How to explain the major fires that ravage California?

California: How to explain the major fires ravaging the state? - 20 minutes

• Nearly 400,000 hectares burned last week, according to the California firefighting agency.
• 240,000 people had to be evacuated to escape the flames.
• For Dominique Morvan, professor at the University of Aix-Marseille and specialist in fire physics, weather conditions are not the only factors involved.

Almost 400,000 hectares have already burned in California after hundreds of fires started in the region. 240,000 people had to be evacuated to escape the flames. It is the second largest fire in the history of the state of the western United States.

How can we explain such numerous and intense fires? For Dominique Morvan, professor at the University of Aix-Marseille and specialist in fire physics, meteorological conditions are not the only factors that come into play.

What are the factors leading to the occurrence of these fires?

The behavior of a fire can be summarized from an object called the “fire triangle”. Each side represents a factor that has an effect on the dynamics of a fire. The first side is the weather. Temperature, wind, rain, but also water content have an impact. The second side is the topography of the land: when the land is hilly, as is the case in the United States, access is all the more difficult and the firefighters will have all the more difficulty in switch off. The third factor is biomass. For it to burn, there must be vegetation, fuel.

In the specific case of the fires that have occurred in the San Francisco area, it is mainly lightning that is responsible. But with regard to Santa Cruz, there has certainly been a deficit of preventive treatment to reduce biomass. In short, the only way to reduce the risk of fire is to reduce the amount of fuel that is at ground level. When you have a forest with an undergrowth, it is better to eliminate the undergrowth when possible to preserve the trees because a fire does not spread alone from top to top.

How can we explain that there were so many fire starts?

This is explained in particular by the fire swings. Flaming particles, which may be pieces of bark or leaves, are torn off by the updrafts generated by a forest fire. We can have winds of 200km / h above this type of fire. These particles are carried away several kilometers. By falling, they can create new fires. So the firefighters are chasing the fire all the time.

In the United States, there is another factor that is important. The habitat is relatively light. Many of the houses look like mobile homes. They therefore burn more easily. When a house burns down, it generates flaming particles which will settle on the house next door and so on. We then go from a forest fire to an urban fire. In some cases, houses burn but not the trees next to them because a tree collects few particles.

More than 13,000 firefighters have been fighting the blazes for several weeks in California. Why do these fires last so long?

In California, firefighters face a multiplicity of emission sources. There are a lot of points of impact due to lightning. Thunderstorms without rain generate a lot of lightning, which causes a lot of impact on the ground and causes hundreds or even thousands of fires to start. From the moment a fire is so intense, it is very difficult to attack it and the firefighters find themselves overwhelmed.

One of the great characteristics of the United States is that the regions are very large. There are fewer roads, fewer trails. Even if there are big ways in California, fires are more complicated to put out. Hilly regions are much more fragile than others. A fire that develops on sloping ground goes much faster and is much more powerful.

Does global warming have something to do with the scale of these fires?

Global warming has a real impact on these fires. Dry and hot periods last longer. There is therefore less rain and the vegetation is drier. The fire season is expanding and is no longer reduced to summer. Winter normally creates a cut in temperatures which has the effect of killing insects, especially those that attack trees. If the winters are too mild, they proliferate and weaken the trees. If you combine that with a rainfall deficit, you have forests that are weakened, trees that are less resistant. As soon as there is a fire, they will burn.

To sum up, with global warming, there are repeated droughts and a rainfall deficit, which leads to an increase in risk, and therefore fires. Every year, we say it's catastrophic and the following year, it is even more so.

Are there other modern changes that may have led to an increase in fires in recent years?

In the United States, there was an increase in forest fires at the end of the 19th century. At the time, when the country was largely populated by Indians, they maintained the landscapes. They collected wood for heating and cooking their food. In many areas, they cleared forests to allow large herbivores to enter the forest, which made hunting easier. With the European colonization and the massacre of the Indians, the forests closed again. The density of trees in the forests of the American West has increased sharply.

Can something be done to prevent these fires from occurring?

Biomass is the only factor that humans can have an impact on. If the goal is to reduce the impact of forest fires on homes, the number one action to take is clearing around a home. It's not going to be a 100% guarantee that the house will be saved, but it will help.

In France, residents must clear their land over an area of ​​50 meters around their homes, precisely to prevent the spread of fire. This at least ensures that if the firefighters get there, they can defend the house safely. In France, it is an obligation but it is not the case everywhere in the world.

Planet

California fires: "Climate change leads to longer droughts"

World

California fires: energy supplier pleads guilty to 84 manslaughter

• 20 minutes video
• Environment
• Global warming
• California
• United States
• Fire
• Planet