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 drives suffocating algae blooms.