Six young people from areas in Portugal ravaged by wildfires and heatwaves have gone head-to-head with 32 European governments, alleging in court that their failure to act fast enough on climate change is a violation of their human rights.
The case, filed in September 2020 against the 27 EU member states as well as Britain, Switzerland, Norway, Russia and Turkey, is the largest climate case ever to be heard by the European Court of Human Rights (ECHR) in Strasbourg.
A ruling in the case is expected in the first half of 2024.
The applicants argue inaction to tackle climate change threatens their rights including to life, physical and mental wellbeing.
Backed by the Galway-based Global Legal Action Network (GLAN), the Portuguese applicants, aged between 11 and 24, are seeking a legally-binding decision that would force states to act.
GLAN became involved when a Portuguese staff member in Galway brought the issue to the attention of the organisation.
If the complaint is upheld, it could result in orders from national courts for governments to cut carbon dioxide emissions blamed for climate change faster than currently planned.
Gerry Liston, one of GLAN's lawyers, said that if the case was successful, it would be up to national courts to enforce the rulings and that they would be provided with a roadmap to ensure enforcement was effective.
A lawyer for the Portuguese government told the court that the evidence provided failed to show the specific damages caused by climate change on the lives of the young applicants.
Greece, in a submission sent to the court before the hearing, said "effects of climate change, as recorded so far, do not seem to directly affect human life or human health", an argument another GLAN lawyer, Dr Gearóid Ó Cuinn, described as "climate denialism".
One of the six applicants, 15-year-old Andre Oliveira, previously told Reuters their goal was to force governments to "do what they promised they would do", referring to the 2015 Paris Agreement to cut emissions to limit global warming to two degrees Celsius and ideally 1.5C.
Current policies would fail to meet either goal, according to the United Nations' Intergovernmental Panel on Climate Change.
"Without urgent action to cut emissions, (the place) where I live will soon become an unbearable furnace," another applicant, 20-year-old Martim Agostinho, said in a statement.
Mr Agostinho and three other applicants are from the central Portuguese region of Leiria, where two wildfires killed more than 100 people in 2017.
A lawyer representing Britain said the consequences of climate change were "global" but that the protection of applicants' interests fell under Portugal's jurisdiction. He said the case should be rejected.
More than 80 lawyers are representing the accused countries, while the applicants are being represented by six lawyers, resulting in what GLAN described in a statement as a hearing "unprecedented in scale".
Earlier, Dr Ó Cuinn said Ireland, which is one of the respondent countries, is a laggard when it comes to this issue and that instead of working to rectify that, the State had instead "pushed back" on the action.
He said he hoped the case would prove that European countries were not serious about the threat posed and that they were trying to "escape scrutiny".
A final ruling is expected within seven to 18 months.