Global Economy Rebounding, O.E.C.D. Says BERLIN - The global economy is slowly regaining its strength, driven largely by the rebound of the United States and Japan, but record-high unemployment in Europe continues to drag on efforts to recover in that region, the Organization for Economic Cooperation and Development said Wednesday. The organization, based in Paris, predicted that gross domestic product in its 34 member countries, all of which have developed economies, would grow 1.2 percent this year, slightly below the 1.4 percent it forecast six months ago. Unemployment, especially in Europe, remains a persistent problem contributing to the uneven pace of growth globally, the organization said. It warned European countries that failing to address the issue would undermine the progress made from the fiscal and structural adjustments that many countries have pushed through in recent years. "Protracted weakness could evolve into stagnation with negative implications for the global economy," said Pier Carlo Padoan, the deputy secretary general of the O.E.C.D. "Reform fatigue is mounting as visible results in growth and jobs fail to materialize." Nevertheless, he pointed to "important progress in the euro zone," where the economic policy changes pushed through in the past three years have begun to take hold. But countries need to maintain those policies and find new ways to persuade the public of the need to see through the changes, the organization said. Eckhard Wurzel, an economist within the organization who is responsible for Europe and the euro zone, predicted that a real turnaround in Europe could be seen by 2015. For that to happen, he said, the 17 countries using the common currency need to maintain their pace at overhauling the weaker economies, and European Union-level policies, including implementing a banking union, needed to be strengthened. "We are on the edge of stabilizing the sovereign debt," Mr. Wurzel said. Reflecting this, the organization forecast that the euro-area economy would shrink 0.6 percent this year before expanding 1.1 percent in 2014. U.S. gross domestic product should rise 1.9 percent this year and 2.8 percent next year, while Japan's will increase 1.6 percent this year and 1.4 percent next year, the O.E.C.D. said. It predicted that the economy in China, a nonmember country for which it nevertheless provides figures, would grow 7.8 percent this year, lower than a previous estimate of 8.5 percent. The biggest challenge facing the United States will be to begin unwinding its stimulus programs, the O.E.C.D. said. It warned that if the United States Federal Reserve were to withdraw from its bond-buying program too early, it could jeopardize the fragile recovery. On the other hand, waiting too long could result in a "disorderly exit from the program," which could trigger excessive risk taking, it said. "Exit from unconventional monetary policy, when needed, may be difficult to manage and less smooth than desirable, possibly leading to sharp rises in bond yields and serious negative consequences for growth," Mr. Padoan said. The organization said the European Central Bank could do more to get credits flowing to the euro-area economy, and called on it to buy financial assets from credit-starved small and medium-sized companies, emphasizing the need for monetary policy to remain loose to assist recovery.