As reported in Jones Day’s Second Quarter 2020 edition of The Climate Report, the English Court of Appeal declared in February 2020 that the United Kingdom Government’s Airports National Policy Statement ("ANPS") for a new third runway at Heathrow Airport was unlawful due to its failure to take into account other "government policy" relating to the mitigation of, and adaptation to, climate change, in the context of the 2015 Paris Agreement on climate change ("Paris Agreement"). On December 16, 2020, the UK Supreme Court unanimously reversed the Court of Appeal’s decision R on the Application of Friends of the Earth Ltd & Others v Heathrow Airport Ltd [2020] UKSC 52. The Supreme Court found that the ANPS, and its accompanying environmental report, were not in fact unlawful, paving the way for Heathrow Airport Ltd ("HAL") to make a formal development consent application for its proposed development.

The decision turned on the Supreme Court’s different interpretation of what constitutes "government policy" in the context of the Planning Act of 2008’s requirement that a planning policy statement must explain how existing government policy on climate change is taken into account. The Supreme Court’s judgment confirmed that these words "were words of the ordinary English language" that should be "given a relatively narrow meaning so that the relevant policies can be readily identified." The Court found that the fact that the Paris Agreement had been ratified by the government and that ministers had made statements about it could not constitute "government policy" in this context. It was not a formal written policy cleared by the relevant departments on a government-wide basis. Indeed, at the time of settling the ANPS, the government was still deliberating as to how to reach the Paris Agreement targets, meaning there was no established "policy" beyond that already expounded within the Climate Change Act of 2008. The absolute minimum standard is a statement that is clear, unambiguous, and devoid of relevant qualification. The Court explained that international treaties are binding only as a matter of international law and do not have an effect in domestic law. Treaty commitments continue whether or not a particular government remains in office and do not constitute a statement of "government policy" for the purposes of domestic law.

Moreover, contrary to the Court of Appeal’s findings, the Supreme Court ruled that the Secretary of State had considered the Paris Agreement in the ANPS. The question was whether the Paris Agreement commitments should have been given more weight. But the Supreme Court found that was for the Secretary of State to decide in the first instance, and because the Secretary of State had not acted irrationally in the amount of weight given to the Paris Agreement in the ANPS, it was not challengeable on that ground.

The arguments as to the impact of the future Heathrow development, and the current developments in policy at that time, will still need to be considered at the development consent stage of the project, where its effect on climate change, as well as other impacts, will have to be taken into account. This is a separate issue to the ANPS designation of Heathrow as the airport of growth for southeast England. Thus, any final decision making as to the expansion of Heathrow Airport, with the likelihood of future legal challenges, is still not over.