Marine biomass carbon removal and storage: sink it and forget it?

Introduction

In recent years, the scientific community has increasingly emphasized that atmospheric carbon dioxide removal is a critical complementary approach to aggressive decarbonization of the world economy in order to meet the temperature objectives of the Paris Agreement. The treaty seeks to “[h]old … the increase in the global average temperature to well below 2°C above pre-industrial levels” and to pursue “efforts to limit the temperature increase to 1.5°C above pre-industrial levels”  

It’s been estimated that in addition to reducing emissions by 90%  or more by 2050, we will need to remove a whopping 10 gigatons of carbon dioxide annually from the atmosphere by 2050, and perhaps 20 million tons every year by 2100. An array of carbon dioxide removal (CDR) options have been proposed to effectuate this goal, with the primary emphasis to date on land-based methods, including direct air capture, bioenergy and carbon capture with storage (BECCS), and afforestation.

However, growing concerns about the sustainability and economics of large-scale deployment of terrestrial CDR methods have led to increasing interest in the potential role of the world’s oceans in removing carbon dioxide from the atmosphere. This partially reflects the fact that ocean ecosystems have sequestered approximately 25-30% of anthropogenic carbon dioxide over the course of the past few centuries and take up approximately 2.5 gigatons of carbon from the atmosphere annually. 

While a lot of the focus of marine-based CDR approaches has been on macroalgae (seaweed) cultivation and sinking, ocean iron fertilization, and ocean alkalinity enhancement, I will focus in this piece on an emerging approach that could potentially play a substantial role in the CDR ecosystem, which I denominate here as marine biomass carbon removal and storage (mBiCRS). In this article, I will explain the science behind mBiCRS, the potential effectiveness of the approach, and potential ocean ecosystem risks.

The science of mBiCRS

Globally, land vegetation, including trees and crops, sequester approximately 60 billion tons of carbon per year. However, this carbon is continuously released back into the atmosphere when the vegetation dies and decomposes. In recent years, a number of proposals have been made to lock up this source of carbon dioxide by burying plant biomass at the bottom of the ocean. In so doing, the method would seek to enhance the “natural ‘carbon pump’” whereby land plants take up carbon dioxide via photosynthesis, with a fraction of the resulting non-respired biomass transported via rivers to oceans, and subsequent sequestration of carbon dioxide in sediments.

Ocean biomass burial seeks to “separate ‘carbon removal’ via photosynthesis from ‘carbon storage.’” For example, one study concluded that bundling up 30% of global crop residues and sinking them to ocean bottoms could reduce atmospheric concentrations of carbon dioxide by 15%. The process could also ensure sequestration on millennial time scales. 

A number of start-up companies, embracing different approaches, have begun to explore the potential of ocean biomass burial. Rewind, an Israeli company, has proposed to collect agricultural and forest residues and orchard trimmings to transport this matter into deep portions of the Black Sea. The company contends that the “special chemistry” of the Black Sea, “no oxygen and high sulfide,” would ensure the long-term preservation of biomass, and “effectively remov[e] CO2 for thousands of years”. The company asserts that in anoxic environments, such as those that characterize the deep waters of the Black Sea, the decomposition rates of organic matter “becomes thermodynamically limited and less efficient”. 

This proposition is supported by the fact that microbes that decompose biomass into carbon dioxide are inhibited in environments devoid of oxygen.  Rewind cites the preservation of a number of shipwrecks at the bottom of the Black Sea, some dating back to 450 A.D., as empirical evidence to validate its approach. The company contemplates availing itself of millions of tons of biomass residue from the six countries that surround the Black Sea to reach a gigaton scale of carbon removal by 2030. 

Another Israeli company, BlueGreen Water Technologies, takes a very different approach. Instead of collecting biomass from terrestrial sources, it administers a solution of hydrogen peroxide to kill, and ultimately sink, toxic harmful algal blooms made up of cyanobacteria, also known as blue-green algae. This approach may also enhance ecosystems by eliminating blooms that can devastate aquatic environments by creating low-oxygen dead zones. And because algae sequester substantial amounts of carbon, the company claims that this approach could concomitantly remove billions of carbon from the atmosphere in both ocean and freshwater ecosystems by burying the blooms in deep-ocean sediments. 

The potential effectiveness of mBiCRS

One of the most challenging aspects of ocean biomass burial, as is a consistent theme with marine approaches more generally, is how to accurately quantify sequestration, with companies in the field seeking certification by carbon accreditation organizations. Puro.earth, a carbon crediting agency focused on carbon removal methods, launched a working group in late 2023 to develop a methodology that could eventually facilitate the sale of certified carbon credits in voluntary and/or compliance markets. 

Also, serious questions abound as to whether ocean burial of terrestrial biomass is the optimal use of such materials from the perspective of climate mitigation. Ultimately, it may make more sense to utilize scrap biomass for incorporation into sustainable building materials or biochar, a process that thermodynamically converts biomass in a low-oxygen environment to lock away carbon dioxide in a charcoal-like substance.

Another important consideration is whether biomass burial could trigger releases of substantial amounts of methane, a gas that has a much more potent global warming potential than carbon dioxide over the course of a few decades. Some experts have expressed concern that microbes called methanogens might consume organic matter in anoxic environments, releasing large amounts of methane that could ultimately make it to the atmosphere. This could obviate the climate benefits of this approach.

Proponents of biomass burial in the Black Sea contend that the absence of vertical mixing will preclude the escape of methane into the atmosphere, but additional research is required in this context. BlueGreen Water Technologies’ accreditation entity, Social Carbon, has concluded that methane emissions associated with the approach would be negligible, at least for some water bodies. However, some researchers contend that additional research is necessary in this context.

Finally, terrestrial biomass sinking only makes sense from a carbon emissions lifecycle assessment perspective where supplies of waste are in reasonable proximity to ocean bodies where this waste might be sunk. Otherwise, the greenhouse gas emissions associated with the transport of the biomass might negate potential climate benefits. Given this constraint, biomass burial might only be capable of sequestering a few tens of billions of tons of carbon dioxide, or a few years of emissions. 

The potential risks of mBiCRS

Some concerns have been expressed that removing crop residues from agricultural fields for the purpose of ocean burial might rob soils of nutrients that could ultimately reduce crop yields. During its ocean descent, terrestrial biomass could release particulate or dissolved organic matter. This could alter the production of microbes, increase oxygen consumption in the water column, and produce additional sources of food that could alter the structure of intermediate ocean depths. Moreover, buried biomass would create large new sources of food that could attract a host of decomposing organisms and predators on the seabed, also potentially impacting ocean ecosystems, including commercial fisheries. Deposit of large amounts of terrestrial biomass can also potentially alter benthic ocean sediment communities. However, potential impacts, and methods to ameliorate such threats, require additional research.

Conclusion

As is true with all marine-based carbon dioxide removal methods, mBiCRS is in an early stage of research, with many abiding questions about potential effectiveness and potential negative impacts. Rewind has recently announced that it has raised $5 million in seed funding, which will help it support an upcoming pilot project 400 times larger than previous experiments. This should make a substantial contribution to the research field. Moreover, another company focused on mBiCRS, Carboniferous, is partnering with Isometric, a carbon removal standard and registry. Isometric is recognized by many as the most stringent registry in the world in terms of the development of protocols for measuring, reporting and verifying carbon removal claims. The next few years should tell us a lot about the potential of mBiCRS as part of a broader portfolio of carbon removal approaches. 

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