More than half of the total amount of oxygen in our atmosphere is created by creatures in the ocean. At the same time, oceans store 50 times more carbon dioxide than what's currently found in our atmosphere. The warmer the ocean gets, the less CO2 it can store. It's a vicious cycle: the warmer it gets, the less our oceans can protect the planet from even more extreme weather events. If temperatures keep increasing at their current speed, scientists believe many shellfish such as mussels and snails will not survive. That's due to ocean acidification: if the CO2 content in the seawater increases, the PH level in the water changes. The increasing acidity hampers the creation of the chalky shells of the animals. This throws entire biospheres off-balance, and could threaten entire economic sectors, such as the breeding of oysters and mussels. The rising temperatures in the atmosphere triggered by the burning of coal, oil and gas also change ocean currents as the water gets warmer. This can already mean death for many creatures, such as corals. Corals live in symbiosis with colorful algae which help feed them. The warming of the water can lead to algae death, which means more stress for corals, leading to many losing their color, which is also known as coral bleaching.
Tim Schauenberg, ‘How to save our high seas from overfishing, pollution’, BBC