In Edinburgh last month I went to the theatre. The play was sprawling, crude and, in my opinion, not very good. Yet when the curtain fell at the end much of the audience jumped to their feet and whooped, whistled and cheered as though they had just witnessed a coruscating rendering of a work of genius – which this play, though interesting, assuredly was not.

This is only a guess, because I did not ask the whoopers and the whistlers why their reaction was so much more enthusiastic than mine. But I am reasonably confident it was down to the topicality of Tim Barrow's play rather than its stage attributes. Titled simply Union, it depicts a few real and a lot of imaginary events surrounding the passing of the Act of Union between Scotland and England in 1707.

The Act of Union is the foundational event of modern Scotland, and not just for nationalists – although they see 1707 as a defining act of grand historic treachery and corruption. In reality the long negotiations on the union were very much more complicated and interesting than that, as Barrow's play acknowledges. Nevertheless in 2014, with the independence referendum just around the corner, the mood in Scotland is highly charged. The audience's reaction in Edinburgh was one theatrical roar in the current political clamour.

One way or another, the UK is shifting beneath our feet right now, and we all need to be clear about two things. The first is that Scotland is significantly more likely to vote yes to independence in five months' time than it was five months ago. The polls have moved in the nationalists' direction, and may continue to move that way. That does not mean that the nationalists will win on 18 September. At the moment the share of the poll for independence, after you exclude the don't-knows and undecideds, is only in the low 40s, compared with the upper 30s before the turn of the year; not even the most favourable poll shows a win for the yes vote. But there has been a clear shift nevertheless. The odds have shortened and this needs to be both understood and respected.

It is clear that the change has followed the UK government's attempts to confront Scottish voters with some of the possible realities that would follow a yes vote. Some dismiss these attempts as threats, others as mere bluster. My own view is that they are legitimate and mostly rational warnings of some of the risks that may, and in some cases would, follow from a yes vote. But it is clear they have boosted the nationalist cause.

One weakness of the warnings is that too many of them have been delivered by Conservatives. The nationalists' always frame the independence argument as a battle between the Scots and the Tories. In a very real sense, Alex Salmond is fighting this campaign against Margaret Thatcher. When David Cameron says he loves Scotland or George Osborne warns about the currency, or Philip Hammond, just this week, says serious things about defence, the yes campaign's response is to monster the Tory. The polls show the response works. The no campaign needs to balance worse-apart with a lot more better-together.

My instinct, as a rationalist not a nationalist, is that some of what the UK ministers say actually gets through all the same. Voters who would never admit in public to agreeing with Cameron or Osborne may nevertheless log their warnings in private – especially in a Scotland where it is so completely not cool to be a unionist. In the 1980s and 1990s, pollsters identified a category of voters who would not admit to voting Tory but who nevertheless did so in large numbers when the moment came. It is not impossible that something similar – closet no voters – now exists in Scotland.

But there is a second thing to be clearer about too. If Scotland votes yes, the consequences could be messier and nastier for longer than most of us have allowed ourselves to consider. That is partly because there is a conspiracy of decorum surrounding the referendum campaign. The no campaign doesn't want to attack the nationalists too hard because that plays to the nationalist message of bullying and victimhood. But the yes campaign is equally bland about pretending that every problem triggered by independence will be sorted pragmatically, amicably and quickly.

I think this is misleading at almost every level. If a yes victory is declared, how will the British Labour party, meeting for its party conference on the following day in Manchester, react? By promptly agreeing to expedite Scotland's departure? Dream on. A yes vote would explode into the UK party conference season. All the main parties would be destabilised in major ways.

Salmond talks as though the negotiations following a yes vote would be straightforward, respectful and informed by mutual trust. Why should that be so? They would more likely be devious, antagonistic and riddled with mutual suspicion, as well as largely meaningless until after the 2015 general election.

Whether Salmond was negotiating with Cameron or Ed Miliband (and it is worth remembering that if Labour wins in the UK in 2015 and then wins in Scotland in 2016, Labour could in fact be negotiating with itself), the process would be likely to be prolonged. The UK government would have every possible incentive to drive a hard bargain with Scotland, as Hammond made clear in the defence context this week, and it would be backed by public opinion.

Meanwhile, what about the public mood? Views will not remain frozen unchangingly once the result is in. Nor will they inevitably remain benign and peaceful. Nationalist opinion could become more militant if the talks become bogged down. Even acts of violence are not inconceivable in certain circumstances or places, as anyone with a smattering of knowledge of the Irish treaty of 1921 will grasp.

The psychological impact in England, Wales and Northern Ireland of Scotland's rejection of the union, meanwhile, could be very unpredictable, and possibly nastily so.

I do not want any of these things to happen. But the possibility that some of them may happen has moved a bit closer with the shift in the Scottish polls this spring. We can no longer simply assume that Scots are certain to vote no and that the UK will survive. Once things change they can change very fast, and in ways for which few of us are in any way prepared.