For generations, the ocean has appeared to most humans as vast and impenetrable. Mysterious, dangerous, unfathomably large, but certainly not susceptible to being significantly altered by something humans might do.
But over the past century, that understanding has changed—first slowly and then more dramatically—and now, the ocean appears smaller and more fragile than we once thought. We have unimpeachable evidence that humans can have a devastating footprint on the ocean: 20th century whalers decimated global populations of blue, fin, and sperm whales over just a few decades; factory trawlers are depleting today’s fisheries; agricultural runoff has created enormous dead zones in the ocean; and plastic waste litters most of the ocean.
And there is a far more dangerous human-caused stressor that has largely gotten a pass from scrutiny, even though it is creating existential threats to the ocean. That stressor is the carbon dioxide pollution we have released into the atmosphere over the past 200 years, with a significant portion finding its way into the ocean’s upper layer.
Although we may not normally think of CO₂ as pollution, human enterprise since the dawn of the industrial revolution has emitted about 2 trillion tons of this invisible gas into Earth’s atmosphere that otherwise would not have been there. Over this time, we have increased CO₂ levels by 50% in the atmosphere and by 30% in the upper layer of the ocean.
This enormous amount of CO₂ pollution has already had, and will continue to have, dangerous effects on the ocean.
The effects of CO₂ pollution on the ocean can be grouped into two large categories: thermal stress and chemical stress.
Thermal stress comes as the massive amounts of excess CO₂ we have put into the air trap an enormous amount of energy from the sun that would otherwise have dissipated into space—about 93% of all this excess heat is absorbed into the ocean. The quantity we are talking about is staggering, calculated at about 14 zettajoules of heat every year. For context, a joule is a basic measure of heat energy, and a zettajoule is that single unit with 21 zeros after it.
As that amount is still difficult to comprehend, let’s give it some additional context. Researchers have translated this into an equivalent measure: the amount of heat energy released by an atomic bomb the size of the one that detonated over Hiroshima. Fourteen zettajoules of heat energy going into the ocean each year is roughly equivalent to five atomic bombs’ worth of heat energy going into the ocean every second of every minute of every day, year after year. This means that, every day, 432,000 atomic bombs’ worth of excess heat energy enters the ocean. And the quantity of heat has risen as atmospheric concentrations of CO₂ have increased.
All this excess heat going into the ocean is literally unraveling the fabric of the system. Warmer ocean water holds less oxygen; already, there has been about a 2% average decrease in dissolved oxygen throughout the ocean. Warmer upper layers of the ocean inhibit mixing with the middle layer of the ocean, which is a primary exchange system that brings nutrients into the global food web. These warmer waters expand, and that expansion is causing a significant portion of the sea-level rise that coastal ecosystems and communities have been experiencing. Warmer waters also lead to marine heat waves that decimate coral reefs; we have already lost more than half of the Earth’s tropical coral reefs primarily due to heating and bleaching. In addition, warmer waters are driving species that can migrate to do so; their move toward cooler water is leading to large-scale migrations of fish stocks poleward. And warmer waters mean less Arctic sea ice, which has functioned like a planetary air conditioning system that we all depend on.
Chemical stress is caused by about 25% of the total CO₂ pollution emissions being absorbed into the upper layer of the ocean, creating a chemical reaction known as ocean acidification. As the ocean absorbs this excess CO₂, it becomes increasingly acidic; today, the global ocean has become about 30% more acidic, on average, than it was in preindustrial times. As this massive shift in ocean chemistry increases, the ocean becomes less hospitable to all life that forms a shell. This notably includes many phytoplankton and zooplankton—the microscopic life forms that sit at the base of a number of oceanic food webs and are major producers of the oxygen we rely on. It also of course includes the shellfish that so many of us love to eat.
These two systemwide threats to the ocean—thermal and chemical stress caused by CO₂ pollution—outweigh, in terms of their ultimate risk, anything else that we have done or are doing to the ocean. The threats are also growing and will continue to do so as atmospheric concentrations of CO₂ and other greenhouse gases increase.
And worse, as these stresses mount, they further fuel “positive feedback loops”—reactions to ocean warming and acidification that further increase warming. For example, the loss of Arctic sea ice means that more heat is being trapped on the planet rather than reflected back into space. This is leading to thawing of permafrost, which contains enormous stores of greenhouse gases. Regional warming is also accelerating the melting of the Greenland ice sheet, which appears to be slowing a critical ocean current in the north Atlantic—altering how the waters naturally circulate.
We can now see and measure the harm that humans have done, and the conclusion is inevitable: The climate crisis is an ocean crisis, and the ocean crisis is a climate crisis. Now, the question is, what can we do about it?
Until recently, most efforts to mitigate climate change have focused on stemming the flow of greenhouse gas emissions into the atmosphere. But the world’s leading body on climate science, the Intergovernmental Panel on Climate Change, has made it clear that at this late stage, even dramatically reducing CO₂ pollution emissions won’t keep global warming from exceeding 1.5 degrees Celsius, which scientists describe as the tipping point of dangerous and potentially irreversible climate disruptions. We also need to remove billions of tons of “legacy” carbon dioxide pollution that is already overheating the planet and acidifying the ocean.
This means quickly developing a host of technologies that can remove CO₂ from the air and water and safely store it as permanently as possible. These tools and techniques will range from the most natural, such as planting trees, to the most industrial, like employing direct air capture plants that operate as CO₂ removal factories and using electrochemistry to essentially strip CO₂ from the atmosphere. And there will be many other options in between, some of which have not been invented yet.
When all is said and done, cleaning up the legacy carbon pollution that we have created is essential to slowing and ultimately reversing the ocean-climate crisis. Carbon cleanup will also have to become an enormous economic sector if it is to achieve a scale relevant to the problem. And the ocean will likely have an important role to play.
Why? The ocean has enormous potential to safely draw down and sequester additional CO₂. The reasons are simple: The ocean is already the largest carbon cycler on the planet, using both biological and biogeochemical processes to move CO₂ from the air and land into the deep ocean. In fact, there is already roughly 50 times more carbon at the bottom of the sea than in the atmosphere. If we could increase that ratio by 1% to 2%, we could have a massive impact on reducing atmospheric concentrations of CO₂ and slowing, and potentially even reversing, the climate and ocean crises.
Ocean-based carbon dioxide removal (CDR) is a field that is in its infancy but growing up fast. Ocean-based CDR techniques essentially mimic, enhance, and/or accelerate oceanic biological and geological processes already underway.
There are five large “domains” of ocean-based CDR approaches that encompass almost every way that the ocean might be engaged to sequester more carbon:
- Seaweed growth for carbon sequestration. Marine macroalgae (aka seaweeds) have tremendous potential to sequester carbon given their staggering growth rates (some kelp can grow 2 feet a day!) and the fact that they do not require additional energy or nutrients to grow. Long-term CO₂ storage may be achieved by making long-lived products from biomass (biochar, bioplastics, etc.) and/or bundling and sinking the biomass into the deep ocean. Many approaches are being tested to accelerate seaweed growth for carbon removal.
- Microalgae cultivation and carbon sequestration. The most productive organisms in the ocean are microscopic algae (phytoplankton). As these microalgae grow, they take up carbon dioxide. Increasing their overall mass could lead to increased CO₂ fixation and, ultimately, transfer of that CO₂ into the deep ocean. Technologies being tested include using ships to distribute nutrients into nutrient-limited areas of the sea to foster growth and pumping nutrient-rich waters to the surface to fuel growth of microalgae.
- Ocean alkalinity enhancement. Over geological time scales, the ocean has become a major store of carbon due to the weathering of natural rocks, which washes alkaline molecules into the ocean. Ocean alkalinity enhancement technologies can speed up this natural process to sequester carbon dioxide and, at the same time, reduce ocean acidification. Many methods are under development to add alkaline material or liquid to the ocean to enhance the natural carbon cycle.
- Direct ocean capture. This approach is like direct air capture plants on land, using electrochemistry to essentially strip CO₂ from seawater. The acidic stream of CO₂ can be stored in deep rock layers or used to weather alkaline rocks to increase alkalinity. The resulting alkaline seawater can enhance ocean alkalinity, allowing the ocean to absorb more CO₂ from the atmosphere.
- Blue carbon ecosystems. Coastal ecosystems, including tidal salt marshes, mangrove forests, and seagrass meadows, fix CO₂ via photosynthesis and can trap organic carbon in their roots and the marine sediments for thousands of years. Disturbance and loss of these ecosystems has led to the release of carbon into the atmosphere. The restoration of these degraded systems can increase carbon sequestration and provide a host of other benefits for nature and people.
Technologies are being developed in each of these five domains to test the potential for permanent carbon removal and the associated economic, social, and environmental costs. For example, some people are building large autonomously operated platforms for growing seaweed in the open ocean and then using robotic harvesters to cut, bale, and sink it to the deep. Others are using wave-powered upwelling pumps that bring nutrients from the deep to the upper photic zone of the ocean to drive blooms of phytoplankton. And yet others are testing an array of ways to disperse alkaline material and liquids into the ocean to create chemical reactions that lead to CO₂ being moved to the deep. And this is just the beginning; as humans come to grips with the massive opportunity and challenge we face in cleaning up carbon, we will see a host of iterations as well as new ideas.
But we are now in a race against time. There are only two ways to reduce atmospheric concentrations of CO₂—reduction and removal—and they have to be done together.
To chart the ways forward, Ocean Visions, working with experts from around the globe, developed a series of technology roadmaps that assess the current state of various technologies that could help reduce carbon and illuminate the diverse obstacles and opportunities to quickly advance the development and testing of these technologies. The roadmaps are not just about science and engineering challenges; they also include critical policy, governance, economic, and social challenges. Work is needed in a number of disciplines and sectors for successful outcomes.
We have now turned our attention to catalyzing efforts to aid these priorities and expanding the overall investment of time, energy, and money into this emerging sector. We desperately need to engage an ever-growing cadre of scientists, engineers, managers, environmentalists, businesses, investors, and others to tackle critical obstacles and pursue key opportunities.
This work has one critical intermediate goal: to advance the development and testing of all viable approaches in the ocean and at scales significant enough to answer the critical question of whether these approaches will be able to contribute to the massive carbon cleanup and ocean regeneration challenges that we face. It is no longer a question of whether we need carbon removal; it is only a question of where society will find the most cost-efficient paths to achieving carbon removal at a climate-relevant scale. To answer these questions that are so important to our future, we must have rigorous and credible science on which climate and ocean policy can be based.
The good news is that solutions to this ocean-climate crisis are possible—if we add carbon removal as a tool in our climate toolbox and if we fully consider all the potential ways the ocean can contribute to carbon removal.
With intensified focus, expanded effort, and application of a suite of new technological tools, we can clean up the CO₂ pollution unraveling our world and regenerate our ocean and our climate so that humans and nature can thrive.
Brad Ack is executive director and chief innovation officer for Ocean Visions.
The Earth’s climate and ocean crises are inextricably linked, and we must quickly develop solutions that work to save them both.