Stratosphere amplifies the global climate effect of wildfires

Abstract

The summer of 2019 witnessed a great number of wildfires around the globe. For example, wildfires scorched huge swaths of Alaska, British Columbia in Canada, and parts of Greenland. And forest fires in Siberia and the Far East of Russia were also very serious, affecting nearly 3 million hectares of land, and putting some parts of Russia into a state of emergency. In particular, forest fires in the Amazon region had attracted global attention. According to Science, as of August 24, Brazil’s National Institute for Space Research had counted more than 41000 fires this year, 86% more than last year, compared with 22000 in the same period last year. Scientists in Brazil and elsewhere believe that the recent increase of wildfires is closely related to the increasing activities of deforestation (Arruda et al., 2019; Escobar, 2019). According to the Global Fire Emission Database and the Global Fire Atlas dataset of NASA Moderate Resolution Imaging Spectrometer (MODIS), a total of 13.3 million fires was identified during 2003–2016, with an average of about 1 million fires per year (Andela et al., 2019). Among them, Africa’s grasslands and savannas regions have the highest density of ignitions in the world, accounting for about 64% of the total number of fires in the world, while the Amazon region accounts for about 10%. The world’s largest individual fires are mainly found in sparsely populated arid and semi-arid grasslands and shrublands of interior Australia, Africa, and Central Asia, and high densities of fire points and relatively small fires in Africa are related to the widespread use of fire for land management in these regions. Eastern China is also a fire-prone area. The number of fires increased from about 25000 in 2001 to about 63000 in 2007. The implement of “the Agricultural Residues Burning Reduction Act” in China during the 2000s, which bans the open-air burning of crop residue, has effectively curbed the rapid increase trend of fire points. In 2016, the number of fires dropped to about 53000 in China (Earl and Simmonds, 2018). Wildfires can burn through forests, threaten the lives and properties of nearby residents, disrupt air transport, cause serious air pollution, and also destroy ecosystems, affect biodiversity and generate a large amount of carbon dioxide. The burning of permafrost in high latitudes will also affect the stability of permafrost, release methane trapped in the permafrost. Meanwhile, the soot from the fires can deposit on Arctic ice, darken ice and reduce the ice albedo, which may speed up its melting and accelerate the global greenhouse warming effect. In particular, wildfire combustion may also inject large quantities of black carbon into the stratosphere, thus affect the ozone layer and wider climate. A recent study (Yu et al., 2019) has monitored the evolution of soot particles formed by wildfires in British Columbia, western Canada, in August 2017. In that month, there were large number of fires in this area. On August 3 alone, 446 fires were recorded. Among them, powerful wildfires can generate pyrocumulonimbus (pyroCb), which are violent storms that act as a funnel, injecting smoke particles into the stratosphere. The strato-