Poor countries in the global south. They are likely to have revolutions and build a new society the soonest due to decreasing material conditions and a lack of alternatives. Wealthier countries will be able to use their money and power to maintain the status quo for much longer imo

Edit: I forgot I also wanted to mention Rojava, they have already begun building a society based around liberation and ecology. Their ideology is based off of Democratic Municipalism as imagined by Abdullah Öcalan and Murray Bookchin. This is a good start but obviously not an ending point for a “solarpunk” society