Eelco Rohling and Joseph Ortiz write:

On January 5, 2018, a paper published in the journal Science delivered a sobering message: The oxygenation of open oceans and coastal seas has been steadily declining during the past half century. The volume of ocean with no oxygen at all has quadrupled, and the volume where oxygen levels are falling dangerously low has increased even more.

We’re seeing the same thing happen in major lakes.

The main culprits are warming and — especially in coastal seas and lakes — eutrophication caused by enhanced nutrient loads in runoff. The findings reaffirm that we urgently need to address global warming, and that we are in need of an updated Clean Water Act. We only need to look to the Mediterranean Sea and, more recently, the North American Great Lakes region for dramatic illustrations of what lies in store if we don’t act now.

Around 8,000 years ago, the entire eastern half of the Mediterranean Sea became severely oxygen-starved between 300 and 1,500 meters, and lost all oxygen, or became ‘anoxic,’ below that. It wasn’t warming that caused the oxygen decline then, as is happening in today’s oceans, but the amplification of the African monsoon, which drove intense flooding of the Nile River, full of nutrients from decomposing organic matter. The freshwater itself inhibited deep-water formation, while its nutrient-load led to wild-growth of algae, cyanobacteria, and animals grazing on them. Upon their death, decomposition sapped oxygen from the water, rapidly turning it oxygen-starved, anoxic, and in extreme cases rendered it ‘euxinic’ (containing hydrogen sulfide, infamous for its rotten-eggs smell).

The conditions wiped out virtually the entire ecosystem from a few hundred meters below the surface of the water to the seafloor. A devastating 4,000-year period of anoxic ‘dead zone’ conditions ensued, which all started within a century of the flooding. [Continue reading…]