OTTAWA — In less than a month, Canada has executed a complete about-face on global climate change.

With the defeat of the Conservatives in the October general election, out went nearly a decade of Canada making itself something of a global outcast on the issue. The record of the former prime minister, Stephen Harper, on climate change was marked by retreat, foot-dragging, and hand-wringing over the economic consequences of moving too quickly.

The new prime minister, Justin Trudeau, and his Liberals are now trying to make up for lost time. And along with the left-leaning New Democrats, who wrested power from the Conservatives in the oil-rich province of Alberta last spring, they have now moved climate-change policy to the top of the country’s political agenda.

The swift about-face put Ed Whittingham, one of the country’s most prominent environmentalists, briefly at a loss for words in a recent interview. “What adjective can I use?” Mr. Whittingham, the executive director of the Pembina Institute, asked after a moment. “Marked? Dramatic? Significant? They all apply.”

Mr. Harper skipped last year’s United Nations climate summit meeting in New York. But Mr. Trudeau will attend this year’s meeting in Paris next week, and not only that: He has invited his political opponents and all 10 of the country’s provincial premiers to join the delegation. He gathered the premiers in Ottawa on Monday for a meeting largely devoted to developing a national strategy.