"We believe that this approach is a step towards a better understanding of the complex links between fire and climates," Julien Ruffault of UR RECOVER Aix-en-Provence and Columbia University, US, told environmentalresearchweb. "Additionally, our method provides a useful conceptual framework to evaluate the impact of global changes on fire regimes."

As an example, the results suggest that in Mediterranean France important changes in fire suppression practices at the end of the 1980s led to a large decrease in the near-normal fire weather type. At the same time, climate change towards drier conditions led to the emergence of the heat-driven fire weather type.

The near-normal fire weather type has near-normal atmospheric conditions whilst the wind-driven weather type is associated with faster winds but cooler temperatures.

There has been an increase in heat-driven potential fire days, the team found, indicating that the dominant fire weather type for the region could be shifting.

Ruffault and colleagues assessed the local-scale weather conditions preceding fires that burnt more than 120 hectares in Mediterranean France in 1973–2012.

"The climatology of large Mediterranean wildfires is not unique," said Ruffault. "It can rather be seen as the result of the coexistence of a limited number of synergic conditions that we called fire weather types. As the atmospheric configurations and physical processes of fire spread behind these fire weather types are considerably different, we designed an objective method to classify them, based on the analysis of climatic conditions over a wide range of temporal scales associated with each fire."

Atmospheric conditions in the years or seasons before the wildfire can affect the growth of fine fuels, whilst monthly to daily atmospheric variability change the moisture content of vegetation, and hourly to daily fluctuations in relative humidity, wind speed and temperature influence fire ignition and propagation.

The team had two main reasons for performing the study. "We received a lot of feedback from French fire services about the existence of the different behaviour of forests fires in the field," said Ruffault. And the results the researchers published in the International Journal of Climatology showed that large Mediterranean wildfires could occur under very different large-scale synoptic conditions.

"Several other research groups [had] already proposed the existence of favourable fire weather types that were appropriate for different regions and ecosystems worldwide," said Ruffault. "But so far a general conceptual framework was missing to unite these approaches, namely by considering the variable role played by different weather variables at the daily, weekly, monthly and seasonal scales prior to large-fires."

Now the researchers plan to extend their approach to other regions and ecosystems, starting within the Mediterranean basin, and to assess whether climate change will affect the occurrence of these fire weather types.

The team reported their work in Environmental Research Letters (ERL).

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