Prime Minister Justin Trudeau of Canada promised to make climate change a priority when he campaigned for office last year. This week he made progress on that promise with a plan to reduce greenhouse gas emissions by putting a price on carbon. This marks a big turning point for his country, whose previous government resisted aggressive steps on climate change because it wanted to protect the oil industry.

Mr. Trudeau’s timing could not have been better. On Wednesday, the number of countries ratifying last December’s global climate agreement in Paris passed the minimum thresholds — 55 nations representing 55 percent of global emissions — necessary for the accord to take effect next month. Individual countries must now start putting in place the policies required to meet the national targets they agreed to in Paris.

The plan will allow each of Canada’s 10 provinces and three territories to choose its own method of carbon pricing, whether a direct tax on emissions or a cap-and-trade system under which governments put a ceiling on emissions and allow utilities, manufacturing plants, fuel distributors and others to buy and sell permits to emit greenhouse gases. Whatever system the provinces and territories choose, they must establish a price by 2018, starting at a minimum of 10 Canadian dollars per ton of carbon (about $7.60 in American dollars) and rising to at least 50 Canadian dollars in 2022. They can use the revenue generated to lower other taxes or invest in renewable energy and other climate projects. The federal government will impose fines on provinces that fail to act.