The global economy faces a new threat from an old enemy: oil. A spike in the price of crude foreshadowed economic slumps in each of the last four decades. The rule of thumb favored by many economists is that every $10 increase in the price of a barrel of oil ends up reducing global growth by about two-tenths of a percentage point. The World Bank on June 10 cut its outlook for 2014 global growth to 2.8% from 3.2% in January. Energy importers such as China and Japan would suffer the most from any jump in price, though exporters in the Middle East would benefit, according to Neil MacKinnon, a global macro strategist at VTB Capital in London. Oil now has the probability of all-out civil war in Iraq baked into its price, as well as the prospect of more violence in Ukraine. Those two risks add $10 to $15 to the price of a barrel of crude.