Demystifying Ocean-based Carbon Dioxide Removal: An Explainer

Climate change has become the biggest and fastest-growing threat to the ocean, due to temperature increases, acidification, sea level rise, and hypoxic conditions. Mitigation efforts alone will be insufficient to stabilize the climate, as we must remove 100–1000 billion tons of carbon dioxide (CO₂) from the atmosphere before the end of the century to reach long-term climate goals.¹ This is an enormous challenge, calling for an “all cards on the table” approach to identifying scalable, affordable, and safe carbon dioxide removal (CDR) mechanisms. The ocean’s natural carbon pumps play a significant role in controlling the climate and will, ultimately, absorb much of the excess heat and CO₂ in the atmosphere, but not in our lifetime or the lifetime of our children and grandchildren.

Ocean-based CDR mechanisms are currently being explored as part of a growing portfolio of CDR approaches. Philanthropy can play an important role in shaping the maturation process of ocean-based CDR approaches by ensuring transparent, inclusive, and responsible research and policy frameworks.

Philanthropy can play an important role in shaping the maturation process of ocean-based CDR approaches by ensuring transparent, inclusive, and responsible research and policy frameworks.

Removing carbon from the atmosphere is necessary to stabilize the climate and guarantee the durability of conservation efforts.

Today, we are on track to increase average global temperatures by 3-4 degrees Celsius compared to pre-industrial levels within the next 80 years alone.² Since the 1850s, we have emitted 1.6 trillion tons of planet-warming carbon dioxide CO₂ into the atmosphere. Based on our global “emissions gap,” the world is projected to reach a catastrophic temperature rise this century (in the absence of rapid decarbonization). The Industrial Revolution fueled a global economy and lifestyle that many in the industrialized world have come to rely on: heated and cooled homes, stocked fridges, global trade and cheap, on-demand personal transportation. The resulting rapidly increasing atmospheric CO₂ concentrations are making the planet warmer and the ocean more acidic, disrupting the global climate system. Concerns about the physical impacts of climate change have grown stronger and louder as heatwaves, wildfires, storms, floods, and failing agricultural yields are affecting more and more people. However, the international response to these threats has largely been slow and insufficient, both in terms of verifiable private sector commitments and nationally binding targets for emission reductions. The reasons leaders are finding climate change so difficult to grapple with are political, economic, ideological, and even psychological in nature.

A 1.5-degree Celsius future has become impossible to achieve without drawing CO₂ out of the atmosphere and storing it safely and permanently. To limit warming to 1.5 degrees C by 2100³ with a 66 percent probability, we must not emit more than 230 Gt of CO₂ between 2020 and 2100. At current rates of emissions, this “carbon budget” will be spent in less than five years. Even the most aggressive projections of emission reductions do not get anywhere close to staying within the 1.5-degree carbon budget. The Intergovernmental Panel on Climate Change (IPCC) suggests that even aggressive mitigation scenarios, that is, preventing the release of CO₂ into the atmosphere—will have to be complemented with carbon dioxide removal (CDR) on the order of 100–1000 billion tons before the end of the 21st century.⁴ Removal is literally bringing back down to Earth the CO₂ that has accumulated since the Industrial Revolution. This legacy “CO₂” will cause the planet to warm, even once mitigatio—solar, wind, electric vehicles—takes new emissions to zero.

Figure 1. Impact of climate change on atmospheric temperature (average above pre-industrial), sea level rise and pH. Source: en-roads.climateinteractive.org, using baseline projections

Marine conservation priorities and past victories may be swamped by climate change. Biodiversity in the ocean has evolved in the absence of human-induced stressors such as overfishing, pollution, shipping, habitat destruction and fragmentation, and invasion of new species. Even without the impacts of climate change and ocean acidification, this ‘cumulative human impact’ on the ocean has considerably diminished marine biodiversity. The effects of climate change have now become the biggest and fastest-growing contributors to cumulative human impact on the ocean.⁵ A recent report from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) found that approximately one million plant and animal species are threatened by extinction, many within decades, and that human activities have significantly altered two-thirds of the ocean.⁶ As a result, many prominent efforts to safeguard biodiversity, sustainably manage fisheries and alleviate poverty in coastal communities around the world are at risk due to climate change: we can work locally and nationally to reduce the detrimental effects of pollution, overfishing or habitat destruction, but ocean acidification, rising temperatures, sea level rise, and hypoxia are largely outside of our control. A thoughtfully examined and determined global effort to remove hundreds of billions of atmospheric CO₂ is an important part of the solution set currently overlooked.

The effects of climate change have now become the biggest and fastest-growing contributors to cumulative human impact on the ocean.

The potential and risks of ocean-based CDR must be explored in detail and with care.

The ocean might be a powerful ally to large-scale CDR. As there is no single silver bullet CDR method, a portfolio approach to negative emission technologies must be pursued. Removing hundreds of billions of tons of CO₂ from the atmosphere is an enormous challenge. Some of the most discussed CDR approaches in the terrestrial realm include direct air capture, bioenergy with carbon capture and storage, reforestation, and agricultural practices that increase the burial rate of organic carbon. While these approaches hold significant promise, there are important impediments to their quick scale-up, including cost, energy requirements, and land use implications. Additionally, some climate-caused “feedbacks” such as drought, fire, and pests already are impairing the ability of land-based solutions such as trees to remove as much CO₂ as once hoped. Direct air capture is considered to be one of the most promising proposals today as the technology has been successfully piloted and seems highly scalable. Nonetheless, costs of net carbon removal remain prohibitively expensive and the technology is water and energy intensive. It is widely acknowledged that we will need a portfolio approach to CDR, given the urgency and scale of the challenge.⁷

It is widely acknowledged that we will need a portfolio approach to CDR, given the urgency and scale of the challenge.

The ocean plays an important role in the global carbon cycle and has contributed to the stabilization of the global climate for millions of years. Primary producers such as phytoplankton, mangroves, and seaweed turn CO₂ into organic carbon, some of which escapes the food web and is permanently sequestered in coastal soils and the seabed. Over geologic timescales, the weathering of rock washes alkaline molecules into the ocean and converts dissolved CO₂ into carbonates and bicarbonates, thereby storing carbon for tens to hundreds of thousands of years in seawater. Overall, the ocean today stores 38 trillion tons of carbon (equivalent to 140 trillion tons of CO₂), approximately 50 times more than is contained in the atmosphere.

Figure 2. CO₂ sources, sinks, and flows. Source: www.globalcarbonproject.org

Several ocean-based approaches have emerged as potentially scalable contributions to a growing CDR portfolio. These approaches aim to boost the biotic and abiotic carbon pumps that naturally remove sequester CO2, thereby “pulling” more CO₂ from the atmosphere into the ocean without increasing ocean acidification. (A detailed overview of approaches can be found at www.oceancdr.net.)

• Biotic approaches aim to boost primary production to increase the net export of organic carbon to the deep ocean and sediments or create harvestable biomass that can be used in other carbon storage approaches; examples include protecting blue carbon stock, artificial upwelling of nutrient-rich waters, ocean fertilization, and seaweed cultivation.
• Abiotic approaches aim to convert dissolved CO₂ into carbonates and bicarbonates (thereby storing the carbon for tens to hundreds of thousands of years) or to physically move CO₂-rich surface waters to the deep ocean. Examples include ocean alkalinity enhancement, seawater CO₂ stripping, and artificial downwelling. Ocean alkalinity is a natural process due to the erosion of rocks into the sea. But enhancement accelerates the process. If humans speed up the natural process can organisms in the sea suddenly adapt to less acidic ocean? Absent large-scale government support, these are questions philanthropy can help to answer.

Figure 3. Overview of Ocean CDR Approaches. Source: Video courtesy of OceanCDR.net

Philanthropy can play an important role in shaping the maturation process of ocean-based CDR approaches by ensuring transparent, inclusive, and responsible research and policy frameworks. Ocean-based CDR approaches might prove to be cost-effective and scalable additions to the emerging CDR portfolio, but their effectiveness and their potential environmental impacts require research, pilot-stage testing and field building. Philanthropy can help to fund early studies in modest amounts but more importantly, philanthropy is best positioned to fund advocates to push the public sector to fund basic research and pursue policies to safeguard the health and integrity of the ocean. In coming years and decades, demand for affordable and scalable CDR approaches will likely grow due to policy targets of net-negative emissions by mid-century, and ocean-based approaches will likely see increased interest. In fact, important conversations are just starting to happen:

• A recent report by the Energy Futures Initiative suggests that ocean CDR approaches might scale to well beyond 10 Gt of net CO₂ removal.⁸
• The National Academies of Sciences, Engineering, and Medicine is currently working on a “Research Strategy for Ocean Carbon Dioxide Removal and Sequestration”.⁹
• There are dozens of research projects underway exploring the effectiveness and environmental risks of ocean CDR approaches.ⁱ

Philanthropy can help to fund early studies in modest amounts but more importantly, philanthropy is best positioned to fund advocates to push the public sector to fund basic research and pursue policies to safeguard the health and integrity of the ocean.

What are the risks involved in ocean CDR and how do they compare with the risk of inaction? Ocean-based CDR covers a range of approaches, from the conservation of mangrove forests to seaweed farming to alkalinity addition and iron fertilization. The large-scale interventions required to draw down gigatons of CO₂ per year has environmental advocates concerned about CDR’s deleterious impact on specific marine species or even irreversible ecosystem change. Moreover, some advocates fear that, by accepting CDR as part of the solution set to combat climate change, we “give up” on mitigation efforts and gives license to the fossil fuel industry to continue to emit CO_₂.

There is no path to meeting our climate goals through mitigation pathways alone, and recent IPCC modeling strongly suggests that only a combination of aggressive mitigation efforts and ambitious CDR can help us stay within biologically safe bounds of CO₂ concentrations. In the absence of quickly scaled responsible research and development in the field of CDR, we risk that less tested and potentially more risky geoengineering approaches are deployed (such as solar radiation management) down the line. The nascent nature of most approaches and the many open questions call for a coordinated effort to test effectiveness, evaluate potential environmental risks, and work closely with coastal communities. Philanthropy’s role will be crucial in supporting the maturation process of promising approaches while building the safety rail guards that prevent irresponsible deployment.

The nascent nature of most approaches and the many open questions call for a coordinated effort to test effectiveness, evaluate potential environmental risks, and work closely with coastal communities.

Notes

1. IPCC (2018). Special Report: Global Warming of 1.5 ºC. Accessed January 2021 at https://www.ipcc.ch/sr15/en-roads.climateinteractive.org
2.  Climate Analytics, 2020. climateanalytics.org
3. https://www.ipcc.ch/sr15/chapter/spm/
4. Halpern, B. et al. (2019). Recent Pace of Change in Human Impact on the World’s Ocean. Scientific Reports 9, no. 1: 11609
5. IPBES. (2019). “IPBES Global Assessment Summary for Policymakers.” https://www.ipbes.net/news/ipbes-global-assessment-summary-policymakers-pdf.
6. National Academies of Sciences, Engineering, and Medicine [NASEM] (2018). Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. The National Academies Press, Washington, D.C.
7. Energy Futures Initiative (2020). Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.
8.  https://www.nationalacademies.org/our-work/a-research-strategy-for-ocean-carbon-dioxide-removal-and-sequestration