At room temperature and atmospheric pressure, carbon dioxide is a gas. However, small amounts of gas can dissolve in water.

There are many ways to separate dissolved CO₂ from water. One way is to raise the temperature of the water, or lower the pressure. Either of these methods will encourage any dissolved gases in the water, including CO₂, to bubble out of the water and enter the surrounding atmosphere.

Another method involves pumping the water through a device called a membrane contactor. A membrane is a barrier that allows some things to pass through it, but not others. For example, air filters often use membranes that trap particles in the air, and allow clean air to pass through. In the case of a membrane contactor for CO₂ removal, the membrane allows CO₂ and other gases to pass through it, but not water. So, when water and dissolved CO₂ flows past the membrane, the CO₂ leaves the water and passes through the membrane, leaving water with much less dissolved gas.

An example flow rate for a membrane contactor, meaning how quickly you can remove gas, is around 100 gallons of water per minute. And, the volume of the ocean has been estimated to be about 0.3 billion cubic miles, or about 330,000,000,000,000,000,000 gallons. So, to filter the CO₂ from the ocean using a single membrane contactor would take approximately 6,000,000,000,000 years, or 6,000 times longer than the lifetime of the universe!