How do we know ocean carbon dioxide levels are rising and pH is decreasing? By taking measurements of seawater over many years. This graph shows rising levels of carbon dioxide (CO2) in the atmosphere, rising CO2 levels in the ocean, and decreasing pH in the water off the coast of Hawaii. As carbon dioxide rises in the atmosphere, some of it dissolves into seawater, increasing the CO2 concentration there as well. As CO2 dissolves in the ocean, it reacts with water to form hydrogen ions, which drive the pH down. In combination, this process is known as ocean acidification.