This is my fourth and final post on the 2021 report: Climate Change: The Physical Science Basis by the United Nations International Panel on Climate Change. I promised four posts covering ocean warming, sea level rise, salinity changes, and finally the subject of this post – ocean acidification. Acidification sounds scary doesn’t it? You might imagine a big rolling sea of water that can burn your skin off. It’s not that bad, but it’s still somewhat frightening. Now I don’t mean to scare people through my blog, that’s big media’s business. I want to inform people who don’t have the time to read thousands of pages of scientific documents about their contents. My blog posts are designed to provide you with a nuts and bolts synopsis of critical water issues plaguing the world today. But sometimes the data can be pretty worrisome.

So let’s get the scary bit out of the way shall we. The IPCC report points to the oceans absorption of carbon dioxide since the 1980s as being in the range of 20 to 30 percent of all human carbon emissions. As a consequence the pH of the ocean surface has declined 0.017 to 0.027 units per decade since the 1980s and subsurface pH up to a mile in depth have declined by 0.003 to 0.026. If you want to know more about how carbon dioxide impacts ocean pH you can find it here https://waterblogger.org/general-information/ocean-acidification-climate-change/ .

You may be asking yourself why such a small decline in pH is so scary. We have to do some chemistry to explain. As the ocean water absorbs carbon dioxide from the air a chemical exchange occurs, producing carbonic acid – H₂CO₃. The carbonic acid dissociates, generating HCO₃ and H⁺. What this means is that carbonate ions (CO₃) in the water are decreased and bicarbonate (HCO₃) ions are increased. It basically changes the whole carbonate chemistry of the ocean. You might not care but all the little critters that make their shells from calcium and carbonate in the ocean’s water do.  Calcium carbonate saturation rates of seawater have been declining at rates of 0.07 to 0.12 per decade.

What I find particularly fascinating in the 2021 IPCC report though is the variation in pH decrease that has been measured in different parts of the ocean now, for example the tropical Pacific Ocean is decreasing in pH at a greater momentum than other ocean areas, while the western tropical Pacific Ocean (renowned for its warm pool of water) shows slower pH declines. Coastal areas are acidifying at a greater rate than elsewhere as waters there are supersaturated with carbon dioxide. What’s happening at depth is interesting too. The well known meridional overturning circulation zones, such as the one in the Atlantic Ocean characterized by the Gulf Stream, bring warm water from the lower latitudes into the Polar Regions where it cools and sinks thereby producing acidification in the deep ocean. Areas like the subpolar North Atlantic Ocean and the Southern Ocean have the highest acidification found at depths greater 2 miles.

Governments are busy trying to decrease carbon dioxide emissions. The results will reverse ocean acidification at the surface, but not at depth due to the long time scales of ocean turnover. This will result in a lasting legacy in the deep ocean and impact biological resources for thousands of years to come.