There are many who are yet to be convinced either way on Scottish independence.
Recent research suggests that, for these "undecideds", the likely state of the Scottish economy will be key. That is why the debate about oil and gas revenues goes right to the heart of the debate.
Last week, a leaked Cabinet briefing paper uncovered the Scottish Government's private concerns about the affordability of state pensions, welfare payments and public services, let alone the much-vaunted Oil Fund, on the back of projections at the time from the Office for Budget Responsibility (OBR). The prospect was not so much an oil bonanza; more additional cuts and a worse budget deficit than the rest of the UK.
Fast forward a few days and we have an altogether sunnier set of projections from the Scottish Government. Instead of £31 billion tax revenue by 2018, its analytical bulletin draws on alternative forecasts of future oil prices, production levels and tax revenues to argue that earnings from the North Sea over the same six-year period could top £57bn. Yesterday First Minister Alex Salmond said there was little doubt Scotland was moving into a second oil boom.
For many, the new figures, covering a variety of scenarios (all more optimistic than the OBR's) merely underline the uncertainties implicit in the economics of oil and the danger of taking a pleasingly upward trend on a graph, then extending it. Projections are a dangerous basis for predictions. That applies on both sides of the argument. Several major factors will affect future production levels, price and therefore tax revenues. These include the politics of the Middle East, the global economy, the technology of exploiting marginal oil and gas fields, the future of shale gas and action to combat climate change. That is why nobody can reliably predict the price of a barrel of oil in 2018, let alone 2025.
There is also the issue of advancing the cause of independence on the basis of short-term predictions about what remains of a finite resource. Mr Salmond is right to point to the recent high level of investment in the North Sea but that is no guarantee it will result in vastly increased production. That will depend on demand for oil and the price.
Secondly, as the Scottish Greens observed yesterday, it is somewhat hypocritical to set world-beating climate change targets and then base the case for a stronger economy on extracting every drop of oil from the North Sea. It has been estimated that burning all remaining 24 billion barrels of oil and gas (the upper estimate) would release 10bn tonnes of carbon dioxide.
In challenging David Cameron's take on the UK economy, the OBR has lived up to its billing as independent. John Swinney is taking a risk in rejecting its figures, especially when they are close to those used by the Norwegian central bank and when the Scottish Government's own fiscal working group urged caution in estimating oil revenues.
Mr Salmond wants to create in the public mind an image of an oil-rich country with plenty to spend on raising living standards. Whether voters will regard these latest figures as justifiably or wildly optimistic is yet to be tested.
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