Extreme wildfires not only destroy ecosystems and biodiversity, but also emit a large amount of harmful pollutants and greenhouse gases into the atmosphere, posing a threat to people's health and the global climate. Recently, an international research team spearheaded by Professor Zhang Qiang from the Department of Earth System Science (DESS) in Tsinghua University and Assistant Professor Zheng Bo from the Shenzhen International Graduate School has, based on the self-developed near-real-time quantitative tracking system of global wildfire carbon emissions, found that with aggravating soil water deficit against the backdrops of global warming, extreme drought events at the middle and high latitudes of the northern hemisphere (north of 50ºN) have increased significantly, making this region gradually become a hot spot of global wildfire activities and carbon emissions. In 2021, the CO2 emissions of wildfires at the middle and high latitudes of the northern hemisphere reached a record high, accounting for 23% of the global carbon emissions of wildfires in that year.

The warming rate of the Arctic amounts to over twice that of other regions in the world. Climate warming leads to the flourishing of vegetation, while high-temperature heat waves make "fuel" extremely dry, causing an increase in the risk of wildfires in the middle and high latitudes of the northern hemisphere (boreal region). To accurately and timely track the global wildfire emissions, the research team has innovatively established a new method to invert the global wildfire CO₂ emissions by using CO satellite remote sensing data, and reconstructed the spatiotemporal dynamic changes of wildfire combustion efficiency based on the CO emission inversion data, thus realizing high-precision dynamic monitoring of wildfire CO₂ emissions. On the basis of this new method, the team independently developed a near-real-time quantitative tracking system for global wildfire carbon emissions, inversed and quantified the spatiotemporal pattern of wildfire CO₂ emissions from 2000 to 2021, systematically analyzed the drivers behind the wildfire emissions in the boreal region in 2021, and revealed the positive feedback mechanism between climate warming and wildfire emissions and potential future risks.

It is found that the CO₂ emissions of boreal wildfires have been on the rise since 2000, reaching a record of 1.76 billion tons in 2021, accounting for 23% of the global wildfire carbon emissions in that year, while the ratio was usually only about 10% twenty years ago. On the contrary, the wildfire burning and carbon emissions in equatorial regions, which have received extensive attention, are in a downward trend, and the interannual amplitude is less than the abnormally high emission value of boreal wildfires in 2021, and the emissions from such fires in 2021 were equivalent to the multi-year average level (Fig. 1).

The results show that wildfire activities have begun to invade the forests in the cold zone in high latitudes, which seldom happened in the past, and the spatiotemporal distribution pattern of boreal wildfires carbon emissions is undergoing drastic changes, posing serious threats to carbon sinks in the northern forests (Fig. 2). In 2000, the carbon emissions of boreal wildfires were mainly concentrated between 50º and 60º N, but in recent years, the growth rate of wildfire carbon emissions in the high latitude zone between 60º and 70º N has obviously accelerated. In 2021, the carbon emissions of wildfires in this region increased by more than 300% compared with the average from 2000 to 2020, while the emissions of wildfires near 50º N only increased by 70%. Combined with the data of vegetation type distribution, the results show that the increase of carbon emission from wildfires in high latitudes in the northern hemisphere mainly occurs in areas with high forest coverage. Forest soil in the northern cold zone is rich in organic carbon. When wildfire breaks out, aboveground vegetation and organic soil burn at the same time, releasing a huge amount of carbon. In the past, the high latitudes were less disturbed by fire, but at present, the impact of wildfire is increasing, which seriously threatens the carbon sink function of forest soil in the north.

It is pointed out in the research that soil water deficit is an important driver for the increase of boreal wildfires combustion, and the expansion of wildfires to forests in high latitudes and cold zone is closely related to the aggravation of soil water deficit. Extreme drought and heat wave in 2021 jointly boosted the wildfire combustion and carbon emissions to break the historical record (Fig. 3). In the high latitude cold zone forest, the areas where soil water deficit has increased year by year over the past two decades are accompanied by the rapid increase of summer fires and carbon emissions. In the years and regions with serious water deficit, such as forests in northern Canada in 2017 and forests in Siberia in 2019, serious wildfires broke out, and the CO₂ emissions were significantly higher than the multi-year average. In the summer of 2021, the temperature in these two areas was high, the precipitation and soil moisture were low, and the heatwave index was high, reaching the extreme level since 2000. Serious soil water deficit occurred in the forests of Siberia, Russia and northern Canada at the same time, and the cumulative water deficit from July to August exceeded 20 mm, which occurred the first time since 2000.

Given the accelerated warming of the Arctic, that is, the amplification effect of the Arctic, high-temperature heat waves and droughts may occur more frequently in the northern hemisphere in the future, and the frequency and intensity of extreme wildfires similar to those in 2021 may continue to increase, and the released CO₂ emissions will further promote global warming, forming a positive feedback between climate warming and wildfire emissions. Arctic warming trend increases forest biomass and aggravates soil water deficit. With the increase of evapotranspiration, air humidity and thunderstorms in the Arctic, the risk of extreme wildfire increases. After the occurrence of extreme wildfire, the recovery and reconstruction of soil microbial community and vegetation are slow, and the function of carbon sink is weakened. A large amount of carbon released by wildfire combustion pushes up the concentration of CO₂ in the atmosphere and promotes global warming, which further increases the risk of extreme wildfires.

The research suggests that in the future we should pay more attention to the risk of wildfire occurrence and its ecological environment impact in the northern forest under the background of global warming, and establish a system for dynamic monitoring of wildfire emission and assessment of ecological environment impact, so as to predict the risk of wildfire occurrence, give early warning against the hazard of wildfire pollution, monitor the flux of wildfire emission, and evaluate the ecological rehabilitation and reconstruction after a fire, thus providing support for formulating scientific and effective wildfire management and regulation policies. Since 2018, the Research Group led by Zheng Bo has started to develop global wildfire emission monitoring and inversion technology based on satellite remote sensing. After five years of constant improvement, the data accuracy and precision have been improved. The near-real-time quantitative tracking system of global wildfire CO₂ emissions built by the team can be used as an important module of the system for wildfire emission dynamic monitoring and ecological environment impact assessment to dynamically assess the impact of wildfires on the atmospheric environment, ecosystem and global climate. With the rapid development of satellite remote sensing monitoring technology, this system will be further improved in the future.

The above results have been published in Science in the form of a research article titled “Record-high CO₂ emissions from boreal fires in 2021”. Zheng Bo is the first author of the article, Zhang Qiang and Zheng Bo are co-corresponding authors. Other authors also include Researcher Philippe Ciais, foreign academician of the Chinese Academy of Sciences, distinguished visiting professor of Tsinghua University, and Laboratoire des Sciences du Climat et de l’Environnement, France, Professor He Kebing, Academician of the Chinese Academy of Engineering and Professor of the School of Environment, Tsinghua University, several domestic cooperators from Tsinghua University and Harbin Industrial University (Shenzhen), as well as over ten overseas collaborators from Germany, Australia, the United States, Denmark, and Spain. The research has been supported by the Young Talent Support Project of the China Association for Science and Technology (CAST), the Innovative Research Group of the National Natural Science Foundation of China, and the Research Starting Fund of the Shenzhen International Graduate School of Tsinghua University.

Full-text link of the article: https://www.science.org/doi/10.1126/science.ade0805

Fig. 1. Inversion estimates of annual and monthly emissions of boreal wildfire carbon emissions and comparison of boreal and tropical fire carbon emissions .

Fig. 2. The 2021 fire season anomaly (relative to the 2000 to 2020 fire season mean) of MOPITT and modeled CO columns, MODIS burned areas, inversion estimates, and weather variables in the boreal region (> 50°N).

Fig. 3. Distribution of boreal fire CO₂ emissions and climatic water deficit in North America and Eurasia

Written by Zhang Qiang

Edited by Wang Jiayin

Reviewed by Lu Hui