Measuring greenhouse gas emissions from wildfires from space

Vast areas of the Northern Hemisphere in Siberia, Canada, and Alaska are covered in forest. Fires (wildfires) in this wilderness are quite common, with the forests being at risk twice a year, first in spring and then in late summer. Occasionally these wildfires reach populated places causing damage to homes and other city buildings. Furthermore, many experts believe that carbon dioxide (CO₂) and methane (CH₄) emissions play a role in modern climate warming. Monitoring these wildfires poses a challenge regarding the remote locations and the potential hazards to researchers. Satellite observations from space (Yurganov and Rakitin, 2022) are free of these constraints.

In recent years, enormous tracts of forest in Russia, the US, and Canada have been burning, and the size of the burned areas has been increasing. As a result, researchers have been asking the question: how much greenhouse gas is released into the atmosphere? CO₂ is the result of the complete combustion of carbon (wood) and is the main product of burning. Under conditions of insufficient oxygen such as during the peat fires, a significant part of organic matter is oxidised to carbon monoxide (CO). In addition, at high temperatures, organic carbon in soil is converted into CH₄, the second most important greenhouse gas after CO₂ .

Measuring greenhouse gas emissions

Measuring greenhouse gas emissions can be carried out in two ways. In the first approach, called a ‘bottom up’ technique, eg, GFED4 (van der Werf et al, 2017), the burned area is determined from satellite observations in the visible spectral regions of the solar light reflected from the ground. The emission is then calculated as the amount of burned organic substance multiplied by known factors for each product. A ‘top-down’ technique (eg, Yurganov and Rakitin, 2022) employs measurements of product concentration in the atmosphere only. Concentrations may then be recalculated into emission.

Carbon monoxide

To learn how much CO₂ is emitted from wildfires, an obvious way is to measure its concentration. So, why do we use CO instead? Fires increase CO₂ concentration by ~2 ppm (2 parts per million parts of air) over the background of ~400 ppm (0.5%). Fires in 2021 led to an increase in CO concentration up to ~170 ppb (170 parts per billion parts of air) compared with the ~90 ppb background, ie, ~90% . Therefore, we have a real chance for verification (validation) of the bottom-up estimates of CO emission by the van der Werf group. If this were to be validated, then CO₂ and CH₄ bottom-up estimates could also be considered trustworthy.

The conversion of the measured atmospheric concentration of gas into its emission from fires is a separate task. 18 years ago, the results of studies by an international group of scientists from Japan, Russia, Belgium, USA, Germany, Sweden, and Canada on CO emissions from catastrophic fires in 2002 and 2003 were published (Yurganov et al, 2005). They employed a so-called box model. The same model was applied to the updated satellite measurements now (Yurganov and Rakitin, 2022).

Results and conclusions

The emission rates in percent per year for two estimation ways were determined as: 4.8 ± 2.7 (top-down CO), 5.1 ± 1.9 (bottom-up CO) and 4.8 ± 1.9 (bottom up CO₂). As concluded by Yurganov and Rakitin (2022), ‘the total CO emissions from biomass burning in the High Northern hemisphere were mostly in the range of 50-100 Mt CO yr⁻¹. For three years (2003, 2012, and 2021) wildfires emitted 125, 135, and 142 Mt CO yr⁻¹, respectively.’ They went on to note that, ‘A similar pattern of yearly CO emissions was found in the GFED4 database, except for 2012. After excluding the marginal values for 2003 and 2021, both approaches showed a statistically significant positive trend of 4.8–5.1 % yr⁻¹. The record high top-down and bottom-up emissions in 2021 supported the finding of an increase in HNH biomass burning (mostly boreal fires) over the past decade. The possibility of further acceleration cannot be ruled out.’

Finally, they commented that, ‘A 20-year data set allowed us to propose a classification of fires depending on their intensity. [….]. The fires of 2002, 2003, and 2021 can be classified as catastrophic (or megafires). Megafires happen from time to time and are likely due to long-lasting blockages in high-pressure systems (heat waves) and severe droughts. Our results can contribute to more reliable forecasting of the various types of forest fires.’