LONDON — A decade after the world descended into a devastating economic crisis, a key marker of revival has finally been achieved. Every major economy on earth is expanding at once, a synchronous wave of growth that is creating jobs, lifting fortunes and tempering fears of popular discontent.

No tidy, all-encompassing narrative explains how the world has finally escaped the global downturn. The United States has been propelled by government spending unleashed during the previous administration, plus a recent $1.5 trillion shot of tax cuts. Europe has finally felt the effects of cheap money pumped out by its central bank.

In general terms, improvement owes less to some newfound wellspring of wealth than the simple fact that many of the destructive forces that felled growth have finally exhausted their potency.

The long convalescence has yielded a global recovery that is far from blistering in pace, and geopolitical risks threaten its demise. Many economists are skeptical that the benefits of growth will reach beyond the educated, affluent, politically connected class that has captured most of the spoils in many countries and left behind working people whose wages have stagnated even as jobless rates have plunged.