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.

Working out how a fire started is, unsurprisingly, not easy.

“In periods of drought such as this one, all you need is a simple lighter,” explains Eric Dufayet, an officer who investigates fires.

Once a fire is out, Eric’s first task is to map its outline along the edge of the burnt vegetation, then trace it back to its point of origin. If the wind has been steady, that outline often forms a “V” shape. From there, his team works backwards along the path the flames took.

“If someone deliberately starts a fire, they may not have used only one device,” he says. It improves the chances of a DNA or fingerprint sample being found.

Experts believe arson accounts for only a small fraction of fires. Most are believed to be accidental, with sparks potentially coming from construction work, faulty electrical equipment, cigarette butts and barbecues.

“With cigarettes, it’s called the ’30’ rule,” says Eric Dufayet. To start a fire, he explains, there must be “a temperature of at least 30C (86F), a maximum of 30% humidity, wind of less than 30km/h (19mph), and at least 30% of the cigarette unburnt.”

In the current drought, he says it is even easier. If those conditions are met, a fire can easily spread, and fast.

According to Michel Lejoyeux, a professor of psychiatry and addictive behaviour at Paris 7 University, people who start fires fall into three categories: those who are careless, those who benefit – with an insurance payout, for example – and pyromaniacs.

“The pyromaniac doesn’t get money, has no spirit of revenge, has no utility of the fire, but he likes the fire,” he says.

However, pyromania is a rare psychiatric disorder that Lejoyeux says is very difficult to diagnose, and accounts for few arson convictions.

Lawyer Muriel Gestas, who has handled several such cases, says some firefighters alleged to have started fires may have wanted to be seen as saviours. For others – notably volunteers earning tax-free overtime – the motive may be financial.

Amid concerns over how officials are pursuing suspects, broader worries about wildfires are far from abating. Forecasts suggest France’s next heatwave is only days away.

President Emmanuel Macron says criticism should not be the priority right now. “We will have to replant and rebuild a different kind of forest because we must adapt to the structural conditions of climate change,” he said this week.

“The time for reviewing lessons learned will come,” he insisted. “In the meantime, the country must remain united.”

Even so, the damage these fires have inflicted on the environment, hundreds of homes, tourism, local businesses and air quality has left the French public with a lot of questions.