Specialist Meric Greenbaum, left, and trader Fred DeMarco, center, work on the floor of the New York Stock Exchange. AP/Richard Drew Global stocks tumbled on Monday, as investors dumped risk assets, with COVID-19 cases rocketing in the US and Europe, while the stimulus saga dragged on.The US marked a new milestone in the past few days, recording around 83,700 new COVID-19 cases each day.