In the 1970s and 1980s, the transboundary acid rain crisis — driven largely by sulphur oxide (SOx) and nitrogen oxide (NOx) emissions — forced Canada and the United States into coordinated action to reduce cross-border air pollution and protect ecosystems and public health. 

Today, Canada is once again at the center of the SOx debate. The shipping industry has long been a major source of SOx emissions, prompting the International Maritime Organization (IMO), the sector’s global regulator, to introduce sulphur limits for marine fuels in 2020. Rather than driving a straightforward transition to cleaner fuels, these rules have opened the door to a controversial compliance mechanism: exhaust gas cleaning systems, commonly known as “scrubbers.” 

Scrubbers allow ships to continue burning the dirtiest marine fuels by ‘scrubbing out’ the sulphur dioxide from its exhaust. However, that scrubbed out sulphur has to go somewhere. The wastewater from the scrubbing process is dumped directly into the ocean, shifting........

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