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Living creatures are amazing at building their homes from just about anything.
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But sea-dwelling creatures are particular wizards: microscopic coccolithophores, coral-building
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algae, and giant snails engineer their own building material, like magic, by pulling
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two dissolved chemicals -- calcium and carbonate -- out of the water to form solid shells of,
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surprise, calcium carbonate.
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The reason those shells don't dissolve back into calcium and carbonate as soon as they're
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built is that ocean water is already holding as much calcium and carbonate as it can, so
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the mineral forms much more easily than it dissolves.
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At least, that's the way it works near the surface, where the shell-builders live.
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But at greater depths, water isn't quite as saturated with calcium and carbonate, and
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thus calcium carbonate is easier to dissolve.
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So, unlike shallow coastal waters where shells of dead creatures build up on the seafloor,
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out in the deep ocean, there's a depth at which calcium carbonate starts to break apart
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and empty shells dissolve before reaching the bottom.
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This "dissolving depth" depends on the concentration of calcium and carbonate already in seawater
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- if the concentration is high, shells sink deeper before their calcium carbonate dissolves.
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And if the concentration is low, the "dissolving depth" moves closer to the surface, meaning
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the deepest intact shells begin to dissolve.
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But this is a feedback loop - shells that dissolve add more calcium carbonate to the
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water, making it harder for other shells to dissolve and lowering the "dissolving depth".
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Basically, chemistry in the deep ocean stabilizes the concentrations of calcium and carbonate
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in seawater, which is why the upper part of the ocean is saturated with calcium and carbonate
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(and perfect for shell-building) to begin with.
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Except, we forgot to take into account the chemistry of another key part of the ocean
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- the atmosphere!
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At the ocean's surface, a small proportion of gases like oxygen and carbon dioxide dissolve
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into the water (dissolved oxygen, for example, allows sea creatures to breathe).
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And when the concentration of the gases in the atmosphere rises or falls, so does the
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amount of gas dissolved in the oceans.
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If it weren't for the ocean's own balancing act, any incoming carbon dioxide would be
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bad news for shell builders, because more CO2 means less CO3.
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That might sound weird, but it's just the way the chemistry plays out: dissolved CO2
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molecules combine with water to form what's called carbonic acid, which in turn combines
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with carbonate to form hydrogen carbonate.
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Simply put, when carbon dioxide in the atmosphere increases, carbonate in the ocean decreases
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and shell-building gets harder to do - at least for a moment.
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Given enough time, the physics and chemistry of the ocean will cause the "dissolving depth"
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to rise, and more shells on the sea floor will return their calcium and carbonate back
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to the water, restoring normal levels.
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There are situations where the oceans can't keep up this balancing act, though.
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For example, if so much carbon dioxide were added to the ocean that the dissolving depth
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rose high enough, all shells everywhere in the ocean might start dissolving.
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While possible, this is a lot less pressing than the risk that, for a time, CO2 levels
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change faster than the ocean can compensate, so that even if it would eventually stabilize
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and allow shell formation at the surface, it would take centuries to do so.
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During that time, the upper reaches of the ocean, where most of the amazing shell-builders
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live, might become a barren wasteland.
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And speaking shellfishly, THAT would be
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a clamity.