The fall in investment and asset prices is all the more harmful because it is so rapid. As oil collapses against the backdrop of a fragile world economy, it could trigger defaults.

The possible financial spillovers are hard to assess. Much of the $650 billion rise in emerging-market corporate debt since 2007 has been in oil and commodity industries. Oil plays a central role in a clutch of emerging markets prone to trouble. With GDP in Russia falling, the government could well face a budgetary crisis within months. Venezuela, where inflation is above 140%, has declared an economic state of emergency.

Other oil producers are prone to a similar, if milder, cycle of weaker growth, a falling currency, imported inflation and tighter monetary policy. Central banks in Colombia and Mexico raised interest rates in December. Nigeria is rationing dollars in a desperate (probably doomed) effort to boost its currency.

There are strains in rich countries, too. Yields on corporate high-yield bonds have jumped from about 6.5% in mid-2015 to 9.7% today. Investors’ aversion spread quickly from energy firms to all borrowers. With bears stalking equity markets, global indices are plumbing 30-month lows. Central bankers in rich countries worry that persistent low inflation will feed expectations of static or falling prices—in effect, raising real interest rates. Policymakers’ ability to respond is constrained because rates, close to zero, cannot be cut much more.

Make the best of it

The oil-price drop creates vast numbers of winners in India and China. It gives oil-dependent economies like Saudi Arabia and Venezuela an urgent reason to embrace reform. It offers oil importers, like South Korea, a chance to tear up wasteful energy subsidies—or boost inflation and curb deficits by raising taxes. But this oil shock comes as the world economy is still coping with the aftermath of the financial crash. You might think that there could be no better time for a boost. In fact, the world could yet be laid low by an oil monster on the prowl.