Harper was in New York all day Thursday, addressing an influential group of American academics at the Council on Foreign Relations and meeting separately with a tight-knit group of business leaders.
With several dozen protesters outside the venue and a couple of probing questions from the floor about Canada's environmental record, Harper defended his regulatory approach to emissions reductions as the most effective, practical way to achieve concrete results.
And he urged other countries, rich and poor alike, to step up with concrete plans of their own.
"It's not just a matter of setting targets. We actually have to have ways of reaching them," a relaxed Harper told a packed room.
The world can't simply rely on economic downturns as a way to diminish greenhouse gases, he added. "We’re not going to simply be able to put caps on economic growth as a way of achieving environmental targets."
Nor will protesting in the streets bring about any actual change, he added.
"It is not a matter of just getting on a street corner and yelling, and that will somehow lead to a solution," Harper said, a reference to the placard-waving activists protesting the Keystone XL pipeline behind a barricade outside the building.
"These are real challenges where environmental needs intersect and often appear to be at cross-purposes with economic and social development. And unless we realize that and take these things seriously, we're going to keep talking around the real issue. I think if we admit there are real problems with real difficult solutions and real difficult choices that have to be made — that everybody has to contribute to — then I think we’ll make progress."
Harper was clearly in his element during the hour-long forum, joking with audience members and offering extensive, analytical answers to questions — a sharp contrast to his clipped, abrupt style at news conferences in Canada.
The key for every country dealing with climate change is to develop low-carbon technology for the energy sector and beyond, he said.
"That is the thing that will allow us to square economic growth with emissions reduction and environmental protection. And I'm convinced that if we cannot square those two things, we cannot make progress globally."
Environmentalists immediately responded, saying the prime minister was being disingenuous because his government has cancelled supports for green energy development, and has supported giving oil companies free rein.
“If Stephen Harper wants to drive clean energy technological development, he should put (a) stop (to) letting oil companies use the atmosphere as a free dumping ground, by putting a price on pollution to level the playing field,” said Keith Stewart of Greenpeace Canada.
At a diplomatic level, Harper said all major emitters need to sign on to an international protocol that commits them to emissions reductions.
As the standard of living in China and India improves, even marginally, Harper said domestic populations will demand improvements to the environment they live in, pushing their governments to act.
Indeed, major emitters are trying to reach such an agreement by 2015 that would commit them to significant action by 2020.
But Harper did not come across as optimistic on that front.
"We'll just keep failing unless we actually get together and realize these are issues that don’t have simple quick answers."
Harper also addressed global growth, the challenges to the North American standard of living, security in the Middle East, and the role of the G20 in global governance.
He defended public medicare, warned the Obama administration against any rash action in Syria, and highlighted the need to improve productivity in both Canada and the U.S. as the centre of economic growth migrates away from the West.
The audience was largely appreciative.
"I think he's got a very coherent and logical presentation on the issues," said Atlanta-based lawyer Gordon Giffin, who was the U.S. ambassador to Canada under Bill Clinton.
Giffin said Harper's explanation of what Canada and the provinces are doing to use technology to reduce emissions intensity was particularly poignant.
"I don't think people here have a good sense of that, and I think Canada gets unfairly criticized in some circles as if Canada doesn't have environmental policies in place that actually in many cases exceed that of the United States."
Harper's trip to New York was part of a blitz by the federal government to bolster Canada's environmental reputation around the world as Ottawa seeks better market access for its oil and gas.
The Obama administration is expected to make a decision in coming months on whether to allow the Keystone XL pipeline, which would carry Alberta bitumen to the Gulf Coast, allowing the Canadian oilpatch to demand better, global prices for its products.
The pipeline has been controversial in the U.S., having become a lightning rod for the debate of jobs against the environment.
Without being asked, Harper launched a strenous defence of the project, arguing that if the oil doesn't flow by pipeline — a relatively safe option — then it will simply flow by rail, a much dodgier environmental prospect.
Scientists, environmental economists and activists around the world have denounced the project in droves, saying the oil would be better left in the ground to avoid provoking a large increase in emissions.