The ocean is a great sponge for carbon dioxide, which is usually reported as good news about where our emissions go. Absorbing a gas is a chemical event, though, not a storage event. NOAA’s program sets out what the chemistry actually does, and the first thing it clears up is a wording problem that trips up almost every conversation about it.
It lowers the pH without making the ocean acidic. When seawater absorbs carbon dioxide, chemical reactions create , and the more carbon dioxide the more of those free ions, which lowers pH. The ocean itself is not acidic; its acidity increases. NOAA tracks four parameters, pH, the partial pressure of carbon dioxide, and dissolved inorganic carbon, and needs two of them to calculate the rest.
The ocean is a great sponge for carbon dioxide, and absorbing it is not the same as storing it. When the ocean absorbs carbon dioxide it changes the chemistry of the ocean, which NOAA describes as a fundamental and global change. The first thing the program is careful about is the vocabulary: the absorption increases the ocean’s acidity, and the ocean itself is not acidic.
The mechanism is short. Chemical reactions in seawater create hydrogen ions, which NOAA calls free agents because they can react with other compounds. pH is a measure of how many of these free ions are present, so the more carbon dioxide in the water, the more ions are created and the lower the pH.
Four parameters are tracked, and the structure of the monitoring is informative in itself. They are pH, the partial pressure of carbon dioxide, total alkalinity and dissolved inorganic carbon. The partial pressure says how much gas is in the water. Total alkalinity measures the buffering molecules, carbonate and bicarbonate, that can neutralize an acid, while dissolved inorganic carbon captures the non-biological carbon in its three main forms. Researchers need to measure two of them to calculate the others.
Two other kinds of acidification sit alongside the open-ocean case. arises from local processes, including upwelling of deeper more acidic water and nutrient runoff from land driving algal blooms that release carbon dioxide as they die. And freshwater bodies acidify too: researchers project pH in the Great Lakes will decline at a rate similar to the oceans, on top of a separate recovery from twentieth-century acid deposition. Impacts depend on the timing, duration and severity of the change and on how marine life responds.
wat. Dissolving carbon dioxide in seawater makes hydrogen ions, which lowers the pH of an ocean that is still not acidic.
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Alkalinity is the part that decides how much headroom there is. Carbonate and bicarbonate in seawater buffer against rising acidity, so the question is not only how much carbon dioxide arrives but how much neutralizing capacity is left.
How do we know?
The NOAA Ocean Acidification Program describes the chemistry, the four measured parameters and why two suffice to characterize conditions, and distinguishes open-ocean from coastal and freshwater acidification.
What is still unknown
An agency program page describing established chemistry and its monitoring framework rather than reporting measurements. It describes impacts on marine life as depending on timing, duration and severity, without quantifying them.
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