By Srivathsan Karanai
Climate Engineering: Carbon Dioxide Removal
“Not only is the Universe stranger than we think, it is stranger than we can think.”
The continuing emission of anthropogenic greenhouse gases (GHGs) in massive quantities since the industrial era is the primary reason for the increasing global average surface temperature. In 2015, the Paris Agreement—a legally binding international treaty—was adopted to limit the increase in the global average temperature to below 2°C above pre-industrial levels and pursue efforts to limit the increase to 1.5°C. To meet this target, GHG emissions must reduce globally by 45% by 2030 and reach net zero by 2050. Fossil fuels drive our economies, and it is an extremely demanding requirement to move away from them completely when the available alternatives are not yet mature.
The most practical approach is to look for solutions to remove GHGs—especially carbon dioxide (CO2), which overwhelmingly dominates anthropogenic emissions—while we continue to pursue the path of reducing GHG emissions. This article discusses carbon dioxide removal (CDR), an important climate engineering technique focused on not only achieving net-zero emission targets but also ushering in negative emissions by removing excess CO2 from the atmosphere.
On the brink
GHGs include gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and water vapor (H2O). Except for water vapor, anthropogenic activities since the industrial era began in the 1850s have directly caused a massive increase in GHGs. Water vapor is mostly produced by the evaporation of water bodies. The increased presence of GHGs warms the surface of the earth, which further increases the process of evaporation, thus triggering a water-vapor feedback cycle. It is the nature of GHGs to allow the passage of short-wave radiation (sunlight) but absorb long-wave radiation (heat) and radiate it back to the surface of the earth. This property, called the greenhouse effect, maintains the average surface temperature on the earth at around 15°C, which would otherwise have been unlivable at a freezing –18°C.
Of all the GHGs, CO2, CH4, and N2O contribute to over 98% of the emissions, and of this, nearly 75% is from CO2 alone. Not only does the emission quantity of the GHGs vary, but so does their average lifetime in the atmosphere, their global warming potential (GWP), and their relative change since 1850 (see Table 1). The emissions are mentioned in carbon dioxide equivalent (CO2-eq) terms, which is a metric measure used to compare the emissions from various greenhouse gases based on their GWP by converting amounts of other gases to the equivalent amount of carbon dioxide with the same GWP.
The global mean surface temperature is currently about 1.1°C higher than what was experienced during the pre-industrial era of 1850–1900. The way in which climate change is unfolding proves that events have a non-linear relationship with an increase in temperature. The extensive use of fossil fuels (coal, oil, and gas) as sources of energy is the prime reason for CO2 being the major GHG emitted. Statistics indicate that since 1850, over 2,500 billion metric tons of CO2 (GtCO₂) have been emitted into the atmosphere from fossil fuel combustion and land-use change, which is the transformation of natural landscapes due to human activities. While the other major GHGs trap more heat than CO2, they have less anthropogenic origins, a shorter lifetime in the atmosphere, and a comparatively low presence in the atmosphere.
The earth has so far experienced five mass extinctions since the origin of life. These extinctions were periods during which a major percentage of life in the oceans and on land vanished completely. It is alarming to note that except for the one that killed the dinosaurs, all the other four involved climate change. The Great Dying, which happened around 250 million years ago, is said to be strikingly similar to what we are going through now, as it was also caused by the enormous presence of CO2 in the atmosphere. The only difference was that during the Great Dying, the surge in CO2 was due to a natural factor—a giant volcanic eruption in today’s Siberia.
To avert a climate crisis, the Paris Agreement has set targets for reducing GHG emissions. However, there are apprehensions about whether we will be able to fix global warming and climate change just by adhering to mitigation plans and decreasing emissions faster. It is feared that even if we could achieve net zero immediately, the CO2 that was dumped earlier in the atmosphere would ensure that the warming continues for several centuries into the future. The despair has prompted the climate response to start considering climate engineering, especially CDR, to remove GHGs from the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) reports elaborate on the ways to capture, transport, and store CO2. They also detail the costs, economic potential, and societal issues of CDR, including public perception and regulatory aspects. The early promise of CDR is that it could help achieve net zero by potentially removing residual CO2. As the technology matures and sees widespread deployment, more CO2 could be removed, resulting in a net negative emission.
Carbon dioxide removal
CDR, also known as negative emissions technology (NET), refers to several methods, practices, and approaches that remove and sequester CO2 from the atmosphere and oceans, and durably store it. CDR approaches could be either nature-based, technology-based, or hybrid. Some of the most popular approaches are described below (see Figure 1).
1. Nature-based solutions
Nature has its own ways of removing CO2 from the atmosphere in massive quantities. Nature-based solutions entail enriching the carbon cycle to make nature absorb more CO2 than it is doing now.
- Forestation. It is well known that trees absorb CO2 from the atmosphere through photosynthesis and sequester it as biomass. Forestation is to plant trees on land that is currently unforested. It comprises reforestation of land that contained forests earlier and afforestation of land that never had a forest before. Trees transfer the biomass containing the absorbed CO2 to the soil through litter fall and dead roots. The limitation of forestation is that it requires enormous land investment, and an intense collective will to implement and manage. It will also take a long time to demonstrate a measurable difference and generate commercial benefits.
- 1.2 Soil carbon sequestration. Soil carbon sequestration is the effective management of land to enhance land carbon sinks. The goal is to deploy practices that either increase carbon input or reduce carbon loss from the soil. The former is achieved through cover crops, crop rotation, and by adding organic matter in the form of manure, residues, or compost. The latter is achieved by switching from annual to perennial crops or by eliminating tillage. The limitation of this approach is that the potential is uncertain, and the effectiveness varies based on soil type and method adopted.
- 1.3 Wetland restoration. Wetlands are natural carbon sinks. The absence of oxygen in the soil reduces decomposition rates and stores a significant amount of CO2 in the soil, thus preventing it from entering the atmosphere. The conservation and restoration of coastal wetlands such as mangroves, seagrass meadows, salt marshes, and freshwater peatlands could sequester more carbon much faster than any other natural method. The challenge with wetlands is that they are themselves sources of CH4.
- 1.4 Ocean fertilization. By absorbing about one-third of CO2 emissions and dissolving them to produce carbonic acid, oceans are the largest natural reservoirs of CO2. However, this process is slowing down as oceans are reaching their maximum absorption limit and are unable to absorb as much CO2 as they did earlier. Ocean fertilization aims to enhance this natural phenomenon by adding micronutrients such as iron, nitrate, phosphate, and urea into high seas to foster algal growth. The algae phytoplankton absorbs CO2 for photosynthesis and, after death, sinks the carbon to the depths of the ocean, where it will not re-enter the atmosphere as a gas for centuries. The limitation of this approach is that the small-scale experiments conducted so far have not been as effective as theorized. Even when the approach turns productive, it is thought that CO2 absorption will be only a meager fraction of the emissions. This approach can result in the overgrowth of phytoplankton that has the potential to deplete the oxygen levels of the sea water, resulting in dead zones that will have catastrophic consequences for marine life.
2. Technology-based solutions
Technology-based solutions envisage active and direct intervention rather than following passive and indirect approaches of nature-based solutions. Carbon capture and storage (CCS) is a technology-based approach that is leveraged in many of the technology-based and hybrid CDR solutions. CCS focuses on capturing a pure stream of CO2 directly from industrial sources and fossil-fuel plants where it is generated and transporting it to a storage location for long-term storage. The variant of this is carbon capture utilization and storage (CCUS), where the captured CO2 is used in other products or services including enhanced oil recovery. Despite being widely accepted and deployed for several decades now, CCS or CCUS are considered mitigation strategies and not climate engineering, as the focus is to reduce the addition of CO2 emissions and not directly intervene with the GHG concentrations in the atmosphere. One major risk attributed to CCS is the possibility to create induced seismicity by inducing stresses or increasing pore pressure in areas that are already seismically active. Two approaches—direct air capture (DAC) and bioenergy with carbon capture and storage (BECCS)—that leverage CCS are extensively cited as most promising for carbon removal.
- 2.1 Direct air capture. While CCS focuses on capturing CO2 from industrial sources, DAC is a method that tries to remove it directly from anywhere. DAC removes CO2 from the ambient air with a liquid solvent or solid sorbent that binds CO2 and separates it from other gases in the air. The filtered CO2 is then compressed, transported, and injected into geological formations deep underground or used to make products for long-term storage. Climate scientists strongly believe that DAC is a crucial part of getting to net zero, as it has no upper limit to its technical potential to remove CO2 on a planetary level and thereby reverse climate change.
The advantage of DAC is that building a system requires much less land than nature-based solutions and could be built anywhere. This means that a DAC system could be constructed even at low-carbon energy sources that are near identified CO2 storage sites or locations where the CO2 can be used. The limitation with DAC is that it is expensive, unproven, and requires large energy inputs. This could necessitate the diversion of renewable resources that are meant to replace fossil fuels. On the other hand, the process of DAC could even cause related GHG emissions. Currently, several pilot experiments with DAC and small-scale commercial implementations are underway. Scaling them up to achieve a significant positive impact on the planet will require further research to bring down costs and reduce the energy requirement.
- 2.2 Bioenergy with carbon capture and storage. BECCS is a carbon-removal technique that involves two stages. The first stage is bioenergy, in which biomass is burned in an industrial facility to convert it to heat and power or to liquid fuels such as ethanol, methanol, or biogas. The second stage is CCS, in which the CO2 emitted during combustion is captured and stored, thus effectively removing it from the carbon cycle. Even though the IPCC considers BECCS to be one of the most promising CDR solutions, the technology is still in its infancy and has seen very few large-scale commercial deployments. The availability of land and sustainable biomass feedstocks are major limitations for deploying BECCS.
3. Hybrid solutions
Hybrid solutions leverage the use of certain technologies to accelerate the absorption of CO2 by natural carbon sinks.
- 3.1 Biochar. Biochar is created by heating biomass, such as agriculture residues, to above 250°C under low-oxygen conditions in a closed container, a process called pyrolysis. This biochar is then added to the soil to enhance soil carbon levels and improve soil quality and crop yields. It consists of carbon black, which decomposes very slowly under natural conditions, rendering biochar a more durable carbon storage form than the original biomass. Biochar has the potential to sequester CO2 for centuries under the right conditions. The limitation with biochar is that it has not been tried on a large scale, and there are uncertainties regarding its adverse side effects.
- 3.2 Enhanced weathering. Weathering, also called carbon mineralization, is a natural process through which rocks and minerals break down or dissolve when they interact with atmospheric CO2. Enhanced weathering aims to artificially accelerate the natural weathering process, which is known to be slow, to remove large amounts of CO2 from the atmosphere faster. The approach for enhanced weathering is to increase the reactive surface area by spreading powdered silicate or carbonate minerals over land, coastal areas, or ocean waters. The challenges are to find appropriate rocks and minerals in large quantities and cost- and energy-effective ways to bring CO2 into contact with reactive minerals at scale, without adversely impacting land and water.
- 3.3 Ocean alkalinization. Oceans are natural carbon sinks. With the increased CO2 in the atmosphere, the absorbed amount in the oceans is also increasing, producing more carbonic acid. This decreases the potential hydrogen (pH) value of the ocean water, turning it acidic. Increased acidity could dissolve the hard shells of many ocean animals, like corals, and kill them. Ocean alkalinization involves adding limestones, silicates, and/or calcium hydroxides to ocean water to increase its alkalinity. This would also potentially increase the ability of the oceans to absorb more CO2 without affecting their acidity. However, the major challenge with this approach is to mine, grind, and transport enough alkaline material, which would require massive infrastructure and efficient supply chains. Besides, excessive ocean alkalinization could increase the levels of toxic metals and other minerals or alter the biodiversity of oceans.
Climate scientists and decision-makers agree that CDR is a crucial technique for removing residual CO2 and achieving net zero emissions. Climate models increasingly include various CDR techniques to simulate different pathways to climate change. However, CDR methods are still in the early stages of research, development, or deployment. The true mitigation potential of CDR, its adverse effects, and governance requirements are not yet clear. To see a reversal in climate change on a planetary scale, it requires cooperation from all the major carbon emitters to deploy massive CDR systems across the world to cause a net reduction in the total amount of CO2 in the atmosphere.
Existing CDR methods require heavy investment over the long-term horizon. Research is currently focused on inventing CDR methods that are cheap, need minimal land and energy, have fewer adverse impacts, and could be scaled up faster. Considering the mismatch between aggressive climate change and an uninspiring response, global climate desperation is set to make CDR a critical necessity despite its shortcomings—and very soon. However, even after massive global adoption is achieved, climate change reversal could be minimal or take prolonged time. While global surface temperatures may start to reverse within a few years, permafrost thawing could take decades, and the acidification of deep oceans could take centuries.
The next article in this series will discuss solar radiation management.
SRIVATHSAN KARANAI MARGAN works as an insurance domain consultant at Tata Consultancy Services Limited.
Bjørn Lomborg. (2010). Smart Solutions to Climate Change: Comparing Costs and Benefits. Cambridge University Press.
Goodell, J. (2011). How to Cool the Planet: Geoengineering and the audacious quest to fix Earth’s Climate. Mariner.
Hawken, P. (2017). Drawdown: The most comprehensive plan ever proposed to reverse global warming. Penguin Books.
Horton, J. B. (2011). “Geoengineering and the Myth of Unilateralism.” Climate Change Geoengineering, 168–181.
IPCC. (2022). Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.
Launder, B. E., & Thompson, M. T. (2010). Geo-engineering Climate Change: Environmental Necessity or Pandora’s box? Cambridge University Press.
Morton, O. (2016). The Planet Remade: How Geoengineering Could Change the World. Princeton University Press.
Royal Society. (2009). Geoengineering the Climate: Science, Governance and Uncertainty. The Royal Society.
Ritchie, H., Roser, M., & Rosado, P. (2020). “Greenhouse gas emissions.” Our World in Data.
Sapinski, J. P., Holly Jean Buck, & Malm, A. (2020). Has It Come to This? Rutgers University Press.
Watts, R. G. (2013). Engineering Response to Climate Change. CRC Press, Taylor & Francis.