nitrogen availability in soil, as decomposing organic matter releases nitrogen in a form usable by plants, while deeper soil layers also contribute mobilized nitrogen.
The Role of Thawing Permafrost in Plant Growth and Ecosystem Dynamics
Positive Feedback to Global Warming
Such an increases can substantially promote plant growth and alter ecosystem dynamics. While modest, it acts as a negative feedback to global warming. Liu and his team's model suggests that thawing permafrost would elevate nitrogen stocks in vegetation by 10 and 26 million tons in the two scenarios, and boost carbon stocks in vegetation by 0.4 and 1.6 Gt.
While the carbon increase does not fully compensate for the carbon loss due to permafrost degradation, thawing has already led to considerable alterations in plant species composition and growth. Other effects are more intricate.
Future Climate Change and the Role of Human Emissions
Human Emissions and Global Warming Mitigation
To stop global warming, human emissions must reach zero; merely maintaining them at current levels won't be enough. Continued warming will lead to further permafrost thaw, exacerbating mitigation efforts in the short term and increasing feedback problems in the coming century.
Uncertainties and Complexities in Permafrost Thawing Projections
Accelerated Thawing and Its Implications
The greatest uncertainties in warming occur at high latitudes and altitudes. The research team highlights that deeper processes, such as "abrupt thaw, root deepening, and microbial colonization, may accelerate the breakdown of vast amounts of thawed soil organic carbon in deep soils," introducing more complexities into the carbon and nitrogen cycles to refine the quantification of carbon loss in permafrost soils.
Conclusion: The Uncertainty of Human Actions
Ultimately, the actions of humanity remain the largest source of uncertainty.
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