Abrupt increase in Arctic-Subarctic wildfires caused by future permafrost thaw

The Arctic's cold climate typically results in long fire return intervals. However, unusually warm and dry summers can create wildfire conditions, as seen in northeastern Siberia (2020-21). Arctic and Subarctic wildfires, burning carbon-rich soils, release significant aerosols and carbon, impacting Earth's radiation budget and climate. Fire occurrence is influenced by atmospheric conditions (fire weather) and soil water content, with drier soils increasing fire frequency and extent. Permafrost, acting as a barrier to water drainage, significantly influences high-latitude soil moisture. Recent Arctic warming has initiated permafrost thaw, deepening the active layer and altering hydrological processes and soil moisture. Modeling these complex interactions, particularly projecting future impacts on wildfires, presents a major challenge due to the range of spatial scales involved. While some CMIP6 models incorporate permafrost-soil hydrology-fire coupling, a fully coupled assessment of future interactions remains lacking. This study aims to address this gap by investigating the impact of rapid permafrost thaw on high-latitude wildfires using the Community Earth System Model 2 (CESM2) large ensemble (CESM2-LE) under a historical/Shared Socioeconomic Pathways (SSP) 3-7.0 scenario, focusing on mechanisms triggering abrupt shifts in fire activity.

Previous research has linked unprecedented fire activity above the Arctic Circle to rising temperatures. Studies have highlighted the significant release of aerosols and carbon from Arctic and Subarctic fires, impacting the Earth's radiation budget. The role of soil moisture in controlling fire occurrences has been established, with drier conditions promoting more frequent and extensive fires. Changes in high-latitude soil moisture are governed by the interplay between precipitation, evapotranspiration, snow and ice melt, and runoff. The presence or absence of permafrost influences runoff, creating complex variations in soil moisture and fire occurrences. Recent studies have documented permafrost thaw's gradual deepening of the active layer and resulting changes in hydrological processes. While some models have begun to incorporate the coupling between permafrost, soil hydrology, and fires, a fully coupled assessment of their future interactions and impact on wildfires has been limited. One study identified potential future increases in fire severity following permafrost degradation using the Fire Weather Index (FWI), but without explicitly considering changes in vegetation, fuel, or soil hydrology.

This study utilizes the Community Earth System Model 2 (CESM2) large ensemble (CESM2-LE) forced by historical and SSP3-7.0 greenhouse gas emission scenarios. The CESM2-LE includes 50 ensemble members, allowing the differentiation of forced changes from natural variability. Observed linear trends in 2 m air temperature (T2M), ground temperature (TG), and active layer thickness (ALT) (1997-2019) were compared with simulated trends from the CESM2-LE. Change point analysis was performed on ALT, soil ice content, and soil moisture for each ensemble member to identify the timing of rapid shifts in soil properties. The study examined hydrological responses to rapid permafrost thaw, focusing on a grid cell in western Siberia to elucidate the mechanisms of soil hydrological changes. The time evolution of surface energy budget components (evapotranspiration, Bowen ratio, surface air temperature, relative humidity, sensible and latent heat fluxes, and ground heat fluxes) was analyzed to understand land-atmosphere interactions following permafrost thaw. The relationship between abrupt soil drying and wildfire activity was investigated by analyzing changes in burned area and fire counts. To isolate the impact of soil moisture on wildfires, two idealized experiments were conducted, reducing soil moisture by 20% and 40% in regions poleward of 40°N. The CESM2-LE simulations included the CLM5 land model, which incorporates comprehensive permafrost-related soil thermal and hydrological dynamics, carbon cycle dynamics, and a process-based fire parameterization. Observations used for model evaluation included burned area and biomass burning carbon emissions from GFEDv4, ALT and ground temperature from the ESA Climate Change Initiative permafrost project, and observed ALT from the CALM program network.

The CESM2-LE simulations project a rapid mid-to-late 21st-century increase in active layer thickness (ALT) and decrease in soil ice content, particularly in western Siberia, far eastern Siberia, and Canada. This is accompanied by a decrease in upper soil moisture and an increase in subsurface runoff. Analysis of a western Siberian grid cell revealed a 28% decline in near-surface soil moisture coinciding with rapid permafrost thaw. The vertical soil profiles showed soil temperatures in the upper layers reaching 0°C around 2030, with warming propagating to deeper layers and soil ice melting around 2050. Following rapid permafrost thaw, total evapotranspiration decreased, and the Bowen ratio increased in July and August over Canada and western Siberia. Surface air temperature increased by more than 2°C, and relative humidity decreased. Time evolution analysis in western Siberia showed an abrupt increase in the Bowen ratio around 2050, a decrease in latent heat flux, and an increase in sensible heat flux. Abrupt increases in burned areas were observed across historical permafrost regions, with the burned area after thaw being ~2.6 times greater than the pre-thaw period. The abrupt increase in wildfires followed abrupt soil drying driven by rapid permafrost thaw. Idealized experiments with 20% and 40% soil moisture reduction showed a nonlinear amplification of the burned area. The cumulative carbon emissions from wildfires in permafrost regions experiencing abrupt changes are estimated to reach 322.6 ± 74.7 TgC by the end of the 21st century, while the cumulative net ecosystem exchange would reach about 8.9 ± 256.5 TgC. The increase in convective available potential energy (CAPE) suggests a potential further increase in lightning and fire frequency.

The findings demonstrate that permafrost thaw can trigger abrupt regime shifts in soil hydrological processes and regional wildfires. Rapid thaw in ice-rich permafrost regions increases soil water percolation, causing sudden upper soil drying. Reduced ground evaporation and increased sensible heat fluxes lead to near-surface atmospheric warming and aridity, enhancing wildfire intensity. The results are consistent with previous studies using the FWI, but this study explicitly simulates interactions between climate, vegetation, permafrost, and fires, providing a more comprehensive representation of coupled feedbacks. While the model captures key interactions, limitations exist, including the simplified representation of subgrid-scale permafrost processes and the lack of explicit representation of fire ignition changes. The underestimation of the observed burned area compared to tropical and temperate latitudes might be due to the model's fixed climatological lightning frequency. The significant increase in CAPE suggests a potential further increase in fire frequency beyond the levels simulated explicitly in the model.

This study provides compelling evidence from a large-ensemble climate model that permafrost thaw in the Arctic and Subarctic regions can trigger abrupt increases in wildfires due to soil drying and changes in atmospheric conditions. The results highlight the importance of considering permafrost-hydrology-fire interactions in climate change projections. Future research should focus on improving the representation of subgrid-scale permafrost processes, fire ignition, and vegetation dynamics in Earth system models to enhance the reliability of climate and carbon cycle interaction projections in high latitudes.

The CESM2-LE model used in this study simplifies some subgrid-scale processes, potentially affecting the accuracy of the simulations. The model underestimates the observed burned area in the Arctic-Subarctic compared to tropical and temperate regions, possibly due to the lack of explicit representation of changes in fire ignition. The model uses a fixed climatological lightning frequency, which might not accurately capture future changes in lightning activity that could further influence wildfire occurrence. Additionally, the model doesn't account for all factors influencing wildfire activity, such as ground subsidence and lateral water flow.