A serious ecological consequence of global warming

Ocean acidification as a result of increasing CO2 doesn't get mentioned often lately, although it presumably continues to increase.   (There was, no doubt, some sloppily done lab based experimentation on this with fish and other creatures which I think has perhaps harmed its reputation amongst science journalists.  But the problem is still real.)  

The other big problem in the oceans from global warming is the increase in lack of oxygen as the water warms, and I have mention it from time to time over the years.  (You can search "ocean oxygen" in the search bar at the side, if you like.)

There was an article about it in Science recently, and yeah, maybe it will be a race between it and acidification as to which will cause the most serious ecological collapse within the next few decades.  Some extracts:

Climate change is leaching oxygen from the ocean by warming surface waters. Two other climate-related threats to the seas—ocean acidification and marine heat waves—get more attention from scientists and the public. But some researchers believe deoxygenation could ultimately pose a more significant threat, making vast swaths of ocean less hospitable to sea life, altering ecosystems, and pushing valuable fisheries into unfamiliar waters. As global warming continues, the problem is sure to get worse, with disturbing forecasts that by 2100 ocean oxygen could decline by as much as 20%. Sharks—fast-moving fish that burn lots of oxygen, sit at the top of food chains and crisscross huge ocean expanses—should be sensitive indicators of the effects....

SCIENTISTS FOR YEARS have documented oxygen-starved dead zones in places like the Gulf of Mexico and the Baltic Sea. There, pollution from nutrients running off the land, such as synthetic fertilizer, sparks algae blooms. Microbes feast on the rotting vegetation, consuming oxygen. A surge of low-oxygen water can flood an area so quickly that crabs, sea stars, and even fish suffocate before they escape. Low-oxygen zones also form naturally along the western edges of the Americas and Africa, where oxygen-depleted water that hasn’t seen daylight for decades wells up.

In the open ocean, currents and storms churn the water, keeping oxygen levels higher. Yet since the 1990s climate models have foretold that a warming climate would deplete oxygen there, too. Surface water warmed by rising air temperatures holds less oxygen, and the growing temperature contrast between surface layers and colder, deeper water slows the mixing that transports oxygen into the depths. At higher latitudes, melting ice can flood surface layers with fresh, low-density meltwater, strengthening the layering and reducing mixing.

In 2008, a paper in Science sounded the alarm. German and U.S. scientists found that the low-oxygen zones off Africa and the Americas were growing deeper and losing still more oxygen. Since the 1960s these areas had expanded by about 4.5 million square kilometers, close to the area of the European Union. In the waters frequented by Sims’s sharks off Africa’s northwest coast, the low-oxygen layer had nearly doubled in thickness over 5 decades, from 370 meters to 690 meters. By 2008 its top had risen to less than 150 meters below the surface. The global trend, the scientists warned, “may have dramatic consequences for ecosystems and coastal economies.”

In 2017, scientists delivered more troubling news in Nature. Overall, the world’s oceans had already lost some 2% of their oxygen since 1960, roughly double what climate models predicted.

For Andreas Oschlies, a biogeochemist at the GEOMAR Helmholtz Centre for Ocean Research Kiel and a leading expert on modeling oxygen in the ocean, the implications were staggering. If the trend continues, it could mean a potential loss of 20% by 2100, he says. That’s equal to going from sea level to more than 2000 meters elevation on land. “I thought ‘Wow!’” Oschlies recalls. “That’s the biggest change and maybe the most worrying change that we see in the ocean. Immediately I thought of (past) major extinction events.” For example, at the end of the Permian period 256 million years ago, rising ocean temperatures and an 80% plunge in oxygen levels helped drive the largest extinction in Earth’s history. Up to 96% of all marine species disappeared.

By comparison, the 2% drop in oxygen levels seen so far might not sound like much. But global averages can be misleading, warns Lisa Levin, a biological oceanographer at the Scripps Institution of Oceanography who has studied the effects of low oxygen on ocean ecosystems for more than 30 years. “There are places in the ocean where there’s been much bigger declines,” Levin says. “These changes are probably very important.”