Mediterranean temperature and salinity trends are speeding up 2,500 metres below the surface

The speedup arrives in units that are easy to skim past: degrees Celsius per decade, per decade. In some regions the warming rate itself is climbing by as much as 0.3 C every decade, while each kilogram of water gains roughly a tenth of a gram of salt per decade ^[5]. A patch warming at 0.1 C per decade in the 1990s, if that acceleration held, is warming at 0.4 C per decade ten years later ^[2].

At the surface, the change since 2000 runs about half a degree per decade, two to three times the rate of the global ocean surface ^[6]. Through most of the basin in the second half of the twentieth century it was under a tenth of a degree per decade ^[7]. That is a more than fivefold increase in rate ^[1]. In recent years the upper 100 metres have sat about 2 C above their 1950-1999 average ^[8].

Warming makes seawater lighter and added salt makes it heavier. Both are rising, so the density of Mediterranean water has changed far less than either its temperature or its salinity ^[9]. Across most of the basin the warming prevails, the surface is getting lighter, and it mixes less readily with the water below ^[10]. Where dense water forms, above all in the Adriatic, the added salt still keeps it heavy enough to sink ^[11].

That sinking is how the deep Mediterranean gets its oxygen. The Adriatic is one of the few places where cold winter winds make surface water dense enough to descend, and that water then spreads into the central and eastern basins, carrying oxygen into the deep sea ^[12]. Since 2000 the deep waters of the southern Adriatic have warmed about six times faster than those of a typical Mediterranean basin ^[13]. "Dense water is still forming, but we have indications that the way it forms is changing, and the water that sinks is now warmer and saltier than it used to be," Terzic said ^[14].

The trend and the speedup do not reach the same depth. "For us, what was alarming was the rate at which this is changing, and the depths that such significant changes reach," Terzic said ^[19]. She puts statistical significance at 3,000 to 4,000 metres and the speedup at about 2,500 metres ^[3], which leaves a layer somewhere between 500 and 1,500 metres thick where the water is warming and salting but not measurably faster than before ^[3].

The published account reports one basinwide surface rate and, for the deep Adriatic, a ratio against other basins, so an operator working at 200 or 600 metres cannot take a depth-specific number from it ^[6][13]. The surface still holds the largest absolute rate quoted anywhere here, so deep change outpacing it is not something these numbers show ^[6]. The acceleration does reach past the near-surface layer where earlier work had spotted it, and this is the first test for it below the surface across the whole basin ^[20].

The study did not measure oxygen, though warmer water holds less of it and declines have already been documented in parts of the Mediterranean ^[15]. Species trying to move away from the heat have nowhere obvious to go, since the sea is closed to the north and the deep water is warming too ^[16]. The oceans have absorbed more than a quarter of humanity's carbon dioxide emissions and more than 90% of the excess heat trapped by greenhouse gases ^[17].