Boris Johnson received a very early Christmas present this week when the courts told him that Her Majesty’s Government could not proceed with its plan to build another runway at Heathrow Airport on the grounds that an extension would clash with its legal commitments on climate change. That may not sound like much of a gift, but the court’s ruling saved the Prime Minister from betraying a well-known personal pledge.

As the MP for a constituency in the path of Heathrow, he had told his discontented voters that he would personally lie down in front of the bulldozers rather than let the extension proceed. And though election promises are like pie-crusts, made to be broken, as Lenin remarked, voters tend to remember those promises that directly affect their personal and local interests, especially when they have been expressed with Boris’s trademark vivacity.

As a very new Prime Minister, however, he had inherited an established policy of Heathrow expansion that was supported by the Transport ministry and—moreover—in line with his wider economic policy of letting Britain rip in an orgy of borrowing and infrastructure development. He was trapped in the jaws of a dilemma.

Now, with one bound, our Boris was free.

The Court of Appeal had lain down in front of the bulldozers instead, ruling that it was illegal for HMG to approve a third runway without taking its own climate change commitments under the Paris Treaty (subsequently put into UK law) into account. Its judgment is interesting. It attributes HMG’s error as follows: “the reason why it was never done is that the secretary of state received legal advice that not only did he not have to take the Paris Agreement into account but that he was legally obliged not to take it into account at all.” In particular HMG had not issued a National Planning Statement as it is required to do.

That’s quite a mistake.

Ministers were free to appeal the judgment but decided not to do so. It’s true they have other options. They could now re-embark on the process of developing the case for Heathrow expansion by consulting with interested parties, holding hearings, and, of course, taking the Paris accords into account. But that process would add at least eighteen months to something that has already taken a decade. It would extend the uncertainty that, like the Brexit uncertainty (still quietly continuing), has so far bedeviled the plans of business and exporters. Above all, it would still not guarantee that any future Heathrow expansion plan would be approved by courts examining its fidelity to the Paris agreement.

Those are the immediate costs of saving Boris’s face. So stitch-up or cock-up? My guess is the latter because, as we shall see below, the costs of this judgment go higher and further than Heathrow.

It’s customary at this point for those who, like me, deplore the intrusion of the courts into what have hitherto been political decisions to denounce the rise of undemocratic judicial imperialism. And I shall give mild vent to these opinions lower down. But there is no avoiding the fact that in this case the courts have been invited by government and Parliament to intervene. A succession of laws have laid down that government planning and economic decisions must observe the legally-binding obligation to reduce carbon emissions consistent with holding a global temperature rise to within 1.5 degrees between now and the end of the century.

That’s the Paris agreement, but Britain had already proposed even more drastic carbon reductions in earlier domestic legislation. The Climate Change Act of 2008—passed with only five brave, honest, and intelligent MPs voting against it, among them the Cambridge science graduate, former deputy prime minister, Lord (Peter) Lilley—laid down that Britain must reduce its carbon emissions by 80 per cent by 2050. And this target was made still more unrealistic by Theresa May’s government in 2016 when the 2050 target was raised to 100 per cent of carbon emissions.

Successive governments had kept their fingers crossed and hoped they could argue that these targets are aspirational—and, hey, we made a good faith effort but we just missed again! But if you make such targets legally binding, then both Ministers and the general public must anticipate that the courts may intervene to require their enforcement. An earlier and more deferential generation of British judges might have been more willing to accept ministerial prerogatives on such plainly political matters. But today’s judges are more interventionist than those in the past. And, after all, they can reasonably claim that they were invited to ensure that on climate change HMG would keep its word and fulfill its treaty obligations.

Accordingly, it seems that the courts—other things being equal—will now help to impose a zero-carbon policy on the British economy. As Margaretha Wewerinke-Singh, “an international public law expert at Leiden University” in the Netherlands, told the Guardian: “For the first time, a court has confirmed that the Paris agreement temperature goal has binding effect. This goal was based on overwhelming evidence about the catastrophic risk of exceeding 1.5 C of warming. Yet some have argued that the goal is aspirational only, leaving governments free to ignore it in practice.”

Her judgment seems more authoritative on the law than on science. For activists will certainly mount legal challenges to any future substantial infrastructure or energy exploration projects. The government, other public bodies, and industry will find that many of their legal arguments have been foreclosed by this judgment. All industrial projects will be slowed down; many, perhaps most, will be either abandoned or not proposed in the first place.

Heathrow itself would struggle to meet the requirements of the UK’s climate legislation even without the lawyers’ mistakes. Its third runway is estimated to likely to bring in 700 more planes a day with a consequent large rise in carbon emissions contrary to the law. But that’s true also of the policy now touted as an alternative to another Heathrow runway—namely the expansion of existing British airports, especially Birmingham, to accommodate the rise in passenger and goods traffic that is the aim of Boris’s policies of overall economic expansion and the economic “leveling up” of regions outside the South-East.

It would also prevent his other great personal infrastructure project: the HS2 super-railway between London and Birmingham in the next decade and to Scotland and the North later. Indeed, objectors would be able to kill almost any industrial project that increased carbon emissions overall from a small power-station to a massive national industrial program like HS2—kill either by direct prohibition or, more likely, from endless legal delays as in Canada over pipeline construction. When other nations from the US to China are building runways and exploring new energy sources, oblivious to the fact that a British court had outlawed such things (pace Professor Wewerinke-Singh), Britain will be suffering from a historically unique plague: a legally-binding rule of national impoverishment. Such a policy might have been devised by the European Union to make Brexit an apparent failure.

Not surprisingly, HMG is coy about the likely costs of this. Ministers have outsourced the estimates of a zero-carbon economy by 2050 to the Climate Change Committee established under the 2008 legislation which estimates a £50 billion cost for 2050. But the Committee is composed mainly of enthusiasts for climate puritanism, and its estimates have been challenged both within and outside government. An overall review of cost estimates has been made by Harry Wilkinson of the Global Policy Warming Institute which includes a warning from the then-Chancellor of the Exchequer, Philip Hammond, to Prime Minister Theresa May that the cost was likely to be 40 per cent higher, at £70 billion per annum—with households paying an average of £2,400 every year between now and 2050. Ministers have since gone quiet on making estimates about the costs of their policies, but Wilkinson quotes an overall estimate by Andrew Mountford that total costs would probably exceed £3 trillion when the investments needed for decarbonising transport and industry are taken into account.

Cut this estimate in half and it is still staggering!

If this policy continues unchanged, it will prove extraordinarily unpopular with the voters who have been given no warning at all that the idyllic Green future they have been promised (and which they have largely embraced) would come with such a massive bill both in money and in job losses. Only climate zealots from the parliamentary climate committee to Extinction Rebellion will be happy with the outcome—and perhaps only Extinction Rebellion, which actually wants the pre-industrial world of equality in misery that a zero-carbon planetimplies in the absence of major scientific advances such as nuclear fission on which it would be reckless to depend.

More than half of the blame for this extraordinary leap into the carbon-free unknown, however, has to be placed on the politicians—all but the gallant five MPs who rejected such dangerous extremism in 2008. Politicians of all parties in recent years have wanted to appease Green hordes—which in practice meant never seriously challenging their wildest forecasts. Their method of doing so has been to pass “aspirational” laws that mandated massive emission cuts, and even to make modest bows to the Green agenda where it was easy -- as Boris Johnson did when he ruled out “fracking” in the last Tory manifesto because a non-existent industry has few supporters, while ignoring such laws when they obstructed existing policies they considered important such as his infrastructure agenda. That way they could claim to be following environmentalist policies while putting off their actual more painful implementation until another party took power.

As Ross Kemp points out in the Spectator, moreover, this habit of exiling serious policy commitment to enforcement by the courts or semi-independent bodies like the Climate Committee has grown without anyone really thinking through the consequences: “Is the government going to be able to freeze fuel duty in the Budget? That too could be said to be inconsistent with working towards zero emissions. Then again, is Rishi Sunak going to be able to announce any spending increase? I am not a lawyer, but there doesn’t seem to me to be any reason why, say, the Taxpayers’ Alliance should not go full Gina Miller and take the Chancellor to court on the basis that he has failed to run a surplus – something which was written into George Osborne’s Charter for Budgetary Responsibility."

Boris escaped one minor trap when the court decided to declare the expansion of Heathrow illegal, but it placed him and Britain in a much more restraining one when it made his entire policy of infrastructure expansion dependent upon the approval of activist judges. To escape from that trap, he will have to repeal or amend laws that passed overwhelmingly. And that’s a bigger battle than Brexit.