The grim forecast in its 2020 World Economic Outlook is an extraordinary reversal from January, when the I.M.F. predicted moderate growth at 3.3 percent. The revised forecast takes into account disrupted supply chains and sustained standstills in industries like tourism.

“The world has been put in a Great Lockdown,” said Gita Gopinath, the fund’s chief economist. “The magnitude and speed of collapse in activity that has followed is unlike anything experienced in our lifetimes.”

Though it predicted a partial rebound next year, the fund said there was still a chance the outcome could be even worse. This year’s decline in output, the I.M.F. predicts, would be worse than that of the 2008 recession, when the economy contracted by less than 1 percent, but less severe than that of the Great Depression, when output contracted by 10 percent.

In context: In this best-case scenario, the world would lose a cumulative $9 trillion in output over two years — greater than the combined gross domestic products of Germany and Japan.

Markets: The S&P 500 rose nearly 3 percent, after upticks in European and Asian markets, as investors saw signs that outbreaks were peaking and businesses slowly reawakening.