Let's say there's some incredibly deadly airborne disease. Its only weakness is that below a certain air pressure, it goes inert and dies rapidly. As a consequence, it wipes out most of human civilization except for the bits that are really high up. Populated lowlands are wiped out, and humanity now exists on a series of disconnected mountain slopes.
With fossil fuels out of reach deep in the zone of death, the only way to travel between the safe areas is by airship. Mostly hot air balloons, but there's a few hydrogen-filled blimps and zeppelins around. Gliders are an option for the foolhardy, if the air currents are favorable.
So what's left if you chop off the bottom 4000 metres? El Alto in Bolivia is now Capital of the World, with more than a million inhabitants. The major inhabited areas are now in the mountains of South America, India and Tibet. In a less extreme scenario, where we only slice off the bottom two kilometres, large chunks of the Americas and Asia are just fine, whereas in Europe, only isolated mountain villages would survive. Alphorns and Airships.
And of course, air pressure isn't constant. A high-pressure system coming your way? Better abandon the lower slopes. Low pressure? An excellent opportunity to go scavenging in the death zone. During a thunderstorm. Enjoy!