Doubled seasonal amplitude of Mediterranean surface acidity by 2100

Chair: Samantha Siedlecki

James C. Orr (1)*, Briac Le Vu (1), Julien Palmieri (1), Jean-Claude Dutay (1), Florence Sevault (2), and Samuel Somot (2)

1 LSCE/IPSL, Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ, Gif-sur-Yvette, France<o:p></o:p>
2 CNRM/Météo-France, Toulouse, France

The Mediterranean Sea’s relatively high alkalinity and rapid ventilation of deep waters make it more capable of absorbing anthropogenic CO2, but exactly how much is absorbed and the intensity of associated acidification is under debate. Here we provide the first projections of Mediterranean Sea acidification over the 21st century. Our projections rely on a high-resolution 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. Results from that coupled-climate model were used here to drive an offline configuration of the same ocean model that is coupled to a biogeochemical model (PISCES), which includes both natural and anthropogenic carbon. Simulations were made with and without increasing CO2 and with and without climate change. Under that A2 scenario, the projected level of free acidity ([H+]) doubles during 1860 to 2100 as does the seasonal amplitude of [H+]. Yet the seasonal amplitude of pH actually declines slightly. Because of its log scale, a change in pH represents a relative change in [H+], whereas a change in [H+] is an absolute change. And although the projected change in surface pH is quite similar between average ocean waters and the Mediterranean, absolute changes in [H+] are lower in the Mediterranean. The pH changes are driven mostly by increasing atmospheric CO2, but effects from climate change vary spatially and are prominent in the western basin The Mediterranean Sea’s enhanced seasonal extremes during the 21st century further exacerbates summertime acidification but mitigates wintertime acidification relative to the case where the seasonal cycle would remain unchanged. Beyond the Mediterranean, future changes in the seasonal amplitude of [H+] could be much larger still (e.g., in the Arctic) based on preliminary analysis of the CMIP5 earth system models.