The title of this blog comes from a frustration I had, back in 2010, about the absurdly optimistic climate scenarios that were being described by politicians and pundits as “alarmist”. At that point in time, I remember people scoffing at the notion that we might see two feet of sea level rise by the end of the century, and most of the other effects of a warming planet, like crop failures, were largely ignored. Honestly, climate change in general was being largely ignored, which is part of how we got where we are today.

So I started out to write about worst-case scenarios, and the possibility of warming oceans leading to widespread anoxic conditions struck me as particularly worrisome. More on this later, but that exact scenario is the most likely cause of the Permian-Triassic extinction event, also known as The Great Dying. All the food we get from the sea requires oxygen to survive. Specifically, it requires dissolved oxygen, which means oxygen molecules in the water that are not part of the H2O molecules, but that saturate the space between water molecules. The air we breathe is around 78% nitrogen and 21% oxygen. In aquatic environments, H2O is roughly analogous to nitrogen in air, when it comes to respiration, but the concentration of oxygen available for “breathing” is much lower. This isn’t a problem – gills tend to be very efficient, and aquatic life has evolved to deal with that. What it does mean is that despite oxygen being a major component of water, most of what’s there isn’t actually usable for breathing. There are a number of factors that affect how much there is, but temperature is a big one, and it’s feeding into a rise in so-called “dead zones”.

As ocean and atmospheric scientists focused 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.

I think it’s important to understand that not all life on Earth has the requirement for oxygen that is common among the organisms that surround us. Because we require an environment with a certain oxygen saturation, we simply don’t interact with life forms that rely on, for example, sulfur as an electron receptor. This applies both on land and in the water, but possibly the most common anoxic environment on Earth is the muck under bodies of water. When oxygen concentration dips low enough, anaerobic life gets to come out and play, and that’s bad for us aerobic critters. If a dead zone becomes permanent, rather than seasonal like the ones described in the article I quoted, then the rise in anaerobic life, and its waste products, will mean that lack of oxygen will not be the only problem – you also see a rise in sulfur compounds that are poisonous to us all by themselves. It may also be possible for sulfide-saturated waters to release toxic gas that could suffocate coastal areas.

Our oceans are so huge that it’s hard to wrap our heads around the scale. From everything I’ve read, it should be thousands of years before heat-driven dead zones in the oceans start gassing low-lying land, but it does seem to be a real possibility. Furthermore, I think it’s worth remembering a few things:

The first predictions of global warming due to fossil fuel emissions, in the 1890s, projected a timeline of 3,000 years to the “palm trees growing in Sweden” mark. As it stands today, I think we will be lucky if it takes 300 from the same starting point (meaning we’re over a century in), and the most common refrain from living climate scientists seems to be that everything is happening faster than anyone expected.

Chemical changes like this aren’t really something a lot of people are talking about, but I think if they do happen, it will be similar to how sea level rise has been happening – unevenly, and more or less destructive depending on local conditions. It will be a long time before we get to the water levels in my “flooded NYC” stories, but we’ve already seen the city’s subways flooded a number of times in the last few years. I don’t think it’s out of the question for there to be localized “gas events” driven by aquatic dead zones centuries before anoxic conditions exist in a majority of our oceans.

We are living through an event unlike anything our species, or any of those with which we share the planet, have experienced. Knowledge of our history can help us, but it cannot guide us. The reality is that things are moving fast, and the only certainty we have is that the world in which our civilization was born no longer exists, and this new one is not pulling its punches.

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