Perhaps the most levelheaded defense of Donald Trump’s misguided plan for steep global tariffs is that they’ll never be imposed. Trump’s new global tariff threat, however, would likely be far less successful, and would impose significant costs even if the tariffs were never applied. Various measures of what economists refer to as “trade policy uncertainty,” or TPU, spiked during Trump’s time in office as he routinely announced or teased radical changes to U.S. tariff policy on Twitter. Until Congress changes the law, trade policy will be vulnerable to abuse and will therefore continue to thicken the fog surrounding trillions of dollars in annual U.S. trade. That fog is, unfortunately, again building up as trade-policy uncertainty has climbed back to levels not seen since Trump’s time in office.