With the publication of the 2012/13 GERS figures this morning, the fiscal case for Scottish independence has ‘collapsed’. Or so says Iain Gray, Scottish Labour’s shadow finance secretary.

The reality, as ever, is rather different.

GERS shows that North Sea oil revenues declined by 41 per cent over the last year, falling from £11.3bn to £6.5bn. Clearly, this has consequences for Scotland’s public finances. Where Scotland used to have a smaller deficit than the UK as a whole, it now has a larger one (8.3 per cent for Scotland compared to 7.3 per cent for the UK). Where Scotland used to generate more tax than it received in spending, it now generates marginally less (9.3 per cent expenditure to 9.1 per cent revenue).

None of this is good news for the Yes campaign. It makes it harder to argue that Scottish public services are not subsidised by English taxes. It makes it harder to argue that Scotland would have greater room for fiscal manoeuvre outside the Union. It means there will be a flood of gloating Daily Mail / Daily Telegraph / Times editorials claiming Scotland is, as suspected, a ‘mendicant’ nation.

However, behind the hysteria, all the GERS figures really tell us is that the strength of Scotland’s public finances relative to the rest of the UK depend quite heavily on oil revenues – and that oil revenues fluctuate.

But here’s the thing: oil revenues have always fluctuated. In the mid-1980s, North Sea tax receipts hit a high of more than £12bn (at 2009/10 prices) per year. By the early 1990s, they were down to two or three billion a year. By 2011/12, they were back up to £11.3bn.

Gray and co present these fluctuations as a devastating challenge to Scotland’s capacity to manage its own economy.

Yet, putting aside the fact the oil accounts for just 16 per cent (approximately) of annual Scottish public revenue (well below the 25 per cent ‘instability’ threshold set by Standard and Poor’s), it isn’t helpful to look at North Sea resources on such a short-term basis. They should be looked at over a five to ten year stretch. Why? Because low revenues one year can be – and frequently are – compensated by high revenues the next.

In 2011, Alex Kemp, professor of petro-economics at Aberdeen University, estimated that North Sea production was likely to generate between £50bn and £100bn in tax over the next ten years. His projections were dismissed, indirectly, by the OBR and the IFS as being too optimistic.

In fact, they have proved remarkably accurate so far. In 2010/11 revenues were £8.8bn, in ‘11/12 they were £11.3bn and in ‘12/13 they were £6.5bn.That amounts to an annual average, over three years, of £8.7bn, which is at the (very) high end of Kemp’s estimates.

If this trend continue, Scotland’s overall fiscal position is likely to be stronger than the rest of the UK’s over the decade from 2011 to 2021.

Between 1976 and 2011, total North Sea oil and gas tax revenues amounted to £285bn, of which Scotland’s share – according to a median line division of North Sea territory – was £257bn. According to unionist logic, an independent Scotland wouldn’t have benefited from this enormous sum because it didn’t arrive in a consistent stream of £7.3bn every year for 35 years.

That is obviously a very silly argument. But it is precisely what the No campaign is saying about future revenues.