Don't be an activist, things are too large for one person.
Be a socialist
jokes aside, we all can start doing something, but
1) what exactly ?
2) how to coordinate
3) without falling into politics hell
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1) I really wonder how we could help purify air. More plants ? that might absorb some CO2, what about other molecules ? What about vacuums in sewer pipes ? enough to cause a little depression near ground that would suck part of the exhaust gas in places where they can be processed.
2) see above for planting and tweaking cities then 3) .. have no idea