The Baltic Sea’s Oxygen Levels Are At a 1,500-Year Low

Parts of the Baltic Sea are suffocating.

The sea cuts deep into Europe, and nine countries line its coast. Due to human activity over the past century, the Baltic Sea contains one of the largest “dead zones” of oxygen-starved water in the world—over 60,000 square kilometres.

Oxygen levels in a coastal area of the Baltic that the study looked at are at a 1,500-year low and are “unprecedentedly severe,” according to a new study published on Thursday in Biogeosciences. Oxygen deprivation of this kind occurs when waters are overloaded with sewage, fertilizers, and other nutrients. The oxygen-deprived waters suffocate fish and other marine life and give rise to blooms of toxic bacteria—this isn’t just bad for the environment, but threatens already-dwindling fish stocks for human consumption too.

The study concluded that human-led trends have accelerated oxygen depletion in the Baltic over the past century—and that climate change is playing a role.

“Climate change was not the main cause of the current dead zone, but it is an important factor delaying the recovery,” Tom Jilbert, an assistant professor at the University of Helsinki in Finland and one of the study’s co-authors, said in a statement. This is because warm water is less effective when it comes to holding oxygen.

For the study, scientists extracted two core samples from the Archipelago Sea, an area in the larger Baltic Sea between Finland and Sweden. Analyzing these cores allowed the researchers to look at oxygen depletion trends over a 1,500-year time span. Despite recent efforts to improve oxygen levels in the area—by reducing the dumping of polluting nutrients, for example—the study found “no evidence of recovery.”

Oxygen starvation and “dead zones” aren’t just a problem in the Baltic. Last year, scientists measured a dead zone the size of New Jersey in the Gulf of Mexico. Understanding historical trends in the creation of dead zones due to oxygen depletion, and their causes, can help efforts to improve water quality.

Countries lining the Baltic have tried to reduce nutrient loading for years through an intergovernmental organization called the Baltic Marine Environment Protection Commission. But the new study should be a wake-up call that much more is needed to help the Baltic Sea recover from its current oxygen-starved state.

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