Projected acidification of the Mediterranean Sea

The Mediterranean Sea's acidification rate is under debate. Its 10\% higher alkalinity relative to that of average ocean waters, makes it chemically more capable to absorb CO$_2$. That capacity is further enhanced by the rapid ventilation of its deep waters. Some data-based studies suggest that that extra carbon uptake causes larger changes in surface and deep pH than found in open-ocean waters. Yet simplified model simulations over the industrial era suggest otherwise. Here we offer the first projections of Mediterranean Sea acidification over the 21st century. Our projections rely on a regional climate model, which couples the ARPEGE atmospheric model to the NEMO-MED8 regional eddying ocean model under historical forcing followed by the IPCC A2 scenario over the 21st century. Those model results are used here to drive an offline configuration of the same ocean model including natural and anthropogenic carbon. Simulations were made with and without increasing CO$_2$ and with and without climate change. With both, the level of free acidity ([H$^+$]) doubles during 1860 to 2100 as does the seasonal amplitude of [H$^+$]. But the seasonal amplitude of pH actually declines slightly (because of its log scale, $\Delta$pH represents a relative change in [H$^+$]). And although the projected change in surface pH is quite similar between average ocean waters and the Mediterranean, corresponding changes in [H$^+$] are somewhat lower in the Med. Once again, changes on a log scale can be misleading. The pH changes are mostly due to increasing atmospheric CO$_2$, but climate change substantially worsens the perturbation in parts of the western basin; conversely, previous studies suggest that climate change has virtually no overall effect on pH. Thus under the above scenario, the Mediterranean Sea is projected to experience a doubling of its natural level of free acidity along with similar enhancement to its seasonal extremes during the 21st century.