CLIMATE CHANGE IS DRIVING OCEAN OXYGEN LEVELS DOWN, AND THAT’S A BIG PROBLEM FOR MARINE ECOSYSTEMS

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Scientists know that climate change is slowly robbing the oceans of their oxygen, but historically, it’s been hard to differentiate oxygen loss that’s due to natural ocean cycles and warming-driven loss. Now, a new study predicts that within the next 15 to 25 years, warming-caused oxygen loss will be detectable across the worlds’ oceans.

The study, published in Global Biogeochemical Cycles, used modelling to determine that, between 2030 and 2040, warming-caused oxygen loss will be severe enough — and data will be comprehensive enough — for scientists to see what parts of the ocean are being affected by human-caused deoxygenation.

“Oxygen varies naturally in the ocean quite substantially,” Matthew Long, lead author of the study and scientist at the National Center for Atmospheric Research. “Without any human-driven climate change we could expect oxygen levels at a particular location to go up and down in such a way that low levels may be persistent for a number of years, followed by a period of high levels.”

That means that, if scientists today measure oxygen levels in part of the ocean for a short period of time and observe a decreasing trend, they can’t say for sure if that trend is caused by climate change, Long said. But that should change by 2030.

“While there’s some ambiguity now, in the not too distant future, that ambiguity will be eliminated in places where we have long records,” he said.

As the planet warms, the oceans are absorbing a lot of heat — so much so that ocean heat sinks were responsible for the slowdown in atmospheric temperature increase earlier this century. But warmer waters, as the study explains, don’t absorb as much oxygen as cooler waters do. Higher surface water temperature also “stratifies the ocean,” the study states: As the surface water warms, it becomes more buoyant and less dense, meaning that it mixes less and oxygen at the surface doesn’t make it to the middle of the ocean.

“It’s that mixing that’s responsible for sustaining oxygen levels at depth,” Long said.

Lack of oxygen is a serious problem for ocean ecosystems. Just like animals on land, marine creatures like fish and crabs depend on oxygen for survival. In the last 50 years alone, National Geographic reports, areas of the ocean with low oxygen have increased by 1.7 million square miles. Many species can’t survive in these regions, so they hover towards the top of the ocean, avoiding the low-oxygen regions closer to the middle.

“What is very clear is that if the trend of human warming continues — which it seems likely to do given the relative inactivity on curtaining CO2 emissions — oxygen levels in the ocean at depth will continue to decline and there will be significant impacts on marine ecosystems,” Long said. “As oxygen levels decline, more and more of the ocean is going to be uninhabitable by certain organisms. Habitat will become more fragmented, and the ecosystem will become more vulnerable to other stressors.”

And the oceans are facing a multitude of other stressors. Take coral bleaching: When ocean temperatures get too high, corals become stressed and expel the photosynthetic algae living symbiotically in their tissues. Without this algae, the coral turns white and is at a much higher risk of death if temperatures don’t fall quickly enough. Australia’s Great Barrier Reef is seeing the worst coral bleaching in 15 years due to high ocean temperatures, and reefs at Kiritimati Atoll, located about 1,300 miles south of Hawaii, have seen such severe bleaching in recent months that 80 percent of their coral has died.

Reefs are hugely important to the ocean ecosystem: according to NOAA, they provide habitat to “more species per unit area than any other marine environment, including about 4,000 species of fish, 800 species of hard corals and hundreds of other species.” There could also be as many as 8 million other yet-to-be-discovered species living in and near coral reefs. And reefs are key to the livelihoods of 500 million people around the world.

And though ocean oxygen levels are being driven down, carbon dioxide levels in the ocean are going up. Ocean acidification, which is driven by increasing levels of carbon dioxide in the oceans, is a major problem for marine ecosystems. It’s causing coral reefs to grow more slowly, and it can make it difficult for shellfish larvae to grow shells.

We need to address the cause of oxygen loss — and warming and acidification — if we want to slow it, Long said. That means decreasing CO2 emissions. But more investment in our monitoring abilities is also key if we want to understand where and how low oxygen levels are impacting the ocean.

“Being able to really quantify what’s going on is really important,” he said.