The ocean is losing its oxygen. Last week, in a sweeping analysis in the journal Science, scientists put it starkly: Over the past 50 years, the volume of the ocean with no oxygen at all has quadrupled, while oxygen-deprived swaths of the open seas have expanded by the size of the European Union. The culprits are familiar: global warming and pollution. Warmer seawater both holds less oxygen and turbocharges the worldwide consumption of oxygen by microorganisms. Meanwhile, agricultural runoff and sewage drive suffocating algae blooms.
The analysis builds on a growing body of research pointing to increasingly sick seas pummeled by the effluent of civilization. In one landmark paper published last year, a research team led by the German oceanographer Sunke Schmidtko quantified for the first time just how much oxygen human civilization has already drained from the oceans. Compiling more than 50 years of disparate data, gathered on research cruises, from floating palaces of ice in the arctic to twilit coral reefs in the South Pacific, Schmidtko’s team calculated that the Earth’s oceans had lost 2 percent of their oxygen since 1960.
Two percent might not sound that dramatic, but small changes in the oxygen content of the Earth’s oceans and atmosphere in the ancient past are thought to be responsible for some of the most profound events in the history of life. Some paleontologists have pointed to rising oxygen as the fuse for the supernova of biology at the Cambrian explosion 543 million years ago. Similarly, the fever-dream world of the later Carboniferous period is thought to be the product of an oxygen spike, which subsidized the lifestyles of preposterous animals, like dragonflies the size of seagulls. On the other hand, dramatically declining oxygen in the oceans like we see today is a feature of many of the worst mass extinctions in Earth’s history.