Gather with other persons to invest a few shares in the major groups responsible for CO2 emissions.
Not to have weight per se, but to be invited in shareholders assembly. And to be regularly loud about the topic of the group's responsibility. It's enough for 4, 5 distinct individuals to bring up related questions in the same assembly so that it bothers other investors.
Bothering from within is a start.
Guilt is not a valid reason not to object to, to criticise the compromission.
Would I be drunk, that wouldn't disqualify me from criticising all of: my own responsibility, alcohol, policies regarding liquors.
(in the same way, would I be a believer, that wouldn't make it impossible or irrelevant or even hypocritical to criticise faith, religion, church, customs, etc.).
That's the good thing with reason: you are allowed and you can detach from what you're talking about.
Plus, 1 share is enough to be invited to the shareholders meeting to get to ask questions. That's not out of touch.
(edit for clarification)