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The science we need to assess marine carbon dioxide removal

Various strategies for removing carbon dioxide from seawater and sequestering it for long periods of time have been proposed, each with its own considerations, complications, and potential durabilities. INFOGRAPHIC/Mary Heinrichs/AGU


As companies begin selling credits for marine carbon dioxide removal in largely unregulated marketplaces, scientists now say new standards must be developed for assessing the effectiveness of removal methods.

Several carbon removal methods are now being deployed on land, such as producing energy with biological material and capturing and storing the CO2 produced (i.e., bioenergy with carbon capture and storage), planting more trees, or drawing massive amounts of air through absorbent filters.

Scientists from AGU-Advancing Earth and Space Science say because the ocean covers 71% of Earth’s surface and, apart from the atmosphere, already serves as the largest net sink for anthropogenic CO2  emissions, various ocean-based, or marine, CO2 removal (mCDR) strategies are being proposed must meet the required standards and be transparent.

“The window to limit global warming to Paris Agreement targets by reducing greenhouse gas emissions alone is rapidly closing. According to the Intergovernmental Panel on Climate Change, to have a 50% chance of keeping warming below 1.5°C, the whole of society will need to limit all future carbon dioxide (CO2) emissions to less than a few hundred billion tons,” the scientists note.

The scientists who include Jaime B. PalterJessica CrossMatthew C. LongPatrick A. Rafter and Clare E. Reimers add that with global emissions of CO2 in 2021 totaling 36 billion tons, this limit implies a need to halve emissions within a decade, on our way to eliminating nearly all emissions by midcentury.

This is a formidable challenge that becomes harder to overcome with each passing month. Therefore, many scientists, policymakers, entrepreneurs, and others have begun grappling with the reality that staving off intergenerational harms of climate change—from increasingly intense heat waves to more severe droughts and floods to rising risks from wildfire and tropical cyclones—will also require removing legacy CO2 from the atmosphere.

Three categories of mCDR approaches—ocean iron fertilizationartificial upwelling, and seaweed cultivation—aim to stimulate primary productivity at the ocean’s surface with the expectation that some of the additional biomass produced will sink into and remain in the deep ocean.

They add in contrast, ocean alkalinity enhancement (OAE) involves intentionally dispersing alkaline materials such as lime on the ocean’s surface to shift the chemical equilibrium of the seawater carbon system and thereby increase uptake of atmospheric CO2. Still another approach proposes to remove CO2 directly from seawater through electrochemical reactions and then store it underground.

“Commercial, philanthropic, and government resources are increasingly being directed toward the development of these mCDR strategies, with several pilot deployments planned. But how effective might these pathways be at increasing ocean carbon uptake?” they ask.

Key to any carbon dioxide removal (CDR) method is that it must durably store billions of tons of CO2 in places where it cannot easily return to the atmosphere. Quantifying the effectiveness and durability of these processes in the ocean requires robust science for monitoring, reporting, and verification.

For a detailed analysis of this issue, read here.


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