HOW MUCH LESS OXYGEN CAN THE WARMING CASPIAN SEA HOLD? THE LOSS OF OXYGEN SOLUBILITY IN THE NORTHERN, MIDDLE AND SOUTHERN CASPIAN, 1982-2022

Warming lowers the amount of oxygen that sea water can hold, even if nothing else changes. Using April-October sea-surface temperature from two satellite-based records, OISST v2 and Pathfinder, both based mainly on the AVHRR instruments, this study estimates this warming-only loss of equilibrium oxygen solubility in the three basins of the Caspian Sea over 1982-2022; the northern winter, with no measured OISST temperatures under the ice, is left out. Solubility fell by 5.8%, 3.6% and 3.3% in the north, middle and south in OISST and by 3.8%, 3.4% and 2.6% in Pathfinder. The two records agree in the middle basin, differ slightly in the south and clearly in the north, where the spring is doubtful in both. The northern loss is significantly larger than the southern one only when April, the month with ice in OISST and with the most doubtful Pathfinder values, is included, and even then not in the full Pathfinder record, so the author's hypothesis that the north loses more is not robustly supported. Over 1982-2022, in both records, in every basin and in the three seasons compared (April-October, May-October, June-October), the loss was faster in 1982-2001 than in 2002-2022, but significantly so only in single basins and seasons, never in the same ones in both records, so a departure from a steady loss is not established. The result is the equilibrium level that warming has lowered; measured upper-layer oxygen lies at 69-109% of it off the Iranian coast (117% for one doubtful value) and, in one 2009 study in the north-east that could not be checked, at 125-175%.