Climate change warning: Greenland once melted and raised the ocean 5 feet.

OTTAWA — Prime Minister Mark Carney says Canada is under attack, bombarded by U.S. tariffs and a trade partner that moves goalposts on a whim. The federal government’s initial response was to offer short-term support payments for companies and workers hit by this latest round of American tariffs. But economists say it’s the long-term policy moves that will do much more to inoculate the Canadian economy against the Trump tariffs, not to mention fuel a higher national standard of living. The most powerful policy play, they say, would be an overhaul of Canada’s tax system because...