PARIS — The world economy continues to heal at a disappointingly slow pace, the Organization for Economic Cooperation and Development said on Wednesday, but it predicted that growth should return to a healthier pace close to its long-term goal by the end of 2016.

“Global growth is improving, but it’s not good enough,” Catherine L. Mann, the organization’s chief economist, said in a conference call held before the release of the forecast by the O.E.C.D., the research and policy organization of the world’s richest countries. “It’s a B-minus performance.”

The slow growth has had harmful consequences, Ms. Mann said, contributing to weak labor markets and rising inequality in many countries.

With energy prices relatively low and monetary policy accommodative, growth next year should reach 3.8 percent, she said. That would be the strongest level since before the 2008 credit crisis, Ms. Mann noted, though she expressed concern about the weak start to 2015 and the continued poor investment climate.

Ms. Mann said she expected global fixed investment, which she described as “a key component of potential output,” to increase by about 4 percent next year, the highest since the financial crisis started in 2008. Even that level would fall short of the amount needed to return labor markets to normal and raise living standards for those who have missed out on the recovery, she said.

She cited several risks, however, to that relatively optimistic picture. Among them are the Greek economic and fiscal crisis, which still remains unresolved; the possibility of a sharp slowdown in China; and the danger that financial markets will slump when the Federal Reserve begins raising interest rates.

The United States, which lost ground in the first quarter by one key measure — changes in gross domestic product — is expected to eke out an advance of about 2 percent for the year as a whole, accelerating to 2.8 percent next year, according to the O.E.C.D. forecast.

“U.S. growth projections remain weak compared with recoveries we’ve seen in the past,” Ms. Mann said.

The organization projected that the Federal Reserve would raise interest rates to 2 percent by December 2016, from a current level near zero.

Despite its relatively meager outlook, the United States economy, by most measures, continues to outperform nearly all other advanced industrial nations.

The eurozone will also start to perk up, the O.E.C.D. forecast, growing 1.4 percent in 2015 and 2.1 percent in 2016, after a 0.9 percent expansion in 2014. It warned that high unemployment would continue to weigh on the euro currency bloc’s prospects, with only a gradual decline, to around 10 percent, by the end of 2016. It is currently over 11 percent.

The organization predicted that the European Central Bank and Bank of Japan would continue to hold rates near zero over that span.

The O.E.C.D. forecast that China, which has posted growth well above 7 percent in recent years, would fall short of that level, which is considered necessary to keep unemployment from rising. It said the increase in output could slow to 6.8 percent this year and 6.7 percent in 2016.

The organization, based in Paris, is the official research arm for 34 of the world’s most-developed economies, including the United States, the European Union’s 28 member states and Japan. Mexico and South Korea are members, but several nations with rising economies, including Brazil, China, India and Russia, have not joined.