Color me optimistic, but we'll learn to survive and adapt. Humans in temperate climates (Chicago, Toronto, etc.) already live in places where it's too cold to survive outside for long in winter. Just replace heaters with ACs in this case.
If you just go +20C from comfortable temperature, no amount of clothing will help you survive for very long out in the open. You also have to drink copious amounts of water and salts to avoid dehydrating to death.
Secondly, heating up a closed space is a lot cheaper than cooling it down. Heating can always be done by burning fuel, but cooling greater than 5C requires heat pumps.
The economics and practicalities of heat and cold adaptions are completely different.
You are also leaving snow and ice out of it. There are economics and externalities of moving tons of solid matter around with relatively low latency (people have to work, trucks have to ship). There are also the safety implications and costs associated. Traffic and accident rates do not go up because the precipitation was unexpectedly ten degrees warmer.
So for heating, you can just max out on insulation, decrease your surface area to volume ratio, and the waste heat from interior equipment will be more then enough to provide 'free' heating.
If you outdoor temperature is high (so natural ventilation doesn't do anything), there really isn't much we can do other than try and find higher and higher efficiency heat pumps to cycle out the heat, but even then you're just dumping the heat outside which creates its own issues. We can reflect/control short/long-wave radiation, but the sensible heat from the environment is a problem. Not an impossible problem, but requires more sophistication to resolve.