Marine Oxygen Levels are the Next Great Casualty of Climate Change

Last summer, more than 100 miles of Florida’s coastal waters became an oxygen-depleted dead zone, littered with fish that could be seen even into Tampa Bay. On the other side of the country, Dungeness crabs were washing onto Oregon’s shoreline, unable to escape from water that has, in dramatic episodes, become seasonally depleted of oxygen over the past two decades.  

While much of the conversation around our climate crisis focuses on the emission of greenhouse gases and their effect on warming, precipitation, sea level rise and ocean acidification, little is said about the effect of climate change on oxygen levels, particularly in oceans and lakes. Water without adequate oxygen cannot support life, and for the three billion people who depend on coastal fisheries for income, declining ocean oxygen levels are catastrophic.

As ocean and atmospheric scientists focus on climate, we believe that oceanic oxygen levels are the next big casualty of global warming. To stop this, we need to build on the momentum of the recent COP26 summit and expand our attention to the perilous state of oceanic oxygen levels—the life support system of our planet. We need to accelerate ocean-based climate solutions that boost oxygen, including nature-based solutions like those discussed at COP26

As the amount of CO2 increases in the atmosphere, not only does it warm air by trapping radiation, it warms water. The interplay between oceans and the atmosphere is complex and interwoven, but simply,oceans have taken up about 90 percent of the excess heat created by climate change during the Anthropocene. Bodies of water can absorb CO2 and O2, but only to a temperature-dependent limit. Gas solubility decreases with warming temperatures; that is, warmer water holds less oxygen. This decrease in oxygen content, coupled with a large-scale die-off of oxygen-generating phytoplankton resulting not just from climate change, but from plastic pollution and industrial run-off, compromises ecosystems, asphyxiating marine life and leading to further die-offs. Large swaths of the oceans have lost 10–40 percent of their oxygen, and that loss is expected to accelerate with climate change.

The dramatic loss of oxygen from our bodies of water is compounding climate-related feedback mechanisms described by scientists in many fields, hundreds of whom signed the 2018 Kiel Declaration on Ocean Deoxygenation. This declaration has culminated in the new Global Ocean Oxygen Decade, a project under the U.N. Global Ocean Decade (2021–2030). Yet, despite years of research into climate change and its effect on temperature, we know comparatively little about its effect on oxygen levels and what falling oxygen levels, in turn, may do to the atmosphere. To address this unfolding crisis, we need more research and more data.

In the past 200 years, humans have shown remarkable ability to change the planet by altering the timescales in which the Earth cycles chemicals such as CO2. We need to evaluate any possible solutions for their impact on not just greenhouse gases but other critical elements of life, such as oxygen levels. As the financial world invests in climate change solutions focused on CO2 drawdown, and possibly including future geoengineering efforts such as iron fertilization, we run the risk of causing secondary harm by exacerbating oxygen loss. We need to evaluate potential unintended consequences of climate solutions on the full life support system.

Beyond enhanced monitoring of oxygen and the establishment of an oxygen accounting system, such an agenda encompasses fully valuing the ecosystem co-benefits of carbon sequestration by our ocean’s seaweed, seagrasses, mangroves and other wetlands. These so-called “blue carbon” nature-based solutions are also remarkable at oxygenating our planet through photosynthesis. The theme of COP26 chosen by the host country (U.K.) was “nature-based solutions.” And we saw a lot of primarily terrestrial focused (forestry) initiatives and commitments that are an excellent step forward. We hope this year’s conference and next year’s COP27 help oceanic nature-based solutions to come into their own, propelled by the U.N. Global Ocean Decade.

Putting oxygen into the climate story motivates us to do the work to understand the deep systemic changes happening in our complex atmospheric and oceanic systems. Even as we celebrated the return of humpback whales in 2020 to an increasingly clean New York Harbor and Hudson River, dead fish littered the Hudson River in the summer as warmer waters carried less oxygen. Ecosystem changes connected to physical and chemical systems-level data may point the way to new approaches to climate solutions—ones that encompass an enhanced understanding of the life support system of our planet and that complement our understanding of drawdown to reduce emissions of carbon dioxide. Roughly 40 percent of the world depends on the ocean for their livelihoods. If we do not stop marine life from oxygen-starvation, we propagate a further travesty on ourselves.

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