In a study published in the Proceedings of the National Academy of Sciences, researchers from Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have provided a critical assessment of how climate change, through rising temperatures and increased aridity, has directly fueled the surge in fire damage and toxic smoke experienced over the last 30 years.
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In the western United States, the occurrence and severity of wildfires, along with the hazardous smoke they produce, have escalated since the 1990s – this is evident. However, the precise reasons behind this trend are less clear: Although global warming linked to greenhouse gas emissions is frequently mentioned as a factor, how can one accurately measure this assertion?
Researchers, under the leadership of Loretta Mickley, a senior research fellow specializing in chemistry-climate interactions and head of the Atmospheric Chemistry Modeling Group at Harvard SEAS, were involved in the study that indicated that climate change is responsible for 60-82 % of the total burned area in western U.S. forests and 33 % in central and southern California since the early 1990s. On average, this represents 65 % of the total fire emissions in the U.S. from 1997 to 2020.
Furthermore, between 1997 and 2020, nearly half of the most hazardous types of wildfire smoke in the western US, known as fine particulate matter or PM2.5 due to its minuscule particle size and its capacity to infiltrate the lungs and bloodstream, can be directly attributed to climate change. From 2010 to 2020, climate change accounted for 58 % of the rise in this specific type of smoke pollution.
A combination of observations, machine learning algorithms, and extensive climate models led to these findings, with the analysis conducted by the Harvard team distinctly illustrating the impact of shifting weather patterns and increasing temperatures on wildfire behavior since the 1990s. Additionally, the researchers employed a chemical transport model, GEOS-Chem, to assess the extent to which human-induced climate change contributes to smoke PM2.5 levels.
Our goal was to quantify how much climate change has amplified and exacerbated smoke exposure in the western U.S. Our hope is that this work will spur efforts to think more deeply about how we manage land and wildfires in the western U.S., as well as greater interest toward doing something about the increase in greenhouse gases.
Loretta Mickley, Senior Research Fellow, Harvard John A. Paulson School of Engineering and Applied Sciences
The researchers initially mapped various ecosystems throughout the western United States, including the forested mountains of the northwest, the Mediterranean regions of California, and the frigid deserts of the interior. They compiled decades of data regarding weather patterns, vegetation levels, and the extent of areas affected by fire in each ecosystem. Utilizing machine learning models, they were able to infer how the temperature, humidity, and aridity of vegetation correlated with fire activity.
The analysis revealed that pollution from alternative sources, such as industrial facilities, experienced a significant decline of approximately 44 % from 1997 to 2020, underscoring the effectiveness of the Clean Air Act. In contrast, wildfire smoke exhibited a contrary trend, showing a consistent increase over the same period.
The regions most severely impacted by wildfire smoke included northern California and portions of Oregon, Washington, and Idaho, where climate-induced smoke constituted 44-66 % of total PM2.5 levels from 2010 to 2020.
If you lived in these areas from 2010 to 2020, at least half of all of the fine PM you were breathing came from smoke.
Loretta Mickley, Senior Research Fellow, Harvard John A. Paulson School of Engineering and Applied Sciences
The team is currently engaged in efforts to quantify the extent to which the legacy of fire suppression from the 20th century may have exacerbated the impacts of climate change on contemporary wildfire activity. The accumulation of underbrush and the increasing density of forests have likely resulted in a greater availability of fuel for these fires, thereby contributing to heightened smoke exposure.
Mickley and colleagues' research continues to highlight the critical necessity for land managers and communities in the western United States to implement counteractive strategies, particularly prescribed burning in areas most susceptible to fire. Prescribed burning has the potential to remove underbrush and mitigate the spread of fires that could be catastrophic in the future.
This study received support from the Modeling, Analysis, Prediction, and Projection program within the Climate Program Office, which is part of the National Oceanic and Atmospheric Administration. The lead author of the paper is Xu Feng, a former postdoctoral researcher at Harvard, with co-authors including Jed O. Kaplan, Makoto Kelp, Yang Li, and Tianjia Liu.
Journal Reference:
Feng, X., et al. (2025) Large role of anthropogenic climate change in driving smoke concentrations across the western United States from 1992 to 2020. DOI: 10.1073/pnas.242190312. https://www.pnas.org/doi/10.1073/pnas.2421903122.