After wildfires caused more damage in France this year than at any time since World War Two, many are left asking a single question: Why?

A major driver is that climate is changing as a result of the well-documented way humanity is burning fossil fuels. Temperatures are only rising.

But with records showing that nine out of 10 French wildfires are started by humans, the government in Paris is trying to attribute blame – although it is itself accused of being underprepared for the infernos that have now scorched more than 300,000 acres this summer.

The interior ministry has announced 420 arrests of people alleged to have either started the fires deliberately or by accident. Among them are 166 minors.

In one case, a 23-year-old man has been accused of deliberately starting 10 fires in the space of five days in the south-eastern Var area last month. He faces 15 years in prison and a €150,000 (£128,000) fine, if convicted.

Many others have been tried and immediately sentenced, such as a man who received a three-year jail sentence for starting three recent fires in the Raon-L’Étape forest in the Vosges region.

“Only a few square metres of vegetation went up in smoke,” noted L’Est Républicain, the newspaper that reported the story.

Other prosecutions have been perceived as heavy-handed and unjust. A homeless man and someone with severe autism have reportedly been handed jail terms for throwing cigarette butts.

“I was amazed at the number of arrests, it feels excessive,” admits Muriel Gestas, a lawyer who has represented several people accused of arson, including two firefighters. She doubts many will lead to a conviction.

The newspaper Le Monde is also unconvinced. It questions to what extent police have examined these cases, given the short time frame.

The authorities’ silence on the details – the specific charges, notably whether they involve intent and how the numbers compare with past seasons – rules out any meaningful comparison with previous years, it argues.

But Bordeaux prosecutor Renaud Gaudeul told Le Monde that a tough approach was justified by current circumstances.

“The conditions for a charge of endangering lives are met for those who throw a cigarette butt or light a barbecue in wooded areas. It only takes a tiny spark for it to start,” the newspaper quoted him as saying.