I want to make a world with a great deal of variation in habitability in terms of temperature and pressure. Some areas would be habitable; others less so, some would require protective clothing and some might not be accessible without pressure suits / protective vehicles.
The plan is to start with an approximately Earth sized world with roughly 1g, but different surface features and vastly less ocean. I want a world with a range of altitudes from the highest mountains to the abyssal plains with most of the comfortable habitable areas on the mountain tops and their immediate surroundings.
The atmosphere would consist mostly of carbon dioxide, oxygen and nitrogen but would be predominantly carbon dioxide, however given that carbon dioxide is heavier than oxygen or nitrogen I would expect to find a higher proportion at the lower altitudes.
The net result being high pressure, temperature and carbon dioxide concentration in the low lands and a gradation to lower pressure, temperature and carbon dioxide levels in the highlands and an interesting variety of environments in between. So a little like mt Everest in reverse liveable at to top but not at the bottom.
The proportion of the gases in the atmosphere and total pressure at the lowest point can be adjusted to fit as required, but unprotected humans must be able to live comfortably outside at high altitudes, uncomfortably at lower altitudes and not for long at the lowest altitudes.
Is this sort of world possible and if not how else might it be adjusted to give this effect?