President Donald Trump looks on as his nominee for the chairman of the Federal Reserve Jerome Powell takes to the podium during a press event in the Rose Garden at the White House, November 2, 2017 in Washington, DC.

Former Indian central bank chief Raghuram Rajan warned that the biggest threat to the U.S. economy will come from President Donald Trump's trade policies.

In an opinion piece titled "Is Economic Winter coming?" published on the Project Syndicate website Tuesday, Rajan said the old rules governing macroeconomic cycles no longer seem to apply and it remains to be seen what might cause the next recession in the U.S.