For the first time in a decade, the world’s major economies are growing in sync, a result of lingering low-interest-rate stimulus from central banks and the gradual fading of crises that for years ricocheted from the U.S. to Greece, Brazil and beyond. All 45 countries tracked by the OECD are set to grow this year, and 33 of them are poised to accelerate from a year ago. It is the first time since 2007 that the world has recorded such synchronous performance. In the past 50 years, simultaneous growth among all the OECD-tracked countries has been rare. Other than in 2007, it has only happened in the late 1980s, and for a few years before the 1973 oil crisis. The development comes just as nationalist movements in the U.S., Europe and beyond have gotten a new life. The harmonized growth rate poses a risk as well as an opportunity — there will be no country to pick up the slack if they all turn down in unison.