Ozone hole over Antarctica.
It is possible for us to work together and fix these things.
Edit:
Moving away from leaded gas might be another example.
(CFCs and leaded gas invented by the same dude, fun fact: https://en.wikipedia.org/wiki/Thomas_Midgley_Jr. Quite a legacy...)
We haven't fixed the ozone hole yet. It'll take another 40 years for it to return to the levels prior to 1980:
https://science.nasa.gov/earth/earth-observatory/ozone-hole-...
When the costs are low and require no change in personal behaviour, nor even belief in the problem. Billions of people had no idea about changes to aerosol propellants or refrigerants and they were not required to know or agree in order to solve the problem.