This method involves the addition of alkaline materials to seawater to accelerate the ocean’s natural carbon uptake. Natural carbon uptake occurs when carbon dioxide from the air dissolves in seawater, and then combines with molecules already in the water to form bicarbonate and carbonate molecules, which are stable forms of carbon that do not re-enter the atmosphere. Adding alkalinity can increase the rate at which this process occurs, in turn enabling the ocean to absorb more carbon dioxide from the air to restore equilibrium. 

This approach involves the addition of micronutrients (e.g. iron) and/or macronutrients (e.g. nitrogen or phosphorous) to the surface ocean with the deliberate intent to (1) increase photosynthesis by marine phytoplankton, and thus enhance uptake of carbon dioxide from surface waters, and to (2) enhance the transfer of the newly formed organic carbon to the deep sea, away from the surface layer that is in immediate contact with the atmosphere.

Artificial upwelling (AU) and downwelling (AD) could enhance the ocean’s natural vertical circulation patterns via a system of long vertical pipes equipped with circulation pumps. In AU, these pumps circulate nutrient-rich seawater upwards from the ocean’s depth into nutrient-poor surface waters, where (it is hoped) these nutrients can stimulate phytoplankton blooms. In AD,  pumps circulate plankton-rich surface waters downwards into the ocean’s depths, where (it is hoped) the plankton biomass will remain deep underwater for thousands of years.

Direct Ocean Capture (DOC) involves the capture and removal of dissolved CO2 directly from seawater using electrochemical treatments, which can include electrolysis or electrodialysis. Under these treatments, an acidic solution is applied to seawater, converting bicarbonate to CO2, which can be subsequently stripped from the ocean water using membranes and vacuum pumps. The end products of DOC include CO2 gas and CO2-depleted seawater.  

To count as carbon removal, the carbon sequestered by seaweed via photosynthesis must be locked away somewhere where those carbon molecules cannot re-enter the atmosphere for at least hundreds of years. The most cost effective and scalable use of seaweed as a carbon dioxide removal technique would be to simply cultivate large amounts of seaweed and sink it directly to the ocean floor.