Canada's first commitment to reduce greenhouse gas emissions was made by Brian Mulroney in 1988, at an international conference on the "changing atmosphere" in Toronto. It was pledged then that Canada would seek a 20-per-cent reduction in its annual greenhouse gas emissions by 2005.

Two years later, that target was adjusted to merely stabilizing GHGs at 1990 levels by 2005. Still, that would have kept emissions to 613 megatonnes per year.

Instead, in 2014, the last full year for which data is available, Canada emitted a total of 732 megatonnes of greenhouse gases, a 20-per-cent increase since 2005.

If Mulroney had put Canada on a path to achieving that target of 1990, if Jean Chrétien or Paul Martin or Stephen Harper had set Canada on its way to achieving any of the targets they subsequently set, Justin Trudeau would now be heading into a merely interesting fall, the biggest issue of which would be the negotiation of new health accords with the provinces or the consideration of a new electoral system.

A pivotal fall ahead

Instead, as MPs return to within shouting distance of each other in Ottawa on Monday, this fall feels potentially pivotal, for the prime minister and for the country.

In November, he is due to meet the premiers to finalize a national plan on climate change, or at least the makings thereof. By Dec. 19, his cabinet must decide whether to approve the Trans Mountain pipeline proposal that would transfer oil from Alberta to the port of Vancouver.

Leadership does not consist of imposing unpopular ideas on the public, but of making unpopular ideas acceptable to the nation - Brian Mulroney, former prime minister

And between those two, Trudeau gets to wrestle with questions of federalism, the national economy and the future of humanity on a warming planet.

The climate change plan seems likely to include some kind of mechanism for pricing carbon. And while putting a price on carbon has become the focal point of debate about what to do about climate change, pipelines have, fairly or not, become a focus of attention for those who worry about the impact of GHGs on the planet.

The prime minister has, either explicitly or implicitly, committed to doing both.

A single pipeline might not be the difference between Canada succeeding or failing to make meaningful reductions in GHG emissions, but without a plan to make those reductions, a pipeline is easier to oppose. That, for instance, would be the easiest lesson to draw from the Keystone XL rejection by the U.S.

In the case of Trans Mountain, there would still be concerns about a spill, particularly in the waters off the coast of British Columbia, and so the Trudeau government will presumably need to address that if it has any interest in approving the pipeline.

If it was easy, someone would have done it

But if Trans Mountain can fit within the mission of Canada hitting its international targets, the Trudeau government will need to explain how.

And regardless, there will be complaint.

Grand Chief Stewart Phillip of the Union of B.C. Indian Chiefs leads a protest against expansion of the Kinder Morgan Trans Mountain pipeline on Jan. 19. (CBC)

That the federal government has failed over the last 28 years to adequately address the threat of climate change is a matter of political will, but also public will. If it were easy and wildly popular to do something, someone would have done it already.

The moment a price on carbon is set, Conservatives (perhaps with the notable exceptions of Patrick Brown and Michael Chong) will howl. But environmentalists will cry foul if the plan to deal with climate change is not sufficiently robust and also condemn any decision to approve a pipeline.

Trudeau has political capital to burn

Hanging over the pipeline question is some possibility Trudeau will have to choose between his suggestion that "communities grant permission" for pipelines and his view that getting national resources to market is a fundamental responsibility of any prime minister.

Suggestions of the honeymoon's end are now cliche: the public's regard for Trudeau has held up so far despite several moments at which the shine seemed to be in danger of coming off.

But now Trudeau is coming up to decisions that will attract louder complaints than he has so far faced. Which brings up another notion that is now verging on the cliché: that Trudeau has capital to burn and might have to set some of it alight.

Mulroney has lectured Trudeau on the need to bring all interested parties to agreement on pipelines, an idea that is complicated by the regulatory process. In a speech two years ago, the former prime minister presented an alluring definition of what constitutes real leadership.

"Leadership is the process, not only of foreseeing the need for change, but of making the case for change," he said. "Leadership does not consist of imposing unpopular ideas on the public, but of making unpopular ideas acceptable to the nation."

Former prime minister Brian Mulroney spoke about pipelines at the University of Calgary last week. (Larry MacDougal/Canadian Press)

That is arguably what Mulroney did with free trade. Real action on climate change might surpass that accomplishment, and a pipeline would add extra credit.

Trudeau has succeeded so far in winning approval and building trust, and he certainly has some room to disappoint and still remain electable.

But a greater question emerges from Mulroney's theory. Beyond merely withstanding or minimizing the damage, can Justin Trudeau use whatever faith he has attracted to bring the Canadian public along with him? Can he win the arguments and establish wide acceptance for contested ideas like pricing carbon and building pipelines?

The profound decisions that loom will no doubt burden the prime minister. But there is also an opportunity for him to demonstrate a willingness and ability to lead.