Ask HN: What can I do to mitigate climate change?

Recently, I have been increasingly concerned about the harm done by my first-world lifestyle. I'm looking for suggestions for things I can do to actually help, either things I can stop doing or things I can start doing. Is anybody aware of a quantified comparison of the effectiveness of lifestyle changes? Is there something in particular I can do as a programmer? Maybe an open source project that climate scientists rely on? I'm an American, so is there an activist group in particular that could use my help? I'm open to lifestyle changes like not eating meat, but I'm interested in how much this would actually help.