It’s the fate of Chancellors to spend a good portion of their lives saying “no” to colleagues, but either circumstance or character has led Philip Hammond to suffer that role a bit more than most.

At the moment he’s engaged in a two-front battle. We learn today of Theresa May’s attempt to cobble together a legacy, aside from her Brexit failure, by splashing cash before she leaves the building.

That isn’t so much a fiscal red flag to the Treasury as a huge banner unfurled immediately outside their windows. The conditions in Downing Street, and the apparent priority of building a personal legacy, are hardly likely to encourage rational and long-term decision-making.

Beyond the practicality is the eternal question of political control. The Treasury has a spending review planned, thank you very much, and does not want to have its wriggle-room frittered away by an outgoing Prime Minister.

On that, at least, Hammond has the firm agreement of the leadership candidates. They each know that if they win they will have to contend with May’s legacy already, without having to fulfil whatever spending plans she comes up with at the last minute. Worse, there are some reports she hopes to hijack and implement some of the candidates’ own policies (such as on school spending) during the leadership contest itself, which is predictably unpopular with those she hopes to pickpocket.

The second battle Hammond is engaged in is more controversial, and pits him against a wider field of colleagues: his concern, stated in a letter to the Prime Minister, about the idea and possible cost of legislating to make the UK a net zero carbon emissions economy by 2050.

He’s right to be concerned.

In general, this whole practice of putting targets into law is a bad one in itself. Legislation is not meant to be used as a fancy form of press release, or as a medium through which to illustrate the virtue of one’s intentions, it is a costly and serious tool to make practical changes.

It’s also meant to be used accountably – pass a law today and be judged on whether it works or not. Passing a law simply declaring that something will happen long after most MPs who vote on it are long retired is an exercise in trying to grab the glory of a promise while imposing the burden of implementation and the eventual judgement of success or failure on others.

At best that is meaningless, and at worst it feels duplicitous, an unconservative grab for privileges without fulfilling the attendant responsibilities.

This brings us to the specific cause for concern in this case: the costs which such legislation would incur are potentially vast. As The Times argues, there are very serious implications – economic, social and human – to the radical action on climate change some propose, and those doing the proposing have a responsibility to explain them to those who will foot the bill. That might be hard but it must happen, particularly in a democracy.

Just legislating for the concept of solving it all by 2050 conveniently skips the difficult and uncomfortable decisions which have to be taken on the intervening 31 years. Hammond is completely justified in popping up to say “hang on, have you told everyone how much this costs?”

It might well be that such costs are consented to in the face of the perceived risk of inaction. It might be that the predicted costs don’t come to pass due to unforeseen technological improvements or voluntary changes in behaviour. It might equally turn out that the cost is far higher even than Hammond expects.

Either way, it isn’t sufficient to simply farm out the decision to John Gummer’s committee then legislate the headline goal into law in order to look good. Those who want this degree of action should be even more keen than others to ensure it is properly explained to the people up front – if it isn’t, then they risk a backlash when the implications become clear later on.

Unlike on May’s spending plans, the difficulty Hammond might face on this question is that he does not have the agreement of all the would-be leaders. Some would very much like to make such a pledge themselves.