You'd be forgiven for doing a double-take when noticing it is two conservative leaders who are responsible for the world's most successful environmental treaty, the Montreal Protocol, signed 30 years ago.

Climate change skepticism, let alone downright denial, has become such a hallmark of contemporary conservatism in North America that the efforts of Brian Mulroney and Ronald Reagan in 1987 now seem like a historical anomaly.

​The White House, after all, is currently occupied by someone who once described climate change as a hoax hatched by the Chinese. As president, Donald Trump has busied himself repealing many of the environmental protections implemented by his predecessors.

Canada's most recent Conservative prime minister, Stephen Harper, earned the ire of environmentalists for pushing oil pipeline development at the expense of action on climate change.

The party's new leader, Andrew Scheer, has so far indicated he will continue the Tories' long-running opposition to carbon pricing.

There was a different sense of urgency among conservatives 30 years ago, when Mulroney hosted an international climate conference in Montreal.

Scientists had just shocked the world with the discovery that made-man chemicals — chlorofluorocarbons (CFCs) — were burning a hole through the Earth's ozone layer.

This image provided by NOAA shows the ozone hole. Scientists believe the hole has begun to repair itself since the Montreal Protocol took effect. (Associated Press)

They predicted that continued use of CFCs would completely collapse the ozone layer by 2050. Without ozone protecting us from the sun's UV rays, skin cancer rates would skyrocket.

Faced with that dire outlook in 1987, 46 countries agreed, in Montreal, to dramatically limit the use and production of CFCs.

Mulroney signed the protocol. So did Reagan, often considered the ur-Republican. Even Margaret Thatcher, the Iron Lady of British Toryism, got on board.

Since then, close to 150 more countries have signed the protocol. It's credited with saving the ozone layer and delaying rising temperatures.

Former United Nations Secretary-General Kofi Annan called it "perhaps the single most successful international agreement to date."

On Monday, Mulroney will join Environment Minister Catherine McKenna in Montreal to mark the agreement's anniversary.

Environmental conservatives?

Historians have speculated about why this conservative troika felt compelled to act on climate change, especially given that it has since become a taboo topic for many on the right.

Some point out that Thatcher had studied chemistry before jumping into politics, and could grasp the science behind the warnings of scientists.

Others note that by the time he signed the protocol, Reagan had already undergone two operations to remove cancerous cells on his nose, likely caused by exposure to the sun.

​But these contingencies aside, conservatism was also made of different stuff back then — more concerned with pragmatism than ideological purity.

In agreeing to crack down on CFCs, Reagan reputedly bucked the recommendations of his advisors and the powerful industrial chemical lobby. And just as there is now, there was skepticism about the science.

It was Brian Mulroney's government that pushed to host the 1987 conference that led to the Montreal Protocol. (Christopher Katsarov/Canadian Press)

But Reagan sold the Montreal Protocol as an "insurance policy," something worth the effort in case the science turned out to be right, according to George P. Shultz, the U.S. secretary of state at the time.

Shultz often cites Reagan's approach to the Montreal Protocol when attempting to convince Republicans that taking action on climate change doesn't mean sacrificing their conservative values.

"Before you get mugged by reality, take out an insurance policy. It's the Reagan way," he wrote in 2015.

Earlier this year, he joined several other old-guard Republicans issuing a document called "The Conservative Case for Carbon Dividends," a carbon tax plan that would see dividends returned to average Americans.

"For the sake of our children and grandchildren, I believe it is imperative that we set forth a climate solution that embodies long-standing conservative principles," he said at an event announcing the plan.

The Canadian drift to the right

Like it did in the U.S., conservatism north of the border also took a right-ward turn in the last 20 years, growing more suspicious of intellectualism and government intervention.

As part of that movement, the environment dropped low on the Conservative party's list of priorities.

That has made the party unrecognizable to at least one member of the Mulroney government: his environment minister when the Montreal Protocol was signed, Tom McMillan.

"If conservatives are not in the vanguard of the environmental movement, then what are they trying to conserve," he said during a phone interview Sunday.

"I would argue that nothing is more important for conservatives, and for anybody, than saving this fragile planet from destruction in the face of our own activities."

Stephen Harper, seen here in 2006, championed the merger of the Canadian Alliance with the Progressive Conservatives in 2003. (Tom Hanson/Canadian Press)

McMillan recently authored a book, Not My Party: The Rise and Fall of Canadian Tories, from Robert Stanfield to Stephen Harper, which criticizes the 2003 merger of the Canadian Alliance and Progressive Conservatives.

He attributes the shift in Canadian conservatism to a variety of factors, chief among them immigration from former Communist countries, the rise of the oil-rich West as a site of political and economic power and atomising modern technologies.

​Together these factors have combined to create a voter-base less receptive to a collectivist vision of government, to government wading into complex global issues, McMillan said.

But the Montreal Protocol, he added, offers a different lesson for Conservatives.

"It was as complex and technical and scientific an issue as the world has ever faced," McMillan said of the ozone hole.

"If that could be solved by political will, then there is not a single problem — economic or environmental — on the planet that cannot be solved with the same courage and political acumen."