> biggest reason why air conditioning is not so widespread in Northern Europe as in the United States

It certainly is a transitional period where each summer more and more people realize that eventually something needs to be done, "maybe a/c next year" for many years until the year of installation finally comes.

In Northern Europe it certainly is still a rare occurrence that everything gets heated so warm that the air does no longer cool during the night and you can't cool down for the next day. Yes, we do get heat waves but they don't last very long. Yes, summers are mostly getting hotter but it's still nothing like in southern Europe. We might have several weeks of 25-28C with a lot of lakes and sea to dip into.

Admittedly it can be tormenting in city apartments where you might not have a place for A/C even if you wanted to, and where you might not have enough outside walls to effectively cross-ventilate. Further, the stone and pavement in a city absorb heat like a sponge which keeps the average a few degrees warmer than greenleaf areas across the hot season.

Yet I think maybe half-ish of households (or at least detached houses) already do have A/C. Installations have been steadily creeping up in the last 25 years. But those units aren't there because of their cooling capacity (which isn't necessarily always even used). Those are air-to-air heat pumps that keep the house warm in the winter, and can be used for cooling in the summer.