Could Scotland just borrow more? I am all for borrowing to cover temporary reductions in income, due to recessions for example, which is why I have been so critical of current austerity. However, as the IFS show, North Sea oil income is falling long term, so this is not a temporary problem. Now it could be that the gap will be covered in the longer term by the kind of increases in productivity and labour supply that the Scottish government assume. Governments that try to borrow today in the hope of a more optimistic future are not behaving very responsibly. However it seems unlikely that Scotland would be able to behave irresponsibly, whatever the currency regime. They would either be stopped by fiscal rules imposed by the remaining UK, or markets that did not share the SNP’s optimism about longer term growth. So this means, over the next five or ten years, either additional spending cuts (to those already planned by the UK government), or (I hope more realistically) tax increases.
There is more here, more Keynesian than I would present my own version of some of those those arguments, but in any case he makes many good points.