"I would tell audiences that we were facing not a bubble but a froth - lots of small, local bubbles that never grew to a scale that could threaten the health of the overall economy." Alan Greenspan, The Age of Turbulence.

That used to be Mr Greenspan's view of the US housing bubble. He was wrong, alas. So how bad might this downturn get? To answer this question we should ask a true bear. My favourite one is Nouriel Roubini of New York University's Stern School of Business, founder of RGE monitor.

Recently, Professor Roubini's scenarios have been dire enough to make the flesh creep. But his thinking deserves to be taken seriously. He first predicted a US recession in July 2006*. At that time, his view was extremely controversial. It is so no longer. Now he states that there is "a rising probability of a 'catastrophic' financial and economic outcome"**. The characteristics of this scenario are, he argues: "A vicious circle where a deep recession makes the financial losses more severe and where, in turn, large and growing financial losses and a financial meltdown make the recession even more severe."

Prof Roubini is even fonder of lists than I am. Here are his 12 - yes, 12 - steps to financial disaster.

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Step one is the worst housing recession in US history. House prices will, he says, fall by 20 to 30 per cent from their peak, which would wipe out between $4,000bn and $6,000bn in household wealth. Ten million households will end up with negative equity and so with a huge incentive to put the house keys in the post and depart for greener fields. Many more home-builders will be bankrupted.

Step two would be further losses, beyond the $250bn-$300bn now estimated, for subprime mortgages. About 60 per cent of all mortgage origination between 2005 and 2007 had "reckless or toxic features", argues Prof Roubini. Goldman Sachs estimates mortgage losses at $400bn. But if home prices fell by more than 20 per cent, losses would be bigger. That would further impair the banks' ability to offer credit.

Step three would be big losses on unsecured consumer debt: credit cards, auto loans, student loans and so forth. The "credit crunch" would then spread from mortgages to a wide range of consumer credit.

Step four would be the downgrading of the monoline insurers, which do not deserve the AAA rating on which their business depends. A further $150bn writedown of asset-backed securities would then ensue.

Step five would be the meltdown of the commercial property market, while step six would be bankruptcy of a large regional or national bank.

Full article here.