Geothermal power is THE answer in places where it is available.

Which includes large parts of the western U.S., I'd note.

Iceland has some particular advantages. Fact known mostly to people who have been there:

Reykjavik has a municipal hot water tank.

No, really.

If you live in Reykjavik, your hot pipes are connected to one of six huge water tanks at Perlan (which also has an amazing, but amazingly expensive restaurant built on top of it because there's a lot of space on top of it and why waste it). These tanks are filled with geothermally heated water straight from the Earth, which is mixed with cold water (because it's too hot) and then piped to everyone's hot taps and heating systems. Hot water radiators are standard.

It's so cheap that some people don't have off switches on their radiators. When it's too hot, you open the windows and heat the outside.

This saves a lot of energy for the city by replacing all of those individual hot water heaters.

There is, however, a downside.

The hot water smells of brimstone.

And because it's not possible to completely prevent hot to cold water contamination in fixtures?

The tap water tastes of brimstone.

It won't hurt you, but it is hugely disconcerting!

Iceland just needs to switch to electric vehicles to become a carbon neutral country and the technology is there.