How would the Earth system be different if ice sank in liquid water?

This ice layer insulates the water below it, allowing it to stay liquid, which allows the life within it to survive. If ice sank, the liquid water on top would also freeze and sink, until all the liquid water became frozen. Water is less dense as a solid, than as a liquid, which is why ice floats.

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if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.