A lawsuit over climate change filed by 21 young Americans has gained the attention of the fossil fuel industry, which is joining the US government to oppose the kids' demands for sharper pollution cuts.

The plaintiffs, aged eight to 19, include the granddaughter of renowned climate scientist James Hansen, formerly of NASA and a well-known advocate of reducing the greenhouse gases that are causing the planet to heat up.

The plaintiffs want the government to commit to significantly reduce carbon dioxide emissions and implement "a science-based climate recovery plan" that protects the Earth for future generations, according to the Oregon-based group, Our Children's Trust.

"This case will put indisputable science about climate change squarely in front of the federal judiciary," said the group, which filed its lawsuit against President Barack Obama's administration in August, and has filed multiple state lawsuits over the past several years.

They are calling on the US District Court of Oregon -- the state where most of the plaintiffs live -- to order the government to "swiftly phase down carbon dioxide emissions" so that atmospheric CO2 concentrations "are no more than 350 parts per million by 2100."

Atmospheric CO2 concentration is currently around 400 ppm, a level unprecedented in modern history and one that has raised alarm among many climate scientists.

Meanwhile, the planet is on track for its hottest year since 1880, amid key climate talks later this month in Paris that will reveal how much world leaders are prepared to do to save the environment.

In a sign that the kids' lawsuit is causing some concern to industry interests, powerful oil and coal companies filed earlier this month for permission to join the US government in opposing it.

They include the American Fuel and Petrochemical Manufacturers -- which represents ExxonMobil, BP, Shell, Koch Industries and more -- the American Petroleum Institute and the National Association of Manufacturers.

"The fossil fuel industry doesn't want additional pressure on the federal government to run a stricter climate change program," said Cornell University law professor Gerald Torres, an expert on environmental law who is not involved in the case.

"It does suggest they are taking this lawsuit seriously. And I think it ought to be taken seriously," Torres told AFP.

- Dangers ignored -

The plaintiffs say the federal government has known about the danger of carbon emissions since 1965, but has not done enough to stem them.

Specifically, pledges in the 1990s by Congress and the Environmental Protection Agency to significantly reduce CO2 emissions and stop global warming were "never implemented."

This lack of action shows that the "federal government has violated the youngest generation's constitutional rights to life, liberty, property, as well as failed to protect essential public trust resources," Our Children's Trust has said.

In other words, the government has jeopardized such vital natural resources as the air, seas, coastlines, water and wildlife.

Instead, the US government has "continued to permit, authorize and subsidize fossil fuel extraction, development, consumption and exportation -- activities producing enormous quantities of CO2 emissions that have substantially caused the rise in the atmospheric concentration of CO2," said the legal documents.

The political atmosphere in the United States is also contentious, with many Republican lawmakers skeptical of -- or outright denying -- the existence of human-caused climate change.

"At the national level, the debate is pretty much stuck," said Torres, adding that he thinks the kids' case "is legally sound."

He said it raised "the fundamental question which every person in a democracy ought to ask every day when they wake up: What is government for?"

The Obama administration has issued a Clean Power Plan which aims to reduce CO2 emissions by 32 percent from 2005 levels by 2030. And the US president has rejected the controversial Keystone XL pipeline from Canada, a project many environmentalists had opposed.

But Hansen and the children involved in the suit say these measures do not go far enough.

According to Alex Loznak, a youth plaintiff in the case from Oregon, the intervention of fossil fuel companies "makes it clear that the industry is scared."

"As Mahatma Gandhi once said, 'First they ignore you, then they laugh at you, then they fight you, then you win.' The fight has begun, and we will win," he said in a statement.