In ocean science, sixty years is a long time. Long enough, it turns out, to observe changes in the deep sea that the scientists who first monitored it in the 1960s would not have anticipated. Teams from the Chinese Academy of Sciences, France’s Mercator Ocean International, and the École Normale Supérieure in Paris collaborated on a study published in Nature Climate Change that accomplished something that oceanographers have been working toward for decades: it integrated several ocean variables into a single monitoring framework and systematically asked how much has actually changed since we started paying close attention. The response exceeds expectations and appears in more locations at once than the models had predicted.
In comparison to sixty years ago, at least two crucial characteristics have already changed significantly in between thirty and forty percent of the ocean’s upper 1,000 meters. The primary author of the study, Dr. Zhetao Tan, explains the significance of that headline statistic with proper gravity: in certain areas, up to a quarter of the ocean is exhibiting simultaneous changes in temperature, salinity, and oxygen all at once. Not in that order. Not in distinct areas. collectively. The researchers’ primary focus is on the compound nature of that shift, which sets this work apart from previous studies that monitored specific factors separately.
The distinction is important because temperature changes do not occur in a vacuum in marine ecosystems. Warming water, shifting salinity, decreasing oxygen, and rising acidity are all changes that a fish, coral, or deep-water organism must cope with simultaneously. Within evolutionary limits that have evolved over millions of years, each stress on its own is bearable. It is different to have several tensions placed on top of one another at the same time. They have the power to drive organisms beyond adaptive boundaries that would have been maintained under pressure from a single variable. According to Dr. Laurent Bopp of ENS-PSL, marine species that are subject to several stresses at once may migrate or decline. At the global fisheries scale, neither result is insignificant.
The tropical and subtropical Atlantic, the North Pacific, the Arabian Sea, and the Mediterranean are the areas with the strongest compound changes. Ocean regions that are already under stress due to their proximity to significant human land use, intensifying weather systems, or decreased water exchange with the larger global circulation exhibit a pattern of convergence when these regions are taken as a whole. The Mediterranean is particularly vulnerable to changes in salinity and stored heat since it is a nearly isolated sea with little interchange with the Atlantic. The monsoon dynamics that govern the Arabian Sea are changing. These aren’t sporadic hotspots. These are locations when several drivers are supporting one another.
The deep ocean responds more quickly than anticipated, according to Professor Lijing Cheng of the Chinese Academy of Sciences. This is significant because the deep sea has traditionally been regarded as a stable baseline; change at the top occurs rapidly, but change thousands of meters below occurs over geological ages. We’re changing that presumption. The statistics indicate that the distribution of nutrients for creatures that rely on those currents for food is being impacted by decreased oxygen levels and changed current velocity in deep water. The ocean’s thermohaline circulation, which transports heat, nutrients, and gasses across the world, is reacting to surface changes, albeit slowly.
