Your worlds start out with normal biomes and climates. But as more and more items are smelted using coal or charcoal, the save’s Overworld dimension will gradually change over time like so:

• Grass in plains biomes will get browner and browner (and eventually die altogether
• Corals in warm ocean biomes start dying
• Rains become shorter and less frequent
• River levels slowly drop
• Snowy mountains become windswept hill biomes and all the snowy taigas become regular taigas (and then burst into flame of course)
• A new weather effect will be added called “heatwave”. During this, all the small water bodies will evaporate, grass and farmland blocks start randomly converting into coarse dirt (rip cpu lol) and you will catch fire if exposed to sunlight without a helmet, much like zombies and skeletons.

The only way to stop all of this is to completely stop using coal or any wood-based fuels, and instead switch to using things like kelp blocks, blaze rods and lava. Once you’ve done that, you can start removing the excess carbon dioxide from the air by spamming trees everywhere; as you do this you’ll notice everything start going back to normal- but you better not plant too many or you’ll get an ice age.