Overfishing began in the nineteenth century and continues, with more meager returns, to this day. The other problem the sea faces is chemical change. This is even harder to see and more global in its sources, and, as a result, even harder to fix. One example is acidification. As the CO2 concentration in the atmosphere rises due to burning fossil fuels, some of the extra CO2 dissolves into the sea. There it changes the water’s pH balance, pushing it away from its usual state of mild alkalinity. The metabolisms of a great many sea animals, including cephalopods, are affected by this, and there
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