Large transboundary health impact of Arctic wildfire smoke

The Arctic is warming at a rate three times faster than the global average, with Siberia experiencing even more rapid warming. This accelerated warming significantly increases the risk of large-scale wildfires, as evidenced by the unusually large fires in 2019 and 2020, which burned nearly half the total area burned in the previous four decades. These fires release substantial amounts of carbon dioxide, potentially turning historically carbon-sink Arctic peatlands into carbon sources. Globally, while the total area burned by fires has declined, the total carbon emitted has remained stable due to an increasing proportion of burned area being forest, which has a greater carbon emission intensity than grassland. Climate change is expected to exacerbate this trend, particularly in the Arctic, increasing the frequency of peatland fires with their high CO2 emission intensity. Wildfires worsen air quality locally and internationally due to long-range atmospheric transport of smoke plumes. Fine particulate matter (PM2.5), a major component of wildfire smoke, is linked to increased mortality from cardiovascular and respiratory diseases. Wildfire-sourced PM2.5 is particularly concerning due to its potential higher toxicity compared to PM2.5 from other sources. This study investigates the transboundary health impacts of PM2.5 originating from wildfires within Arctic Council states, aiming to quantify the attributable mortality burden both within and outside the Arctic region.

Existing research highlights the escalating impact of Arctic wildfires on air quality. Studies have documented significant increases in summer PM2.5 concentrations in Siberia and Canada, exceeding air quality standards in various regions, including Japan. The long-range transport of wildfire smoke plumes has been established, demonstrating the transboundary nature of this pollution. The global health burden associated with PM2.5 exposure is well-documented, with epidemiological evidence linking it to millions of deaths annually from cardiovascular and respiratory illnesses. Studies also indicate a potentially greater toxicity of wildfire-sourced PM2.5 compared to other sources. Previous assessments of wildfire smoke health impacts have provided estimates at the global or regional level, but there is a lack of detailed analysis focusing specifically on the Arctic region and the transboundary effects of Arctic wildfires. This research addresses this gap by providing a comprehensive assessment of the health impacts of Arctic wildfire smoke.

To quantify the health impacts, the researchers used the Community Earth System Model (CESM) with the Quick Fire Emissions Dataset (QFED) to simulate global PM2.5 under two scenarios: 'FIRE ON,' including all fire emissions, and 'ARCTIC WILDFIRE OFF,' excluding wildfire emissions from Arctic Council states. The CESM simulations included the Community Atmosphere Model with chemistry (CAM-Chem), using the MOZART-TS1 chemical mechanism and the MAM4 aerosol model. The model ran at 0.9° by 1.25° resolution, with nudged meteorology from MERRA2 reanalysis data. Because CESM underestimates PM2.5 compared to observations, a bias correction was applied using the Geographically Weighted Regression PM2.5 (GWR PM2.5) reanalysis dataset for regions south of 68°N. Above 68°N the CESM wildfire simulation was used. The "FIRE FRACTION", calculated as the difference in PM2.5 between the two scenarios, was applied to the GWR PM2.5 data to create a "BIAS CORRECTED WILDFIRE OFF" scenario. The Global Exposure Mortality Model (GEMM) was used to estimate excess deaths due to chronic PM2.5 exposure in both scenarios. The difference in excess deaths between the scenarios represents the mortality attributable to Arctic wildfire smoke. Population data came from the Gridded Population of the World (GPW) dataset, and baseline mortality rates from the Global Burden of Disease 2019 study. Theil-Sen trend estimators and Mann-Kendall tests were used for trend analysis.

Wildfires accounted for a substantial proportion of annual mean PM2.5 concentrations across much of the Canadian and Siberian Arctic, reaching over 70% in remote areas. On average, wildfire PM2.5 contributed 21% of annual mean PM2.5 within Arctic Council states, with higher contributions in Russia (22.8%) and Canada (27.1%). The model attributed 21,100 (95% CI: 15,800–27,700) excess deaths annually to Arctic Council wildfire-sourced PM2.5, with over 8,100 deaths (95% CI: 5900–10,800) occurring outside Arctic Council states. Russia experienced the largest mortality burden (9900 (7300–13,100) deaths), followed by China (4800 (3500–6400) deaths) due to its proximity to Siberia and high population density. Despite a positive trend in summer PM2.5 concentrations in some parts of Siberia, the overall number of wildfire-attributed deaths decreased from 2001–2010 to 2011–2020. This is attributed to a northward shift in Siberian wildfires, reducing their impact on more densely populated regions. Analysis showed that the population-weighted mean PM2.5 attributed to wildfires decreased over time in Russia, while the area-weighted mean increased. This indicates that despite increasing wildfire PM2.5 concentrations, the impact on populated areas decreased.

The findings demonstrate the significant transboundary health impact of Arctic wildfires, extending far beyond the Arctic region. The substantial number of excess deaths attributed to Arctic wildfire PM2.5 highlights the urgent need for mitigation strategies. The observed decrease in health impact despite rising PM2.5 concentrations underscores the complex interplay between wildfire location, population density, and atmospheric transport patterns. The northward shift in wildfires might be temporary, and future changes in climate and fire regimes could alter these patterns. The study's results highlight the importance of international cooperation in addressing the transboundary health challenges posed by Arctic wildfires. Future research should investigate the factors driving the northward shift in wildfires and refine the models used to better capture the complex interactions between wildfires, atmospheric transport, and human health.

This study provides the first comprehensive assessment of the transboundary health impacts of Arctic wildfires. The findings quantify a substantial annual mortality burden attributable to wildfire-sourced PM2.5, both within and outside Arctic Council states. A surprising decrease in overall mortality despite increased PM2.5 is linked to a northward shift in Siberian wildfires. This research emphasizes the urgent need for international collaborations to mitigate the effects of Arctic wildfires and improve air quality, thereby reducing the associated health risks. Future research should focus on refining the models used and investigating the sustainability of the observed northward wildfire shift.

The study acknowledges potential limitations in the accuracy of PM2.5 estimations due to model underestimation and the lack of comprehensive Arctic PM2.5 observations. The use of national-level data for population age structure and baseline mortality rates may not fully capture regional variations within countries. The GEMM model, while widely used, may not perfectly reflect the relationship between PM2.5 exposure and health outcomes in Arctic populations due to differences in lifestyles and exposure patterns. The assumption that emissions from agricultural land are not wildfires and emissions from other land uses are wildfires introduces uncertainty. Despite these limitations, the study provides valuable insights and highlights the significant transboundary health impacts of Arctic wildfires.