Others will no doubt analyse Friends of the Earth Ltd & Ors, R (On the Application Of) v Secretary of State for Business, Energy and Industrial Strategy  EWHC 1841 (Admin) at much more length. I just thought I would pen down my thoughts when reading the judgment.
The case is a further judgment holding Governments to account for not addressing climate change challenges properly. The United Kingdom being a dualist country (all the more so following Brexit), the arguments do not much feature the Paris Agreement directly. Rather, claimants aim to hold Government to how Parliament said it should act in addressing climate change in the Climate Change Act 2008 – CCA 2008, and, additionally, through the requirements of the European Convention on Human Rights, whether or not in combination with the UK Human Rights Act. The core of the exercise and judgment therefore is one of statutory interpretation.
Of note first of all is that most of the claimants’ arguments were rejected and one assumes therefore that they will be seeking permission to appeal (just as the Government will).
The judgment kicks off with the oblique reference to trias politica. Holgate J  cites R (Rights: Community: Action) v Secretary of State for Housing Communities and Local Government  PTSR 553 at : –
“It is important to emphasise at the outset what this case is and
is not about. Judicial review is the means of ensuring that public
bodies act within the limits of their legal powers and in
accordance with the relevant procedures and legal principles
governing the exercise of their decision-making functions. The
role of the court in judicial review is concerned with resolving
questions of law. The court is not responsible for making
political, social, or economic choices. Those decisions, and those
choices, are ones that Parliament has entrusted to ministers and
other public bodies. The choices may be matters of legitimate
public debate, but they are not matters for the court to determine.
The court is only concerned with the legal issues raised by the
claimant as to whether the defendant has acted unlawfully.”
And : judicial review in this case must not be merits review and the judge must adopt a ‘light touch’.
Starting with the ECHR arguments, there were summarily dismissed  ff. They engaged with Article 2 ECHR’s right to life, Article 8’s right to family life (these two being the classic anchors for environmental rights in the ECHR) and Article 1 of the first protocol (‘A1P1′)’s right to [protection of property. Holgate J holds that the claimants’ argument on the ECHR ‘goes beyond permissible incremental development of clear and constant Strasbourg case law’  and [269-270] that the Dutch Urgenda decision offers a narrow window of ECHR relevance to climate law which does not open in the current case (with  in fine an explicit warning that Dutch authority, it being a monist country, should not hold much sway in England and Wales).
A first ground discussed the role of quantitative v qualitative assessment and whether and the degree to which the Minister was to show the targets could be met quantitatively. The judge held that ‘the CCA 2008 does not require the Secretary of State to be satisfied that the quantifiable effects of his proposals and policies will enable the whole of the emissions reductions required by the carbon budgets to be met. The [statutory] obligation …does not have to be satisfied by quantitative analysis alone.’ .
However one of the grounds on which the challenge did succeed is the quality of the input for the Minister’s decision: this overall briefing was held to have omitted data the minister was legally obliged to take into account, and which was not insignificant. As a result the Minister failed to take it into account as a material consideration, so that his decision was unlawful (compare ).  the briefing was held to have been wanting, in that it failed to identify under the quantitative analysis the contribution each quantifiable proposal or policy would make to meeting the UK’s carbon budgets; and it failed to identify under the qualitative analysis which proposals and policies would meet the 5% shortfall for one of the carbon budgets and how each would do so.
 ff (where Holgate J does refer, albeit with statutory distinguishing, to relevant Irish cases), another partial ground is upheld namely that of proper information given to Parliament (and therefore also the public; both a sore point in the current UK Government) on the data reached for the Ministerial conclusion and data on the pathways for delivery themselves. : ‘contributions from individual policies which are properly quantifiable must be addressed in’ the report given to Parliament and hence the public.
The result therefore is important in terms of accountability and transparency (where unfortunately no mention was made of the Aarhus Convention which continues to apply to the UK), with the latter element also being inspirational for other jurisdictions where Governments have been told to go back to the climate change drawing board.