The study posits that avoiding just one of the key factors in starting a large wildfire could significantly reduce the likelihood of wildfires. Photo: PIXABAY

• A research co-led by the Desertification Research Centre (CIDE, CSIC-UV-GVA) describes the mechanisms that make large fires possible.

• Climate appears as one of the main triggers by favoring fuel flammability, drought conditions and the effectiveness of ignitions.

An article recently published in the journal Frontiers in Ecology and the Environment of the Ecological Society of America (USA) sets out the ingredients and mechanisms necessary for large wildfires to occur. Four ingredients are required: ignition, fuel, drought and appropriate weather conditions. The study establishes a model showing how these four ingredients are related, and argues that climate change increases the ideal conditions for large fires. The work is co-led by the Desertification Research Centre (CIDE), a joint center of the Spanish National Research Council (CSIC), Universitat de València and the Generalitat Valenciana.

The work proposes a model that describes how large forest fires occur. This model establishes that large fires occur when three thresholds are crossed simultaneously: the ignition threshold, the fuel availability threshold and the drought threshold. According to the study, these three thresholds decrease and are more easily crossed with certain meteorological conditions, such as dry winds (for example, westerlies in the Comunitat Valenciana) and high temperatures (favored by climate change).

Once these three thresholds are crossed, fires are generated that can be of great magnitude (mega-fires), generate their own dynamics (the so-called 'firestorms'), and exceed the firefighting capacity of firefighters. "These conditions have occurred on several occasions in the Comunitat Valenciana, and it is foreseeable that they will be amplified in the coming years as climate change progresses," says Juli G. Pausas, CSIC researcher at the CIDE and co-author of this study with Jon Keeley, of the U.S. Geological Survey.

Weather conditions appear as a triggering factor for fires to occur in a given ecosystem, since they reduce the thresholds of the other three ingredients. According to this study, climate change in the form of drought and high temperatures means that fewer ignitions and less fuel are needed to start large wildfires. In the same direction is the fact that more and more tropical hurricanes are reaching the coasts of the Iberian Peninsula, which favors the spread of fires.

"It is important to bear in mind that an ignition and weather conditions conducive to fires are not enough for large fires; extensive and flammable biomass is also needed," explains the CIDE researcher. The availability of this 'fuel' is affected by the topography, the type of vegetation, its structure and human use of the forest, among other factors. "A certain continuity of fuel is required for large fires to be generated, and in the Comunitat Valenciana, as well as throughout the Mediterranean basin, this continuity is generated mainly by rural abandonment, by the reduction of agriculture and grazing," says Pausas.

Managing the triggers of large fires

Thus, climate change not only affects fire behavior (making them more intense), but also influences the increase in fire size and duration, as well as the time window in which large fires can occur. Among the conclusions drawn from the study that may be useful for managing the triggers of these large fires, the authors posit that avoiding just one of these key factors for a large wildfire to start (ignitions, drought, or fuel continuity) could significantly reduce the likelihood of wildfires.

"It is important to reduce ignitions in areas where wind plays a preponderant role in generating large fires. In contrast, generating fuel discontinuities, so-called mosaics, is more relevant in ecosystems where drought is key to fires," reveals Pausas. The researchers propose that, when modification of these factors is not possible, fire danger zones should be designated where human activity is minimized, just as is now done for areas near active volcanoes or in flood-prone areas.

Reference:
Juli G Pausas, Jon E Keeley, Wildfires and global change, Frontiers in Ecology and the Environment, DOI: https://doi.org/10.1002/fee.2359

CIDE Communication