It's actually common sense and nothing to do with research.

The countries in the far noryh closest to the arctic and the countries in the far south, near the south poles will be safest from heat.

The countries that have cool climates and are islands will be even safer because people will need boats to get there.

The countries that have prepared for climate change stand a better chance than countries thst haven't prepared for climate change.

Balkan countries, just like Spain, Italy, etc aren't mentioned because they are not good places to relocate to. If Balkan countries were good pla es to relocate to, they would have been mentioned.