A new World Meteorological Organization bulletin on Aerosols examines the impact of biomass burning (wildfires and open burning for agriculture) on climate and air quality. It covers the episodes of the 2019/2020 Australian bushfires, the 2015 Indonesia peatfires and smoke transport from boreal forest fires to the Arctic.
Worldwide fire activity evolves continually throughout the year and many regions of the world have relatively clearly defined fire seasons that do not vary significantly from one year to another. This is generally most apparent in tropical regions where fire conditions are driven by the onset and duration of the dry season, when fire is used as a tool for burning off the wet season growth of vegetation for agricultural purposes. In extratropical regions, fire activity typically occurs in the summer months but generally has a higher degree of variability compared to fires in the tropics.
The general trend in global fire activity has been observed to be one of decline over the last 20 years largely driven by changes in the use of fire for agricultural land clearing in tropical regions. Clear trends are difficult to ascertain for extratropical regions due to interannual variability in the distribution and scale of fire activity, according to the WMO Aerosol Bulletin on Biomass Burning.
The Aerosol Bulletin reflects on a number of episodes of forest fires that have led to substantial air pollution events with elevated aerosol concentrations.
Australian Bushfires
The bushfire (wildfire) event of the 2019/2020 southeastern Australian summer has been described as unprecedented. Between September 2019 and February 2020, 12 million hectares (120 000 square km) of land burned, leading to the direct loss of 34 lives, the destruction of 3 500 houses and significant wildlife and habitat losses.
Smoke from fires severely reduced air quality across the entire south-eastern region of Australia, from Queensland in the north to Tasmania in the south. Long-range transport of smoke was observed to reach New Zealand on several occasions.
The maximum daily concentrations of PM2.5 particulate matter (particles with a diameter of smaller than 2.5 micron) in the smoke plumes in Queensland and New South Wales were 4 times higher than the national standard, in Victoria 8 times higher. In the Australian Capital Territory they were almost 40 times greater, and at one air quality monitoring station the daily average PM2.5 concentration exceeded the national standard for 53 days between 1 November 2019 and 28 February 2020.
More than 10 million people are likely to have experienced some exposure to these very hazardous concentrations of PM2.5, according to figures cited in the Aerosol Bulletin. One study estimated that this exposure may be responsible for approximately 400 excess deaths, 1 120 hospital admissions for cardiovascular problems, 2 030 admissions for respiratory problems, and 1 300 emergency department attendances for asthma.
Indonesia peat burning
In 2015, extensive burning of peat occurred throughout large parts of Indonesia from August to November.
Peat fires are extremely difficult to extinguish and can burn continuously until the return of the monsoon rains. The strength and prevalence of these fires are strongly influenced by large-scale climate patterns such as El Niño.
The 2015 fires escalated to an environmental and public health catastrophe. Fire emissions during that period were consistently and extraordinarily strong.
In addition to the air quality and climate implications of such events, which add large amounts of aerosols to the atmosphere, the weather is also altered. During the 2015 Indonesia peat fires, surface cooling related to smoke from the fires was evident, according to the Aerosol Bulletin.
In response to the Indonesian wildfire crisis of 2015, WMO has initiated a Vegetation Fire and Smoke Pollution Warning and Advisory System, with the aim of providing guidance to affected WMO Member States through specialized regional centres. The first Regional Vegetation Fire and Smoke Pollution Warning and Advisory Centre has been established for the WMO South-West Pacific Region, operated by the Meteorological Service of Singapore.
Smoke transport from boreal forest fires to the Arctic
Large fires in the boreal forests of Eurasia and North America are a common occurrence from late spring through to early autumn. They can release large quantities of smoke to the atmosphere where they may be subject to long-range, even intercontinental, transport over thousands of kilometres.
Recent summers have seen significant fires in boreal forests in North America and Siberia and poleward of the Arctic Circle, smoke from which has been observed to be transported high into the Arctic Circle and occasionally to undergo transpolar transport.
Increasing boreal forest fire activity and transport of smoke into the Arctic Circle have potential climate impacts through increased surface deposition of black carbon and particulate matter on to sea ice, affecting the albedo, that is, the amount of solar radiation reflected, leading to enhanced warming and melting, and changes in absorption/reflection of atmospheric radiation with potential impacts on polar meteorology and climate.
Estimates of emissions are available from satellite observations of either the burnt area or fire radiative power (a measure of the rate of emitted radiative energy by the fire at the time of the observation). These observations can be used to estimate the amount of vegetation consumed by fires, information that can be further used to estimate the amount of carbon, gases and aerosols released to the atmosphere. The European Commission's Copernicus Atmosphere Monitoring Service (CAMS), implemented by ECMWF, is one such service and makes use of a wide range of satellite observations to produce analyses and 5-day forecasts. Forecasts from CAMS provide information on black carbon and organic matter aerosols and allow the monitoring of smoke transport from fires worldwide.
Future Directions
The global observation system that has grown and been greatly expanded over the last 20 years, including the availability of active fire data exploitation of the wide range of currently available Earth fire observations that provides essential information on global fire activity and emissions.
Combining data from different satellite sensors will help to increase timeliness of global fire data and provide valuable information for civil protection and air-quality monitoring.
