public class ExampleMod implements ModInitializer {
  @Override
  public void onInitialize() {
    [...]

    OverworldBiomes.addContinentalBiome(OBSILAND_KEY, OverworldClimate.TEMPERATE, 2D);
    OverworldBiomes.addContinentalBiome(OBSILAND_KEY, OverworldClimate.COOL, 2D);
  }
}