One of Mulroney's key legacies is his work on acid rain. While he was quick to address the issue in Canada, he had considerable difficulty prodding President Ronald Reagan to pursue a similar course in the United States. (Roughly half of the pollution causing acid rain in Canada came from sources south of the border, meaning a bilateral solution was necessary.) It wasn't for lack of trying. While considerable lobbying occurred behind the scenes, you can get a sense of Mulroney's dedication to the issue through his public statements at the time. In March 1985 the Prime Minister appeared on the MacNeil/Lehrer NewsHour on PBS. When asked if he was pleased with Reagan's recent admission that acid rain was an important issue to consider -- part of the President's grand scheme of delaying action -- Mulroney replied that "I hope that we will be able to do better than that. In point of fact the United States and Canada have inflicted great savagery upon our common environment. You know this is not something that we own. This is a trust that we must pass on undiminished to our children. And I think that we've done great damage over the years to our environment. We have been thoughtless. We have been careless.”
Nonetheless, Reagan continued to delay action, citing the need for further study of the issue. Two years later, when Mulroney was introducing Reagan to a Special Session of the Canadian Parliament, he once again returned to the theme. As he eloquently stated, "I urgently invite the U.S. Administration and the American Congress to join with this Parliament and the Government of Canada in concluding a firm bilateral accord which will provide a North American solution to acid rain. In this matter, time is not our ally but our enemy. The longer we delay, the greater the cost. For what would be said of a generation that sought the stars but permitted its lakes and streams to languish and die?"
In 1991 Mulroney and Reagan's successor, George H.W. Bush, signed the Canada-United States Air Quality Agreement, obliging the two countries to reduce their sulphur dioxide and nitrogen oxides emissions.