President Vladimir Putin's refusal to rule out using "tactical" nuclear weapons in Russia's war against Ukraine put much of the world on edge last month. But economist Nouriel Roubini says a nuclear escalation of that nearly eight-month conflict could mark just the start of a global battle.

The world could also see nuclear conflict in Iran and, most worrying for the U.S., around China's fight for Taiwan, says Roubini, who was dubbed “Dr. Doom” after the realization of his dire predictions including the 2008 housing crisis that sparked the Great Recession.

The New York University business professor contends a cold war already exists between the U.S. and China, and that it could escalate into a "hot war" as President Xi Jinping aims to unite China and Taiwan. Roubini appeared on Monday at the Yahoo Finance’s 2022 All Markets Summit, ahead of the release of his book, “Megathreats: Ten Dangerous Threats that Imperil our Future and How to Survive Them.”

“In some sense, World War III has already started,” Roubini said. “It started in Ukraine because this conflict has broader implications that go well beyond Russia and Ukraine. It’s the beginning of something else.”

A cold war between the West 'and a number of revisionist powers’

Speaking to Yahoo Finance's editor-in-chief on Monday, Roubini highlighted geopolitical threats and what he calls a new cold war that has emerged between the U.S. and China as well as between the West and "revisionist" powers including Russia, China, Iran, North Korea, and Pakistan.

“They're essentially challenging the economic, social, and geopolitical order that the U.S., and Europe, and the West created after World War II,” Roubini said, explaining this crisis could ensnare the U.S.

In addition to Ukraine, another flash point could also be Iran, Roubini says.

“The negotiations between the U.S. and Iran on the new nuclear deal are not going anywhere, and Israel is saying if [Iran is] going to become nuclear — and they're one step away from it — we're going to attack them,” Roubini said.