In November 2020, a news article reported on a paper by Khaykin et al. concerning the unprecedented extent and magnitude of the recent Australian wildfires and the potential future impacts of increased wildfires under global climate change. There are concerns that climate change will likely result in an increase in frequency and magnitude of wildfires, with 25.3% increase in fire weather season length, and an increased global frequency in fire weather seasons of 53.4% already observed since 1979 (Jolly et. al, 2015).

Natural wildfires can be a key component in ecosystems, clearing dead and decaying plant material, reduce risks of pest, and creating area for new plant growth (Moritz et. al, 2014). Some plant species, such as the lodgepole pine, need regular fires in order to release their seeds (Lotan et. al 1985). Such events occur naturally during “fire weather season”, where a range of contributing factors such as low rainfall, low humidity, wind speed and high temperatures (Hübnerova et. al, 2020) greatly increase the likelihood of a fire. Historically these events are common in North America, Mediterranean Europe, Australia (Hunter et. al, 2020) and Africa (Gatebe et al., 2014). In some cases, controlled fires are used as a prescribed technique to maintain particular areas of land and/or to also reduce risk of much larger fires. Wildfire’s interact with local albedo too, the smoke from the immediate burning can increase albedo, and have a general cooling effect through the reduction of shortwave radiation reaching the surface (Stone et al, 2011). However, aerosol composition variations, such as black carbon, can also absorb incoming radiation (Stone et al., 2011). In the wake of a fire, surface albedo can vary depending on the original land type; in some areas with sparse vegetation, the burning of biomass can reveal brighter material such as sand below (Gatebe et al., 2012). Where there is greater vegetation, albedo is typically lowered due to the burnt remains of biomass absorbing more radiation (Gatebe et al., 2014).

Figure 1: Natural colour image of the Australian Wildfire acquired on January 4th from the MODIS on NASA’s Aqua satellite

Source: NASA Earth Observatory, 2020

The wildfire season in Australia between September 2019 and January 2020 saw mass areas burnt (Fig. 1) at an unprecedented scale of 5.8 million hectares, approximately 21% of Australia’s temperate forest area (Boer et al., 2020). Notably during the fire, there was formation of a pyrocumulonimbus (pyroCb) cloud (Ndalila et al., 2019) as seen in Fig. 2, which Khaykin et al. stated are comparable to volcanic eruptions in size. Typically, volcanoes can release volcanic ash and sulphur dioxide gas, causing an increase in aerosol concentration within the stratosphere, and have been known to offset the warming effect from incoming solar radiation (Ma et al., 2020). A pyroCb is characterised as a cumulonimbus (thunderstorm) cloud that uses the heat from fire to generate lift (Densmore, 2017). The tops of these formations can reach the upper troposphere and lower stratosphere (Ndalia et al., 2020), and are typically are made up of condensed water, smoke aerosols, organic and black carbon, as well as combustion products in their gaseous forms (Khaykin et al., 2020). NASA/CNES CALIPSO satellite recorded that the Australian Wildfire plume reached 25km above the surface, making it the highest recorded wildfire-caused plume tracked by CALIPSO (NASA Earth Observatory, 2020). Khaykin et al. noted that the instantaneous horizontal extent of the PyroCb reached 6.1 million km2 on January 7th.

Figure 2: The pyroCb cloud formation from the Australian Wildfire

Source: NBC News, 2020

Aerosol optical depth is a measure of the amount of aerosol present in the atmosphere. Using Stratospheric Aerosol Optical Depth (SAOD) as a measure, Khaykin et al. found that change in the stratosphere were comparable to two of the largest volcanic eruptions over the past 25 years, with the fires continuing to decay at a similar rate to the volcano’s over 3 months from the start of the event. The North American wildfires in 2017 were of a magnitude larger than previous wildfires, causing stratospheric variations on scale not seen before from pyroCbs, reaching 23km above surface level (Yu, 2019). However as shown in Fig. 3, the Australian wildfires had an even bigger perturbance to the stratosphere aerosol concentration, and were on par with the Calbuco eruption (2015) and the Raikoke eruption (2019).

Figure 3: SAOD perturbation at 746nm for a extended time period following the Australian wildfires (red), Pacific North West/Canadian wildfire (purple) and the Raikoke (blue) and Calbuco (green) eruptions

Source: Khaykin et. al, 2020

Due to the predominantly carbon-based nature of the aerosols within the PyroCb, the smoke cloud absorbed large amounts of sunlight and rose at a rate of 0.45km/day (Khaykin et al., 2020). The authors noted that this rise created a compact smoke bubble that peaked at 36km above surface level, much higher than volcanic aerosols over the past 25 years. Christian et al. (2020) had compared the height of the pyroCb from the North American (Pacific North West) fires, the Calbuco eruption and Puyehu eruption (2011) in Fig. 4, highlighting the vertical extent of the aerosols in the stratosphere over time, and the notable height of the Australian wildfire pyroCb.

Figure 4: The altitude development of the Australian wildfires (black), Pacific North West/Canadian wildfire (red) and the Puyehue (green) and Calbuco (blue) eruptions. The solid line represents data from the CALIOP lidar and dashed from the CATS lidar. Shaded areas are the 25th and 75th percentile.

Source: Christian et al., 2020

Khaykin et al. also found that a localised vortex formed around the bubble, following its movement and rise. Using data from European Centre for Medium-Range Weather Forecasts (ECMWF), an ozone depleting anomaly was identified above the smoke bubble, which Kablick III et al. (2020) found too. The assumed cause was thought to be the rise of ozone-poor tropospheric air, and the potential chemistry of the cloud that could lead to ozone depletion.

In conclusion, wildfires are a common occurrence with a range of benefits and risks, however the Australian wildfires occurred at a scale which had not been seen before. Furthermore, the impacts in terms of aerosol emissions were comparable to volcanic eruptions in the previous 25 years, as well as more novel effects such as ozone depletion. In the face of further climate change, there is greater risk of wildfires, leading to an increased likelihood of significant disturbances within the atmosphere due to wildfire induced aerosols. This will lead to alternations in how prescribed burns are issued, in order to control extreme wildfire events in the future (Di Virgillio et al., 2020).