Does government inaction on climate change violate human rights?
That is the question the European Court of Human Rights will for the first time seek to answer in Strasbourg, France, as it rules this week on three separate climate cases.
The verdicts will set a precedent for future litigation on how rising temperatures affect people's right to a livable planet.
The following lays out what is at stake.
What are the lawsuits?
Six Portuguese youths are suing 32 European countries for allegedly failing to avert catastrophic climate change that they say threatens their right to life.
The case, which has been described by experts as "David v. Goliath," does not ask for financial compensation, but for governments to drastically cut emissions.
At the same time, thousands of elderly Swiss women have argued that their government's "woefully inadequate" efforts to fight global warming put them at risk of dying during heatwaves.
The women's lawyers are seeking a ruling that could force Bern to cut carbon dioxide emissions much faster than planned.
In the third and final case, Damien Carême, a former mayor of the French commune of Grande-Synthe, is challenging France's refusal to take more ambitious climate measures.
What rights have been violated?
This will be the first time the European Court rules on whether allegedly weak climate change policies infringe on people's human rights enshrined in the European Convention.
The Portuguese youths have argued their right to life is threatened by climate change-driven events such as wildfires, and that failure to tackle climate change particularly discriminates against young people who face the prospect of an increasingly unlivable planet.
The Swiss women have said Bern violated their right to life by failing to cut emissions in line with a pathway that limits global warming to 1.5C (2.7F).
Their case cites a U.N. Intergovernmental Panel on Climate Change report that found that women and older adults were among those at highest risk of temperature-related deaths during heatwaves, and uses the applicants' medical records to show their vulnerability.
Carême's application, made in 2019, will assess whether insufficient government action can amount to a violation of the right to life, by exposing people's homes to climate risk.
"We all are trying to achieve the same goal," said 23-year-old Catarina Mota, one of the Portuguese youths. "A win in any one of the three cases will be a win for everyone."
What could the rulings be?
The 17-judge panel could issue very different verdicts for each case. The rulings cannot be appealed.
"The three cases are quite distinct in terms of who's bringing the case, which government or governments is being sued, and what the ask is in the case," said Lucy Maxwell, co-director of the Climate Litigation Network.
Some of the involved governments have argued the cases are inadmissible. Switzerland has said it is not Strasbourg's job to be "supreme court" on environmental matters or to enforce climate treaties.
The court may decide a case is too difficult to fit within the existing framework of the Court and needs to be decided at a national level, Maxwell said. The latter is a common outcome that could give a boost to national accountability.
"The European Court may issue a declaration that those governments have not complied with their human rights obligations because their 2030 targets are too weak and not in line with science," she said.
What can a ruling against the government achieve?
A ruling against the Swiss or Portuguese government would "send a clear message that governments have legal duties to significantly increase their efforts to combat climate change in order to protect human rights," Maxwell said.
This should result in those countries revising their 2030 emissions reductions targets.
If countries do not update their targets, further litigation could be carried out at the national level and courts could issue financial penalties.
Any failure of governments to comply with domestic court orders "sparks major rule of law issues," Maxwell said.
"We rely on the compliance of governments with national court orders."
How will the rulings set a legal precedent?
A regional human rights court has never before ruled on climate cases, and the verdicts are likely to be game-changing.
"If successful..it would be the most important thing to happen for the climate in Europe since the Paris Agreement because it kind of has the effect of a regional European treaty," said Ruth Delbaere, a senior legal campaigner for civic movement Avaaz, which has helped to raise funds to cover the Portuguese youth's legal fees.
All three cases are being decided by the court's top bench — known as the Grand Chamber — where only cases that raise serious questions about the Convention's interpretation are sent.
The cases' outcomes will therefore serve as a blueprint for both the Strasbourg court and national courts considering similar cases.
Gerry Liston, a senior lawyer arguing the Portuguese case, said "the most impactful outcome" would be a ruling that binds the 32 countries that are Europe's major emitters. They include the European Union and neighboring countries.
But a ruling against even just one country could be applied as precedent against all 46 signatories of the European Convention.
A win could embolden more communities to bring similar cases against governments. Equally, a loss for the claimants could deter future legal action.
Six other climate cases have been put on hold by the Strasbourg court pending Tuesday's three rulings, said Joie Chowdhury, a senior attorney at the Center for International Environmental Law.
These include a lawsuit against the Norwegian government that alleges it violated human rights by issuing new licences for oil and gas exploration in the Barents Sea beyond 2035.
Whatever happens this week will also have influence beyond Europe's borders, Maxwell said.
Courts in Australia, Brazil, Peru and South Korea are considering human rights-based climate cases.
"They will be looking at what happens in Europe and there will be ripple effects well outside," she said.