Natural Fires

Among water, air and soil, fire is one of the 4 elements that formed our planet long since the outset of mankind. With the beginning of the plants spreading out onto the continents, fire started to change the landscape and the vegetation. Natural causes for burning vegetation is primary lightning, but also volcanic eruptions can cause fires. Some ecosystems are adapted to the regularly recurring of vegetation fires. The tropical savannahs in Africa, South America and Australia are counted among these as well as the boreal coniferous forests in Russia and Canada/Alaska. Interacting with the regularly recurring fires, a unique variety of plants and animals developed in the tropical savannah. Although individual plants are burnt in a fire, new plants of the same kind can grow rapidly. All plant species in the savannah are adapted to being regularly burnt down. The regularly occuring fires swamp plant species, vulnerable to fire, out of the savanah. Fire in the boreal coniferous forests plays a role of similar importance. Indeed it does not occur as often as in the tropical savannahs, but at intervals of several decades. Also here, plant species are domiciled, which are strongly adapted to the occuring of fires. E.g., the boreal pine needs a fire event to rejuvenate. Only at very high temperatures (above 1000°C), the pine cones open and the seeds drop onto the ground. There they find optimal conditions to grow after a fire.

Manmade Fire

Man uses the fire to form the natural scenery according to his own mind since long ago. It is a common method to alter forest into farmland. The increasing population, especially in the tropical developing countries, imposes continously additional pressure upon the forest and other ecosystems. Nowadays, more than 90% of all vegetation fires throughout the world are caused by human activities. Only in some remote areas of the boreal coniferous forest, lightning is still the primary source of fires. More and more regions, not or only rarely influenced by fires under natural circumstances, are exposed to fire events. This is especially the case for the topical rainforests in Amazonia and Asia. There, huge areas of rainforest are lumbered and burnt every year. As the local plant and animal species are not adapted to the fire, they have no chance of protecting themselves against the fire and might die out in the worst case.

Fire and Climate

The impact of vegetation fires on the climate is multifarious. Due to the dissapearing of the rainforests, the evaporation and as a consequence the precipitiation in the tropical regions, is changing. The gases and particles emitted by the combustion cause air pollution throughout a wide area. The forest fires in Indonesia in 1997/1998, which caused the air quality to decrease dramatically in Southeast Asia (Indonesia, Malaysia, Signapur) and airports to be closed, are only one example of the nationwide impact of vegatation fires.

Besides organic compounds like carbondioxide (CO2), carbonmonoxide (CO) and methane (CH4), the gaseous emissions of fires consist of compounds containing nitrogen, e.g., nitrous oxides (NO and NO2). This mixture of gases leads to the photochemical formation of ozone (O3) in the troposphere. These are the same chemical processes leading to enhanced ozone concentration in our urban regions in summer (photosmog). As the vegatation fires occur regularly, especially in the tropics, on wide areas, they are a globally significant source of ozone. In additon to the gases, of course, also a great number of particles is emitted by the vegetation fires into the atmosphere. As these are mostly relative small sized particles, they can remain for a long period of time in the atmosphere and affect the air quality of a wide region. In addition, the particles increase the reflection of solar energy (radiation) and therefore have a cooling effect on the atmosphere. An other, potentially very important effect, is the interaction of the particles with cloud droplets and their role in cloud formation, which has attracted the attention of the scientific community for the first time some years ago. Because of the presence of the particles, the optical properties as well as the probability of precipitation are changed. The change of the precipition probability by particles from fires could be shown recently using satellites for the first time.

In addition, very detailed information (in english) on the effects of vegetation fires can be found on the site of the Global Fire Monitoring Center.

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Text: Jörg Trentmann (MPI für Chemie, Mainz)
Translation: Axel Lauer, DLR Oberpfaffenhofen 08/10/01