I thought gas was comparatively efficient, compared to other fossil fuels?
That's easy to say, difficult to implement.
E.g. China is cutting back on domestic use of coal, but has decided to build hundreds of new coal plants worldwide: https://www.npr.org/2019/04/29/716347646/why-is-china-placin...
Also, there are other vast large-scale man-made ongoing changes to the world's ecosystem. E.g. the Amazon rainforest continues to be under attack. Same in other countries like Indonesia.
One US city switching away from natural gas is totally insignificant compared to all that.
If you really want to reduce climate change, figure out how to stop the big big crazy things. Banning natural gas in new construction in Berkeley is simply symbolism over substance.
Everything starts with one.
If it turns out that Berkeley doesn't spontaneously erupt into rioting and general disorder because gas is being phased out, well perhaps others that were considering it will see that it's not such a crazy idea.