Even with the ECB's offer to buy short-term bonds if governments adopt austerity programs, the 17-nation euro area still lacks fiscal ties, while budget cuts are compounding recessions and fanning suggestions the group still could splinter. Neither Spain nor Italy has sought the ECB's aid yet, and Greece is struggling to earn its latest tranche of cash.
Emerging markets also are finding it tough to transform their economies, Fels says. China is seeking to shift from a reliance on exports to emphasis on domestic drivers, while India's ability to support consumption through government spending is threatened by fiscal deficits. Commodity-dependent Russia and Brazil may suffer from less foreign demand.
The research by Sheets, director of the Fed's international finance division until last year, suggests the worldwide stall speed is about 3.5 percent, above the 3.2 percent median forecast for 2012 of economists surveyed by Bloomberg News from Sept. 7 to Sept. 12.
At 2 percent, growth traditionally has fallen "significantly further," as in the recent financial crisis, or stagnated for several years, as in the early 1990s, he said. "The global economy is very vulnerable at the moment."