The Atlantic Meridional Overturning Circulation (AMOC) redistributes heat, water, salt, oxygen, nutrients, and carbonate species on our planet. Likely due to global warming, the AOMC is declining rapidly in the mid-twentieth century. Here we show with long-term time-series data that this slowdown could enhance ocean acidification in the deep seas.

Many organizations, such as Wikipedia, YouTube, the International Union for Conservation of Nature, the US National Oceanic and Atmospheric Administration, and the National Aeronautics and Space Administration, the United Nations Development Program, and the World Meteorological Organization (WMO), define the ocean acidification strictly as the consequence of invading atmospheric CO₂. For instance, according to WMO, "Ocean acidification is directly caused by the increase of carbon dioxide (CO₂) levels in the atmosphere". Yet, many coastal processes also cause the seawater to acidify. Even in open oceans, seawater acidification unrelated to atmospheric CO₂ may also occur. For instance, a recent Nature Climate Change article reported that deep oceans might acidify faster than anticipated due to global warming. That study was based on data in the Sea of Japan, where the surface water has increased in temperature and become lighter due to global warming. As a result, it has become more difficult for the deep convection of surface water to occur. Subsequently, deep waters are less ventilated, resulting in more decomposition of organic matter, which releases acid. The result is that the deep waters in the Sea of Japan have acidified faster than found in the surface layer, mainly due to stagnation rather than the increasing atmospheric CO₂. The Nature Climate Change article did not mention any change in pH (pH described here refers to the pH in the total scale at in-situ temperature and depth) based on the Hawaii Ocean Time-series (HOT) data in the Pacific Ocean. Still, it did report a known slight decrease in pH in the deep Atlantic Ocean. What follows is investigating the acidification in the subsurface Atlantic and Pacific Oceans and linking it to reduced AMOC and similar processes in the Pacific Ocean.