Athens, Greece

A thing of beauty is a joy forever—unless it’s upwards of 104 degrees Fahrenheit and smoggy. If you are dying to see what’s left of the Parthenon and the Acropolis in the decades to come, you’ll probably want to rule out a summer trip to this iconic city. The rise in summer temperatures here “will reduce personal comfort and could lead to more heat stress and associated mortality,” Britain's Centre for Future Studies says. Smog will also be a problem, not just in Athens but elsewhere in the Eastern Mediterranean, CFS notes. Also watch out for water restrictions and even more forest fires in this general region. What about Cairo and Istanbul? They’ll also be reeling under the blistering summer heat, so forget the sun hat and consider winter travel instead.