Oceans absorb roughly thirty percent of human CO2 emissions, making them the single largest natural carbon sink for atmospheric CO2 — with the land biosphere being the other major sink. Phytoplankton near the surface photosynthesize like land plants, pulling carbon from the air into ocean food chains.

When marine organisms die, their carbon-rich remains sink to the deep ocean in what scientists call the biological pump. This process has locked away vast amounts of carbon over millennia, and disrupting it would speed up atmospheric warming.