I think it might be <em>natural disasters.</em>
The more the people, the more pollutant they generate which will pollute the the water.
At higher latitudes, climates become colder. The closer you are to the ground, the warmer it will be, but if you're far from the ground, it'll be cold.
see plants are also living beings, they also need oxygen for respiration. There would be less carbon dioxide available for plants for photosynthesis. Atmosphere will be clean, less polluted, clean water. No fighting, peaceful, cool environment