The 2025 wildfire season in Canada has been long and widespread with over 6.8 million hectares burned as of August 6, 2025, placing this year in the top 5 seasons, but less than half the size of the devastating 2023 season, according to the Canadian Wildfire Information System. Smoke from the fires traveled to the USA, Europe and beyond. The June 2025 release of SAGE III/ISS multi-wavelength aerosol extinction coefficient profiles allows scientists to see what smoke was transported into the stratosphere.
Figure 1a is the June 2025 zonal mean vertically resolved aerosol extinction coefficient at a wavelength of 1021 nm. Latitudes north of 40N have values 2 – 4 times larger in the lower stratosphere, < 17 km, when compared to May 2025 (Fig. 1b). Even when compared with the record setting 2023 season, the 2025 aerosol enhancements are 2-4 times over a substantial portion of the lower stratosphere. In contrast, much of the smoke from the 2023 season remained in the upper troposphere.
Another comparison with past perturbations to the Northern Hemisphere lower stratosphere is a time series of the aerosol extinction coefficient in Figure 2 for the latitude band 55N to 60N. Discontinuities that appear are from the occultation sampling at the higher latitudes, which is not continuous throughout the year, but does emphasize the summer months when the wildfires occur. The June 2025 enhancement can be seen for altitudes below 15 km at the right end of the series. At the other end, the 2017 Pacific Northwest fires can be seen with enhancements up to 17 km. The mid-year 2019 Raikoke volcanic eruption produced increases up to 20 km. Further measurements from SAGE will tell whether the June 2025 increases will remain in the stratosphere. Each wildfire event is unique and continued measurements of stratospheric aerosol is the only reliable means to know to what extent an extreme fire has perturbed the stratosphere and altered the energy budget and ozone chemistry.
A detailed review of the greater SAGE II/III record by Thomason and Knepp (2023) indicates that smoke injections into the stratosphere may be more prevalent in recent years. The stable, multi-wavelength measurements from solar occultation enable quantitative evaluation of the global impact by these extreme regional wildfires.
Figure 1a: Monthly zonal mean aerosol extinction coefficient profiles at wavelength of 1021 nm for a) June 2025, b) May 2025, c) June 2023.
Figure 1b: Monthly zonal mean aerosol extinction coefficient profiles at wavelength of 1021 nm for a) June 2025, b) May 2025, c) June 2023.
Figure 1c: Monthly zonal mean aerosol extinction coefficient profiles at wavelength of 1021 nm for a) June 2025, b) May 2025, c) June 2023.
Figure 2: SAGE III/ISS aerosol extinction coefficient time series for latitude range 55N to 60N. Coverage is not continuous because of occultation sampling, which produces some discontinuities. Recent enhancement can be seen below 15 km in June 2025. Other notable enhancements were produced by the Pacific Northwest fires in 2017 (up to 19 km) and Raikoke volcanic eruption in mid 2019 (up to 20 km).
References:
Thomason, L. W. and Knepp, T.: Quantifying SAGE II (1984–2005) and SAGE III/ISS (2017–2022) observations of smoke in the stratosphere, Atmos. Chem. Phys., 23, 10361–10381, https://doi.org/10.5194/acp-23-10361-2023, 2023.
