Increasing large wildfires over the western United States linked to diminishing sea ice in the Arctic

Large wildfires pose a significant threat to the western United States, particularly in wildland-urban interface (WUI) regions. The frequency and extent of large wildfires (≥1000 acres) have increased dramatically in recent decades, resulting in substantial socioeconomic costs, including fatalities, property damage, and public health risks from smoke exposure. While previous research has explored the roles of human activity and anthropogenic climate change in driving these increases, the complex interplay of human and natural factors, along with natural climate variability, makes disentangling these influences challenging. Some studies suggest that climate impacts on fire activity may be masked by human influence on ignition, suppression, land use change, and forest management. However, in less human-impacted areas, climate change is still a major driver, affecting lightning ignitions, fuel availability, and fuel aridity depending on the fire regime. In the western U.S., several studies highlight the significant impact of global climate change on increasing forest wildfires through factors like enhanced fuel aridity and reduced high-elevation flammability barriers. While fire weather variables (temperature, vapor pressure deficit, precipitation) strongly correlate with fire activity in empirical models, these models lack the mechanistic understanding provided by physically-based Earth system models (ESMs) with interactive fire components. Recent advancements in fire modeling within ESMs offer new tools to explore the impact of anthropogenic climate change on fire activity. This study focuses on the potential teleconnection between declining Arctic sea ice and increased wildfire hazards in the western U.S., a relationship supported by some but not all previous studies. The research uses observations, reanalysis data, and climate model sensitivity experiments to explore this connection, analyzing both the mechanistic pathway and the relative importance of this teleconnection compared to other climate drivers.

Existing research highlights the complex relationship between climate change and wildfire activity. Studies have shown a strong correlation between warming temperatures, earlier springs, and increased wildfire activity in the western U.S. Other work emphasizes the role of decreasing fire season precipitation in exacerbating the situation. However, the influence of human activities, including fire suppression and land management practices, is also significant, making it difficult to isolate the specific contribution of climate change. Empirical models utilizing fire weather variables provide strong explanatory power for fire activity, but lack the detailed mechanistic insights offered by process-based Earth system models (ESMs). The decline in Arctic sea ice, a prominent indicator of Arctic amplification, has been linked to changes in mid-latitude weather patterns by several studies, suggesting a potential impact on regional fire weather. While previous research has indicated possible connections between Arctic sea-ice loss and wildfire activity in the western U.S., a comprehensive and quantitative evaluation of this link, considering both interannual and interdecadal timescales, has been lacking. The need for comprehensive study using fire-enabled ESMs is emphasized to explore this complex interaction effectively.

This study employed a multi-faceted approach combining observational data, reanalysis data, and climate model simulations. Observational data included sea-ice concentrations from the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST) and wildfire data from the Monitoring Trends in Burned Severity (MTBS) program. Daily and monthly meteorological variables were obtained from the ERA5 reanalysis dataset. The Fosberg Fire Weather Index (FFWI), based on ERA5 data, was used to assess fire weather conditions. To isolate the impact of Arctic sea-ice loss, the researchers conducted two sensitivity experiments using the Community Earth System Model with a region-specific ecosystem feedback fire model (CESM-RESFire). These experiments perturbed Arctic sea-ice concentrations and sea surface temperatures (SSTs) in the Pacific sector of the Arctic during July-October, representing minimum (SICexp-) and maximum (SICexp+) sea-ice conditions. The resulting changes in regional fire weather and burned area were compared. To assess the role of Arctic sea-ice decline within the context of other climate drivers, the authors analyzed data from the Coupled Model Intercomparison Project Phase 6 (CMIP6), specifically focusing on Atmospheric Model Intercomparison Project (AMIP) and AMIP with pre-industrial forcing (AMIP-piForcing) simulations. These simulations use observed SST and sea ice as boundary conditions while varying the atmospheric forcing levels. The Signal-to-Noise-Maximizing Pattern (S/NP) filtering method was applied to the ERA5 reanalysis and AMIP ensemble data to separate the effects of Arctic sea-ice changes from other climate variability, such as the El Niño-Southern Oscillation (ENSO). Various statistical analyses, including correlation analysis, composite analysis, t-tests, and bootstrap resampling, were employed to determine the statistical significance of the findings. Additionally, dynamic and thermodynamic analyses of atmospheric data were conducted using CESM-RESFire outputs to understand the underlying physical mechanisms.

The observational analysis revealed a strong negative correlation (r = -0.68, p < 0.01) between declining sea-ice concentrations in the Pacific sector of the Arctic during summer and autumn and worsening fire weather conditions in the western U.S. during the following autumn and early winter. Composite analysis of years with minimum (SIC-) and maximum (SIC+) sea-ice concentrations confirmed enhanced fire-favorable weather and increased burned area of large wildfires following Arctic sea-ice decline. This correlation persisted even after removing long-term trends, indicating a robust relationship across interannual and interdecadal timescales. CESM-RESFire sensitivity experiments showed that reduced Arctic sea ice during July-October led to an anomalous dipole pattern in the 500 hPa geopotential height field during the following autumn and early winter, featuring a cyclonic anomaly over Alaska and an anticyclonic anomaly over the western U.S. This pattern was consistent with that observed in the reanalysis data. The model simulations showed a positive shift in a fire-favorable circulation index (Z500i) in response to reduced sea ice, accompanied by suppressed clouds and precipitation, increased solar radiation, and a positive shift in FFWI. This resulted in increased burned area, driven by both more frequent fires and larger fire sizes. Analysis of AMIP and AMIP-piForcing simulations from CMIP6 indicated a dominant role of ocean/sea-ice surface conditions in driving observed fire weather changes, with the response to atmospheric and terrestrial forcing exhibiting distinct differences from the reanalysis-based results. S/NP filtering revealed that both ENSO and Arctic-driven teleconnection patterns contribute to warmer and drier conditions in the western U.S. during years with minimum sea ice, with magnitudes of these contributions being comparable. Analysis across multiple CMIP6 models corroborated the negative correlation between Arctic sea ice and FFWI, with most models capturing the correct sign and spatial patterns of changes in precipitation and FFWI associated with Arctic-driven teleconnections, although with varying magnitude.

The findings strongly support the hypothesis that declining Arctic sea ice plays a significant and synergistic role in exacerbating wildfire hazards in the western U.S. The mechanism involves reduced Arctic sea ice leading to amplified surface warming, increased turbulent heat fluxes, and an anomalous dipole circulation pattern. This pattern shifts the polar jet stream poleward, leading to drier conditions and suppressed precipitation over the western U.S. resulting in enhanced fuel aridity and increased wildfire activity. While the magnitude of this Arctic-driven effect is comparable to that of ENSO, the consistent increasing trend in the Arctic-driven pattern suggests its growing importance in the future. The study acknowledges limitations in the models, including missing interactive processes, but results from multiple sources show consistency and support the overall conclusions. The inclusion of both sensitivity experiments focusing on a single forcing and multi-model ensemble historical simulations with all forcing enables a more robust understanding of the physical mechanisms and relative contributions of various factors.

This study provides robust evidence for a significant teleconnection between declining Arctic sea ice and worsening fire weather in the western U.S. The mechanism involves changes in regional atmospheric circulation and surface conditions driven by reduced Arctic sea ice. This teleconnection has a magnitude comparable to other climate drivers like ENSO and is projected to increase in importance in the future, exacerbating wildfire risk. Further research should focus on improving model representations of interactive processes and expanding on adaptive resilience strategies.

The study acknowledges limitations in the models used, such as the exclusion of certain interactive processes (e.g., full ocean-atmosphere coupling and fire-climate feedbacks). The absence of these feedbacks could potentially underestimate the magnitude of the Arctic-driven effects. The reliance on a specific fire weather index (FFWI) might also introduce some uncertainty; however, sensitivity tests with alternative indices produced qualitatively similar results. Finally, the relatively short observational record for wildfire data could limit the ability to fully capture the long-term trends and variability.