The Mad Planet by Murray Leinster, Science Fiction, Adventure, Fantasy

Thick jungles of rank growth covered the ground. The air was depressing and enervating. Men could live there, but it was a sickly, fever-ridden existence. The whole population of the earth desired the high lands and as the low country became more unbearable, men forgot their two centuries of peace.

They fought destructively, each for a bit of land where he might live and breathe. Then men began to die, men who had persisted in remaining near sea-level. They could not live in the poisonous air. The danger zone crept up as the earth-fissures tirelessly poured out their steady streams of foul gas. Soon men could not live within five hundred feet of sea level. . . .