When Alistair Carmichael replaced the emollient Michael Moore as Scottish secretary last month, it was said that the new minister would bring more bite to the role.

Mr Carmichael has not taken long to bare his teeth. This week he issued two warnings about the state of the independence debate. These were delivered with the kind of snap that was absent from the statements of his predecessor.

The Scottish secretary – an example of that rare breed, the Liberal Democrat bulldog – directed his strictures at both sides. He warned pro-unionists not to read too much into the apparently comforting state of the opinion polls, even if they show solid majorities in favour of retaining the union. He also told nationalists they could not rely on the rest of the UK allowing Scotland to retain the pound after independence. “The only way you can insist on currency union is by remaining within the UK,” he said.

On both issues, Mr Carmichael has valid points to make. The pro-unionists would be foolish indeed to imagine the battle won. The number of those still undecided is large. In Scotland, as in the rest of the UK, there is widespread disaffection with Westminster. It is quite possible to imagine a late swing turning the polls on their head. Moreover, as Mr Carmichael acknowledges, a narrow victory for the union in next year’s referendum would be little better than defeat. Subsequent pressure for a second poll would perpetuate uncertainty over Scotland’s future.

As for the nationalists, the currency question is just one of many thorny issues that would need to be resolved in the event of separation. These include Scotland’s membership of the EU and the equitable division of the UK’s assets and liabilities.

Given the costs and complications of a split, the rump UK cannot be relied upon to be generous in any divorce discussion occasioned by a vote for independence. The Scottish National party has played down the risks of going it alone with its promises to keep the monarchy and the pound. This soothing tone is likely to be echoed in the white paper on independence the Scottish government will publish next week. But these are ultimately promises whose fulfilment depends on British, as well as Scottish, consent.

Since the referendum was called, London’s response has been twofold. It has pointed out the costs of separation to the Scots, while also emphasising the benefits of the union. Mr Carmichael is right to remind Scots that their decision has ramifications beyond their borders, and that they cannot assume that the UK will be redrawn in ways that suit them alone. Few political decisions are potentially as tumultuous as next year’s poll. Those who take it should do so with open eyes.

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