You know that tang when you lick a lemon? That’s due to its acidity . Don’t worry – the Earth isn’t turning into a citrus fruit anytime soon. The ocean is becoming more acidic, but it will probably never be as acidic as a lemon  (luckily)! This is called ocean acidification .
It is caused by CO2 from the air dissolving in the ocean . CO2 reacts with seawater, releasing protons . These protons cause acidity . The acidity of the ocean has increased by about 30% due to human CO2 emissions, and could be 150% higher by 2100 – these conditions have not been seen for over 20 million years !
This has bad consequences for ocean creatures . Perhaps the worst is a decrease in the amount of dissolved minerals (called “carbonate”) [7,12,14]. Oysters use carbonate to make their shells, and corals use it to make their skeletons [2,12]. With less carbonate available, it is harder for these animals to grow and reproduce [2,4,5]. This will worsen the effects of increased water temperature, and lower oxygen concentration [15,16].
‘Sea butterflies’ are small sea creatures (about the size of a pea), and are food for lots of animals including krill, whales, and young salmon . They will likely suffer particularly badly from ocean acidification in the future, which could have widespread effects on the whole marine food chain [12,16].
Animals may also behave differently if the ocean continues to become more acidic [14,17]. For example, crabs may have difficulty finding food , and fish may be worse at detecting predators in acidic water .
Some animals actually benefit from ocean acidification  and some may be able to adapt to it , but scientists agree that most ocean life will suffer . The extent of future damage is highly uncertain . Ocean acidification is not easily reversible , and it might take about 700 years of greenhouse gas emission reductions to restore ocean acidity to historical levels . This is because even if we stopped emitting CO2 today, it would still dissolve in the oceans for many years to come, continuing ocean acidification [1,3,19].
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