The International Monetary Fund issued a stark warning on Tuesday about the coronavirus's economic toll, saying that the world is facing its worst downturn since the Great Depression as shuttered factories, quarantines and national lockdowns cause economic output to collapse. With countries already hoarding medical supplies and international travel curtailed, the I.M.F. warned that the crisis threatened to reverse decades of gains from globalization. In its World Economic Outlook, the I.M.F. projected that the global economy would contract by 3 percent in 2020, an extraordinary reversal from early this year, when the fund forecast that the world economy would outpace 2019 and grow by 3.3 percent. This year's fall in output would be far more severe than the last recession, when the world economy contracted by less than 1 percent between 2008 and 2009. The United States is expected ... contract by about 6 percent in 2020. The global economic contraction from 1929 to 1932 was approximately 10 percent. Advanced economies shrank by 16 percent during that period. Barry Eichengreen, the University of California, Berkeley, economist who is a scholar of the Great Depression, said there were several parallels between now and then. He pointed to the jobless rate in the United States, which he expects could top the 25 percent that was reached in 1933, and the global nature of the downturn, which could prolong the crisis as poor countries struggle to combat the virus.