Carbon Dioxide Sequestration
To prevent global warming to surpass 1.5°C we as a species need to reach net-zero emissions. Reaching and sustaining net zero global anthropogenic CO2 emissions and declining net non-CO2 radiative forcing would halt anthropogenic global warming on multi-decadal times scales.
During the last decade we have focussed our efforts on strategies to reduce emissions, however, to remove and store more carbon dioxide from the air than we emit we require net-zero strategies that actively remove CO2 from the atmosphere.
Both natural and technological strategies exist to remove carbon dioxide from the atmosphere and store it through various means, such as in trees and plants, soils, underground reservoirs, rocks, the ocean and even through products like concrete.
Ocean's Carbon Removal Potential
Proposed methods for increasing the ocean’s ability to remove and store carbon dioxide — including biological, chemical and electrochemical concepts — vary in technical maturity, permanence, public acceptance and risk.
1. Biological Approaches
Biological approaches, which leverage the power of photosynthesis to capture CO2, offer a few approaches for carbon removal.
Ecosystem Restoration: Restoring coastal blue carbon ecosystems, including salt marshes, mangroves and seagrasses, can increase the amount of carbon stored in coastal sediments. Globally, the carbon removal potential of coastal blue carbon ecosystem restoration is around a few hundred million tons of CO2 per year by 2050, which is relatively small compared to the need. However, ample co-benefits — like reducing coastal erosion and flooding, improving water quality and supportinglivelihoods and tourism — make it worth pursuing.
Large-scale Seaweed Cultivation: another proposed approach is large-scale seaweed cultivation, as seaweed captures carbon through photosynthesis. While there is evidence that wild seaweed already contributes to carbon removal, there is potential to cultivate and harvest seaweed for use in a range of products, including food (human and animal), fuel and fertilizer.
The full extent of carbon removal potential from these applications is uncertain, since many of these products would return carbon within the seaweed to the environment during consumption. Yet, these applications could lower emission intensity compared to conventional production processes.
2. Chemical Approaches
Chemical approaches, namely alkalinity enhancement, involve adding different types of minerals to the ocean to react with dissolved carbon dioxide and turn it into dissolved bicarbonates. As dissolved carbon dioxide converts into dissolved bicarbonates, the concentration of dissolved CO2 lowers relative to the air, allowing the ocean to absorb more CO2 from the air at the ocean-air boundary.
3. Electrochemical Approaches
There are a handful of electrochemical concepts that also store carbon as dissolved bicarbonate. Unlike chemical approaches, electrochemical approaches do so by running electric currents through seawater.
Variations of electrochemical approaches could also produce valuable hydrogen or concentrated CO2 for industrial use or storage. Scaling up this approach would depend on the availability of low-carbon energy sources in suitable locations. Additional research will help map such sources and analyze potential benefits, such as hydrogen production.
- Special Report: Global Warming of 1.5 ºC (IPCC)