Predictions matter, not least because they feed directly into the political debate. We saw this all too clearly in 2016. And apocalyptic predictions matter even more when they don’t come true. The Treasury’s gloom-ridden short-term forecast in May 2016 of the impact of Brexit – not to mention its hysterical portrayal by the Treasury website – became the Brexiters’ best friend. And George Osborne’s promised “punishment budget” was rightly derided for being both bad economics and bad politics.

Dire predictions of immediate job losses and indeed recession proved simply to be false. Even when the pound fell precipitately immediately after the referendum, it was easy for proponents of Brexit to point out that things were nowhere near as bad as some had predicted.

The EU has put in place short-term measures. What we do not know is how long these measures will remain in place

So it’s hardly surprising that when the governor of the Bank of England, Mark Carney, warns, as he did on the Today programme this morning, that a no-deal Brexit would cause an “instantaneous shock” to the UK economy, raising prices and reducing household incomes, proponents of no deal such as Iain Duncan Smith would claim he was “reviving project fear”.

And no surprise that the architect of universal credit thinks that when a complex project goes off track the best way to respond is to stick your fingers in your ears. Though it’s slightly more worrying when a supposedly serious newspaper calls for the UK’s independent central bank to stop getting us all down with “gloomy forecasts”.

But if the forecasts were wrong last time, why should we pay any attention to them this time? And what, realistically, can we actually know about the impact of no deal? Well, it’s worth noting that while the Treasury certainly got the Brexit vote very badly wrong, the medium-term impacts haven’t been that different from those projected by credible independent economists. The consensus is that, so far, we’ve lost perhaps 1.5-2% worth of growth and, with uncertainty taking an increasing toll on business confidence and hence investment, next week’s figures are likely to show growth grinding entirely to a halt.

But that doesn’t mean we, or anyone else, know what the economic impact of no deal would actually be. Carney did make that absolutely clear, refusing to produce an economic forecast. There are two good reason to be cautious.

First, while we know some of the consequences of no deal – customs controls, tariffs and new trade barriers – no one really knows how great the short-term disruption will be. Sky News has got hold of a leaked internal Whitehall slide describing the worst-case scenario for the first month of a no-deal Brexit. It talks of UK nationals in other EU states losing residency and other rights; of a risk of increased crime; of the potential for an immediate halting of cross-border agricultural trade on the island of Ireland, of currency and financial market volatility. You get the picture.

But the Cabinet Office doesn’t produce documents like this simply to scare ministers. Its purpose is to put measures into place to mitigate the worst-case scenario. Even before the government ramped up its no-deal planning, much was already in place to ensure we don’t run out of drugs or essential chemicals. And on the other side of the channel, the EU has put in place unilateral short-term measures in areas such as aviation and freight transport to cushion the shock. What we do not know is how long these measures will remain in place.

Second, we don’t know what the impact will be on consumer and business confidence. In 2016, the Treasury and others expected a big hit. As it turned out, the summer of 2016 saw nice weather, a World Cup and generally buoyant spending. Now, as then, we simply cannot be sure. And again, the government can act to mitigate the impacts by cutting taxes or increasing spending.

Exclusion of no deal from analysis renders BoE's projections uncertain Read more

What is important is not what happens on day one or week one after a no-deal exit, but how long it lasts and what happens next. Because no deal is not an event but the start of a process. In an optimistic scenario, the shock of no deal will bring both sides back to the table with strong incentives from domestic constituencies to find a way through. But sadly this seems far less plausible than a political downward spiral, with acrimony over money and the Irish border meaning negotiations go backwards before they can go forwards. And even when serious talking begins, the UK will be a third country. No article 50 and no withdrawal agreement means a lengthier and messier process.

So there’s no point shooting the messenger. When Carney – or other economists – say no deal will damage the UK economy but that it’s hard, if not impossible, to say just how much, he’s not a “gloomster” but simply stating the obvious. And when he says that while the Bank (and the Treasury) can help cushion the immediate impacts but sorting out the mess afterwards will be up to the politicians, he’s not passing the buck but making clear that if politicians take us down that road, they will bear a heavy responsibility.

• Anand Menon is director of UK in a Changing Europe. Jonathan Portes is a senior fellow at UK in a Changing Europe