The oceans are great at absorbing carbon dioxide (CO2) from the air, but when their deep waters are brought to the surface, the oceans themselves can be a source of this prevalent greenhouse gas.
Wind patterns together with the Earth’s rotation drive deep ocean water — and the CO2 it sequesters — upward, replacing surface water moving offshore. A process known as upwelling, it occurs on the west coasts of continents. And it’s part of a never-ending loop in which CO2 levels in the surface ocean rise and fall in a natural rhythm.