As they grow, plants absorb carbon dioxide from the atmosphere during the process of photosynthesis, and this carbon is stored in the vegetation and in organic material present in the soils. Fires can burn in vegetation and also in carbon rich organic soils – and fires are indeed widespread in many of Earth’s environments. For example, landscape fires are part of the natural cycle of activity in boreal and savanna ecosystems.

In many environments humans are the source of most fire ignitions, and they can also alter the characteristics of certain landscapes in ways that make them more fire-prone. This has happened across many tropical peatlands in SE Asia for example, where fires are now quite commonplace in formally moist areas where forest has been cleared and peatlands drained. Purposely lit ‘prescribed burning' can also be used by humans to reduce the risk of catastrophic wildfires, by clearing out highly flammable vegetative litter, burning away surface vegetation, and allowing a patchwork of already burnt areas to be formed by fires conducted at times of the year when fire intensity is lower and the fires more controllable.

When fires burn across a landscape, some of the ‘terrestrially stored’ carbon present in the vegetation and sometimes also in the soil is rapidly re-released back into the atmosphere – mostly as carbon dioxide (CO₂). However, after the fire the vegetation in the burned areas regrows, and photosynthesis once again draws CO₂ back out of the atmosphere as part of the normal post-fire recovery cycle. However, if the fires have been set by humans in a forest as part of land-clearance activity to make way for agriculture, then the post-fire amount of carbon stored per unit area will remain far less than that in the original forest. In this way, land use change primarily in tropical forests is considered to be responsible for perhaps around 20% of current net global anthropogenic greenhouses gas emissions, with the vast majority of the rest coming from the burning of fossil fuels.

Most fires worldwide are lit by humans, and most fire regime changes are therefore related to changing human behaviours – for example those related to the expansion of agriculture and how we are managing land. Climate change may also be making some areas of our planet more fire prone due to changes in temperature and precipitation. For example, parts of Australia in recent years have seen extreme fires that many consider may be linked to climate change delivering a more flammable environment.

Northern latitudes are another region where climate change may be altering fire regimes. During Summer 2019, extreme fires burned in and near the Arctic circle, releasing around 50 megatonnes of carbon dioxide in June alone according to the Copernicus Atmosphere Monitoring Service (CAMS).  This amount of emitted CO₂ exceeds that released by Artic fires in the month of June between 2010 and 2018 combined. Fires were most extreme in Alaska and Siberia and their unusual nature is partly explained by 2019 being the hottest June on record in the Arctic. In parts of Siberia where the fires burned, air temperatures for June were almost ten Celsius higher than the average of those seen between 1981–2010 in the same month.  Smoke from these fires was transported widely, including thousands of kilometres away to affect air quality in North America.

In addition to carbon dioxide, the smoke from fires contains other greenhouse gases – such as methane – and many gaseous air pollutants such as carbon monoxide and numerous volatile organic compounds.

The most harmful air pollutant contained in the smoke is fine particulate matter - PM_2.5 – which are particles that are smaller than 2.5 microns in diameter. These tiny particles can be breathed into human lungs and can then significantly affect human health. This can occur even far from the causal fire due to the fact that these very small particles can be transported a long way from the fire itself. Depending on their composition, the emitted particulates can also scatter or absorb solar radiation, and for example the deposition on ice or snow of particles from Arctic fires can lead to increased melting via changes in surface albedo.

A further example of long-range transport of landscape fire smoke occurred in the US West Coast in September 2020 – where extreme fires generated smoke that greatly affected local air quality but also travelled many thousands of kilometres to affect the European atmosphere.  Toxicological studies have shown important health-related differences in the composition of wildfire-emitted PM_2.5 compared to ambient PM_2.5 sources, and wildfire-specific PM_2.5 is considered likely to cause a greater impact on respiratory health than that from other causes. The long-range transport of smoke containing wildfire-emitted PM_2.5 and the possibility for increased biomass burning in some regions means there is continuing concern for population health beyond directly fire-prone regions. The air quality across many SE Asian countries, even in cities far from any fires, is for example greatly impacted every year by smoke from the massive number of ‘crop residue’ fires lit to remove unwanted vegetation left after harvest – for example the straw that remains after rice production. During the annual ‘burning season’ air quality – particularly levels of PM_2.5 - can quite regularly reach levels considered by the World Health Organisation (WHO) as ‘hazardous to health’.

An example of how widespread landscape burning is, comes from the Copernicus Sentinel-3 satellites. Data from the Sentinel-3 satellites, two of which are operating at any one time, are used to map global fire activity every day, and even a month of data shows how extensive fire activity is around our planet.

In the map you can see a band of fires occurring just north of the equator, for example at the top of South America, in Africa south of the Sahara, and across much of Southeast Asia. Since fires are seasonal phenomena, at different times of year fires will be found at other locations. In many of these regions, the fires are related to agricultural activity, for example many of those in Southeast Asia are likely to be related to the aforementioned burning of crop residues left after harvest.

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Featured Educator

• ‍‍Professor Martin Wooster, Professor of Earth Observation Science, KCL

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