Gases from the atmosphere get absorbed by the ocean and gases dissolved in the ocean are released into the atmosphere. When the two are in equilibrium, roughly the same quantities are being dissolved as are being released. Change the atmosphere’s composition, as we have done, and the exchange becomes lopsided: more carbon dioxide enters the water than comes back out. In this way, humans are constantly adding CO2 to the seas, much as the vents do, but from above rather than below and on a global scale.
