Government agencies are now required to protect citizens from climate change in one US state, following a landmark legal victory for environmental activists.

A Montana judge found state agencies were violating their constitutional right to a clean and healthful environment by allowing fossil fuel development.

The policy being used for evaluating requests for fossil fuel permits — which does not allow agencies to evaluate the effects of greenhouse gas emissions — was ruled to be unconstitutional.

If it stands, the ruling could set an important legal precedent, though experts say its immediate impacts will be limited and state officials have pledged to seek to overturn the decision on appeal.

The ruling adds to a small number of legal decisions around the world that have established a government duty to protect citizens from climate change.

District Court judge Kathy Seeley wrote in the ruling that, “Montana’s emissions and climate change have been proven to be a substantial factor in causing climate impacts to Montana’s environment and harm and injury” to the youth.

Law professor David Dana said the ruling was a “remarkable win” for the young climate activists and predicted it would be used as a guidepost for attorneys bringing similar suits in other states.

Montana is one of the few US states with environmental protections written into its constitution.

State officials tried to derail the case and prevent it from going to trial through numerous motions to dismiss the lawsuit. Judge Seeley rejected those attempts.

Julia Olson, an attorney representing the youth, released a statement calling the ruling a win “for Montana, for youth, for democracy, and for our climate”.

“As fires rage in the West, fuelled by fossil fuel pollution, today’s ruling in Montana is a game changer that marks a turning point in this generation’s efforts to save the planet from the devastating effects of human-caused climate chaos,” said Ms Olson.

The state will appeal

Emily Flower, spokesperson for Montana attorney-general Austin Knudsen, decried the ruling as “absurd” and said the office planned to appeal.

She criticised Judge Seeley for allowing the plaintiffs to put on what Ms Flower called a “taxpayer-funded publicity stunt”.

“Montanans can’t be blamed for changing the climate,” Flower said in an email.

“Their same legal theory has been thrown out of federal court and courts in more than a dozen states.

“It should have been here as well, but they found an ideological judge who bent over backward to allow the case to move forward and earn herself a spot in their next documentary.”

The state argued that even if Montana completely stopped producing carbon dioxide, it would have no effect on a global scale because states and countries around the world contribute to the amount in the atmosphere.

A remedy has to offer relief, the state said, or it’s not a remedy at all.

Attorneys for the 16 plaintiffs, ranging in age from 5 to 22, presented evidence during the two-week trial in June that increasing carbon dioxide emissions were driving hotter temperatures, more drought and wildfires and decreased snowpack (snow that remains on the ground until the arrival of warmer weather).

The plaintiffs said those changes were harming their mental and physical health, with wildfire smoke choking the air they breathed and drought drying out rivers that sustain agriculture, fish, wildlife and recreation.

Carbon dioxide, which is released when fossil fuels are burned, traps heat in the atmosphere and is largely responsible for the warming of the climate.

This spring, carbon dioxide levels in the air reached their highest levels for more than 4 million years, the National Oceanic Atmospheric Administration said earlier this month.

Greenhouse gas emissions also reached a record high last year, according to the International Energy Agency.

July was the hottest month on record globally and likely the warmest that human civilisation has seen, according to scientists.

AP/ABC