Guest Editorial: Is CO2 Removal Technology Key to a Sustainable Future?
Authors: Misra Siddarth , Puri Sateesh Kumar 
Published: July 2023

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The urgent need to address climate change has unleashed a wave of innovative ideas on how to mitigate the greenhouse gas (GHG) emissions that are driving the problem. Among the approaches considered to hold great promise is carbon dioxide removal, or what some call negative emissions technologies.
These strategies have attracted significant attention in recent years and involve removing carbon dioxide (CO2) from the atmosphere and storing it in long-term sinks such as oceans, forests, and underground geological formations.
What follows is a closer look at how these strategies work, their potential to address the global climate crisis, and the challenges they currently face.

Direct Air Capture (DAC)
This technology captures CO2 directly from the air using chemical reactions and/or physical processes. The captured CO2 can then be stored in a long-term sink, used for food processing and cement manufacturing, or combined with hydrogen to create synthetic fuel. There are currently about 20 direct air capture plants operating worldwide.
Climeworks’ Orca plant in Iceland, the largest operating DAC plant in the world, has the capacity to remove 4,000 tons of CO2 per year and store it deep underground. The majority of commercial direct air carbon capture techniques utilize large fans to force ambient air through a filter, with a liquid solvent, often amine-based or caustic, being used to absorb CO2 from the gas.
There are three chemical processes that are being studied extensively for purposes of DAC: causticization using alkali and alkali-earth hydroxides, carbonation, and organic-inorganic hybrid sorbents consisting of amines supported in porous adsorbents. Additionally, others are exploring membrane separation methods based on semipermeable membranes to capture CO2, which require less water and a smaller footprint.
Perhaps the biggest challenge facing this technology’s ability to scale up lies in its ability to be economically successful given current carbon credit pricing regimes balanced with the unknown amount of energy DAC units will need to achieve meaningful capture volumes.

Ocean Fertilization
The aim of ocean fertilization (OF) strategies is to enhance certain biological processes in the oceans that will lead to increased uptake of atmospheric CO2 followed by permanent storage in deep oceanic layers.
The technique involves the addition of nutrients such as iron, phosphorus, and nitrogen to the ocean for stimulating the growth of phytoplankton that absorb CO2 from the atmosphere. When the marine species die, they sink to the ocean floor, taking the absorbed CO2 with them, where it is stored permanently.
However, the implementation of OF has not been thoroughly studied and there are limited long-term experiments. Additionally, certain OF storage approaches are subject to reversibility. In other words, the stored carbon may not remain sequestered permanently and could be released back into the atmosphere due to the constant changes in ocean conditions such as temperature or nutrient availability.
This issue highlights the need for caution and thorough study before implementing OF as a carbon dioxide removal strategy. The Convention on Biological Diversity and the London Protocol have called for appropriate regulations and assessments of risks and impacts related to climate-related geoengineering and OF.