As global temperatures soar and emissions remain higher than ever, scientists are exploring the dramatic, planet-wide interventions we could take to stave off the climate crisis.
One of the most intriguing possibilities involves using the ocean, already the world's largest carbon sink, to suck up even more of the greenhouse gas by removing some of the carbon that it already stores.
Dozens of startups are already experimenting with this form of climate intervention, which is sometimes referred to as marine carbon dioxide removal. What makes it so appealing is that the ocean, in theory, would essentially do the work for us: all we'd have to do is set it into motion and store — or even repurpose — the extracted gases so they doesn't reenter the atmosphere.
But it may be too good to be true. In a new study published in the journal Environmental Research Letters, a team of international researchers warn that this could have dire unintended consequences — like accelerating the decline of the ocean's already plunging oxygen levels.
"What helps the climate is not automatically good for the ocean," lead author Andreas Oschlies, from the GEOMAR Helmholtz Center for Ocean Research Kiel in Germany, said in a statement about the work.
The warmer that water becomes, the less oxygen it can dissolve. In the past fifty years, as global temperatures steadily climbed, the ocean has lost nearly 2 percent of its total dissolved oxygen, a proportion roughly equal to a staggering 77 billion metric tons, according to a 2018 study.
At its worst, this phenomenon, known as ocean deoxygenation, creates entire "dead zones" where there's so little oxygen available that the waters become virtually uninhabitable. Sometimes stretching across thousands of square miles, whatever marine life was once living in the afflicted area either flees or, more grimly, suffocates to death.
Climate change has accelerated the eerie aquatic trend, increasing both the size and number of these dead zones. Clearly, halting global warming would help stymy this — but not if the solution we employ requires putting additional strain on the ocean. In particular, it appears that biotic forms of marine carbon removal could precipitate devastating losses of dissolved oxygen, the researchers caution.
One leading method, called ocean fertilization, proposes seeding the seas with nutrients to boost the growth of oxygen-producing algae. The problem is that when the phytoplanktons perish, their tiny corpses sink to the ocean floor, where the bacteria that feed on them end up consuming even more oxygen.
"Methods that increase biomass production in the ocean, and subsequently lead to oxygen-consuming decomposition, cannot be considered harmless climate solutions," Oschlies said in the statement. "Our model simulations show that such approaches could cause a decrease in dissolved oxygen that is 4 to 40 times greater than the oxygen gain expected from reduced global warming."
But the researchers aren't advocating against using the ocean as a carbon sink entirely. Encouragingly, they found that abiotic methods, including one that involves flushing the waters with minerals like limestone to convert CO2 into a molecule that stays trapped underwater, has minimal effects on oxygen levels.
Instead, the researchers want to stress that going forward, anyone pursuing this research should put assessing the potential oxygen toll of their technique front and center.
"The ocean is a complex system which is already heavily under pressure," Oschlies said. "If we intervene with large-scale measures, we must ensure that, no matter how good our intentions are, we are not further threatening marine environmental conditions that marine life depends on."
More on the ocean: A Strange Darkness Is Spreading Throughout the Oceans
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