Have you considered going vegan? In a recent Oxford study across 119 countries, they concluded that the "single biggest way" an individual can contribute is cutting animal products out of our lives[1]. It is getting easier and easier if you live in the developed western world. And the bonus that if you are already against animal cruelty, then it is probably something that already aligns with your morals too.

This is easily something you could do alongside your professional contribution too :)

[1] https://www.ox.ac.uk/news/2018-06-01-new-estimates-environme... - see the link at the bottom for the full study.

Most people here probably know this. The question is asking specifically what a developer could work on...

This has been asked several times in the past month. You should be able to see those responses via a search on this site.