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Three decades ago, two conservative leaders came together to save the environment from the disastrous effects of pollution generated by coal emissions.

One of those leaders — former U.S. president George H.W. Bush — was honoured in a massive state funeral in Washington, D.C. on Wednesday. The other — former Canadian prime minister Brian Mulroney — was one of the four people selected to speak at his funeral.

Mulroney and Bush worked together for four years as leaders of their respective countries from 1989-1993. In addition to crafting the original North American Free Trade Agreement, they were also the co-authors of a major clean-air plan that saved many lakes and forests in North America from the scourge of acid rain — a major environmental issue at the time.

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Acid rain occurs when sulphur dioxide and other pollutants mix with moisture in the air to form rain droplets with a high level of acidity. This acidity causes aluminum to leach out of the soil and water, potentially poisoning the plants and animals in the impacted ecosystem.

Acid rain has been blamed for significant fish die-offs in the lakes around Sudbury, Ont., and parts of Eastern Canada in the 1980s and ’90s.

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Mulroney has touted that bilateral agreement as one of the more significant accomplishments of his tenure as prime minister — one that he struggled to achieve until Bush succeeded Ronald Reagan as president in 1989.

“Canadians were literally shouting at the rain. But it wasn’t even on the American radar screen,” Mulroney wrote in a 2012 essay marking the 21st anniversary of the agreement. He credited Bush with changing the U.S. stance on acid rain after years of frustratingly slow negotiations with the Reagan administration.

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“In 10 years, we went from yelling at one another, to talking to one another, to negotiating with one another, to making an important agreement with one another,” Mulroney wrote. Tweet This

Bush paid a visit to Ottawa on his first foreign trip as president in 1989, when he vowed to work with Mulroney to take aggressive action on the acid rain problem.

“If we’re to protect our future, we need a new attitude about the environment,” Bush said in a speech to Congress on Feb. 9, 1989, ahead of his meeting with Mulroney. Bush said he planned to honour a promise to Canada by cutting harmful coal pollution.

“We must protect the air we breathe,” Bush said. Tweet This

He proposed sweeping changes to the U.S. Clean Air Act later that year, introducing measures designed to curb acid rain, urban air pollution and toxic air emissions. Congress voted overwhelmingly to pass the amendment in 1990.

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Mulroney and Bush committed to cutting down on the air pollution that causes acid rain in 1991, under the Canada-U.S. Air Quality Agreement. Both nations promised to reduce the emissions of sulphur dioxide and nitrogen oxides — the air pollutants that give rise to acid rain — through a cap-and-trade system.

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“It was everything we had been asking for, and he delivered, big time,” Mulroney said of Bush in 2012. Tweet This

The agreement led to major reductions in dirty fossil-fuel emissions in both countries. Canada slashed its total sulphur dioxide emissions by approximately 63 per cent from 1990 to 2014, while the U.S. cut emissions by 79 per cent. Both countries also recorded major reductions in nitrogen oxide pollution.

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Mulroney hailed Bush’s environmental efforts in a eulogy on Wednesday. He called the acid rain accord “a splendid gift to future generations of American and Canadian to savour.”

Mulroney was Canada’s prime minister throughout Bush’s four years as president. Mulroney also eulogized Bush’s predecessor, Reagan, at a state funeral in 2004.

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