In the 1980s, the threat of acid rain in Canada and the U.S. had become a brewing environmental crisis. In areas of Southern Ontario, lakes that once were teeming with wildlife were on the verge of becoming dead lakes, void of fish and other aquatic species. 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 had been a big issue in Sudbury because of its nickel production, and early on, large smelters were identified as a source of the pollution and all levels of government worked to change regulations and have companies reduce emissions. The acid rain crisis also led to a bilateral Canada-U.S. agreement: the Air Quality Agreement, which was signed in 1991 by former prime minister Brian Mulroney and then-U.S. president George H. W. Bush. 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. 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. At the height of the environmental crisis, 2.5 million tonnes of SO2 emissions were being released in the atmosphere a year from Sudbury. With changes in emission standards, Sudbury now emits 50,000 tonnes of SO2 a year. In Canada, SO2 emissions have decreased by 69 per cent — and in Sudbury, by 98 per cent. On this episode of Global News’ What happened to…?, Erica Vella finds out how emissions causing acid rain were reduced, what is happening now in Ontario lakes, and how can we apply the lessons learned from acid rain to other environmental problems. Contact: Twitter: @ericavella Email: [email protected] See omnystudio.com/listener for privacy information.