Oceanographer: To substantially reduce the amount of carbon dioxide in Earth' s atmosphere, carbon dioxide should be captured and pumped deep into the oceans, where it would dissolve. The cool, dense water in ocean depths takes centuries to mix with the warmer water near the surface, so any carbon dioxide pumped deep into oceans would be trapped there for centuries.
Carbon dioxide dissolved in cool, dense water in ocean depths will not escape back into Earth's atmosphere a long time before the water in which that carbon dioxide is dissolved mixes with warmer water near the surface is an assumption that the oceanographer's argument requires.