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You don't need to use reflection for this. Sorry my code wasn't good. Try this: Climate climate = event.getClimate(); event.setClimate(new Climate(RainType.SNOW, climate.temperature, climate.temperatureModifier, climate.downfall)); FYI: you will not see snow in higher temperature biomes

So it took me a few days, but I managed to figure things out myself. First of all, apparently RegistryObject<Biome> fields used with DeferredRegister cannot be referenced directly. When you want to reference your custom biomes, you need to get them from the biome registry itself. This solves the issues I had with the getNoiseBiome method. Secondly, the codec thing. All I had to do to get that working, was to register the codec in a static initialiser in my biome provider class: static { Registry.register(Registry.BIOME_PROVIDER_CODEC, new ResourceLocation(MOD_ID, "biome_source"), CODEC); } I hope this helps anyone trying this for themselves.