Since many people on HN live in California. The single greatest thing you can do if you live in be an advocate for building more housing in California, especially in locations with public transportation. California has low heating and cooling costs, and existing public transportation networks that can be extended.
Look at the difference in per capita carbon emissions between California and say, Texas, a place where many Californian's are moving to because of high housing costs. https://en.wikipedia.org/wiki/List_of_U.S._states_by_carbon_...