People need food. Food doesn't grow well in very hot and very cold conditions, nor in very wet or very dry conditions. And most other animals don't survive well outside the environment they evolved to live in.
And that’s totally how it works, less food is produced, so people just say, “Oh well, time to die.” They don’t become desperate, they don’t migrate en masse, and they don’t go to war, it’s just a peaceful loss of a few billion lives to make more room for you. All of that assuming we’re not swimming in technological miracles like some others here are saying. Fusion is right around the corner! We’re going to grow cows in test tubes for free!