The deep ocean (deeper than 1km) is a CO2 sink. Deep water masses are cold and dense which minimizes vertical mixing with overlying ocean layers. This means what carbon travels to the deep is sequestered away from the atmosphere. Eric Galbraith, author of recent study in Nature, notes
“It’s like a bottle of Italian salad dressing that hasn’t been shaken,” he said. “You can leave it there forever and it just doesn’t mix.”
So how much CO2 can the deep ocean store?
Looking at the last ice age, 20,000 years ago, Galbraith et al. examined how the deep ocean stored CO2 when atmospheric levels dropped by 30%. During the last ice age the movement of the deep ocean was extremely slow and CO2 extremely isolated from the atmosphere. As things warmed, the deep ocean began to lose its ability to sequester CO2.
Galbraith stressed that his research is historical — he can only see what happened before, not what happens next. But he said it may mean that as the Earth becomes warmer, the ocean’s CO2-disposal system could start breaking down.