TOLUCA, MEXICO, Mexico -- U.S. President Barack Obama used the podium of the Three Amigos summit on Wednesday to push Prime Minister Stephen Harper to work with him on reducing greenhouse gas emissions, saying the science that supports climate change can't be denied.

Obama gave Harper a primer on reducing greenhouse gases as he answered a question about why he has not approved the Keystone XL pipeline.

"Stephen and I, during a break after lunch, discussed a shared interest in working together around dealing with greenhouse gas emissions. And this is something that we have to deal with," Obama said as he shared the stage with Harper and the Mexican prime minister at a joint news conference.

A readout on the 30-minute meeting from the Prime Minister's Office made no mention of that conversation.

"I said previously that how Keystone impacted greenhouse gas emissions would affect our decision. But frankly, it has to affect all of our decisions at this stage because the science is irrefutable," Obama said.

He said increasing "severe weather patterns" has "consequences for our businesses, for our jobs, for our families, for safety and security."

"It has the potential of displacing people in ways that we cannot currently fully anticipate and will be extraordinarily costly. So I welcome the work that we can do together with Canada," Obama added.

Obama said he wants to promote economic growth, but that has to be balanced against eventually transitioning away from the use of fossil fuels.

"We only have one planet," he said, adding "we do have to point to the future" to influence other big emitters such as China and India, and "have leverage" over them.

Harper replied that Canada and the U.S. have a "shared concern" about climate change. He also appeared to make a subtle dig at the long approval process Keystone has faced.

The prime minister also pointed out that the most recent State Department report gave the Alberta oilsands a good grade on environmental impact.

"As you know, a couple of years ago we moved to reform our system so that we have a single (environmental) review wherever possible -- a single review, a multidimensional review that happens over a fixed timeline," Harper said.

"And I think that is a process that is tremendously useful in giving investors greater certainty in terms of the kind of plans they may have in the Canadian economy."

Earlier on Wednesday, the two leaders projected a bit of genuine warmth after arriving at the summit in this Mexican city.

It may have been the sunny, high-altitude climes in the scenic Spanish colonial corner of Toluca, the hometown of their host, Mexican President Enrique Pena Nieto.

Or it may have been the fact that both Harper and Obama had staked out their positions in advance on the divisive Keystone XL pipeline project: the prime minister wants approval, the president wants to respect the approval process.

The president then had affectionate words for Canadians and for Harper, calling him "Stephen" at one point during his remarks after the two leaders strolled through the Cosmo Vital Jardin Botanico, an enormous Toluca greenhouse renowned for its spectacular stained glass windows.

"My brother-in-law is Canadian, so you know I have to like Canadians," Obama said to laughter.

But he added that the fact that Canadian and American men's and women's hockey teams are soon facing off in the ongoing Winter Games could cause him to "not feel as warm toward Canadians until the Olympic matches are over."

Harper echoed Obama's light-hearted tone during his own brief comments.

"Barack, it's always great to see you, and I like my brother-in-law too," Harper said.

Keystone wasn't mentioned at all publicly, in fact, until the final news conference during which the Canadian media contingent raised it with both Harper and Obama.

The president, as expected, noted that the pipeline was at the mercy of an approval process that he acknowledged Harper might find "a little too laborious."

"But these are how we make these decisions," he said.

Earlier, Obama told a business forum that the United States, Canada and Mexico will always have "parochial interests."

Obama said that the trade between the three countries is part of an integrated supply chain that allows them to sell their products and services around the world.

"We have every incentive to make this work," Obama said, urging the business audience at the summit to help push for progress.

"If in fact we're going to continue to build and strengthen ... then you can't just leave it to politicians alone," he said. "When people understand what this means in terms of job creation a when they hear that from you, it's that much more persuasive."

During their meeting, Harper and Obama discussed and strongly condemned the violence taking place on the streets of Ukraine, and discussed issues of shared concern, including the ongoing Trans Pacific Partnership negotiations, the global economy, and border infrastructure and security.

The two leaders held their bilateral meeting a few hours after Harper sat down with TransCanada officials in nearby Mexico City -- and just as a judge struck down a Nebraska law that allowed the pipeline to proceed through the state.

That ruling that will undoubtedly lead to even further delays for the controversial project.