Negative Emission Technologies

Removing carbon dioxide from the atmosphere is a vital response to climate change.

Carbon dioxide emissions caused by humans have accelerated during the 20th century, with global atmospheric concentration reaching 411 ppm in February 2019—by far the highest level in the past 400,000 years. According to the Intergovernmental Panel on Climate Change (IPCC), global warming is still predicted to surpass the 2° C target of the 2015 United Nations Climate Change Conference (COP21), which will have dramatic consequences for ecosystems and societies.

Since the beginning of the industrial era, consumption of fossil fuels has not only amplified the amount of carbon circulating, but also generated imbalance fluxes, leading to a 30 percent increase of the atmospheric carbon stock between 1750 and 2011. Anthropogenic CO2 emissions of about 40 GtCO2 per year contribute to the global carbon cycle. Fossil fuels consumption emits about 35 GtCO2 per year, and land use emits about 5 GtCO2 per year.

Because carbon cycles involve complex exchange mechanisms and various natural reservoirs, understanding them is fundamental to predicting the evolution of the atmospheric concentration of CO2 (see figure 1):

  • Oceans store about 87 percent of the Earth’s carbon. Carbon in the oceans is unevenly distributed and mostly stored in the surface oceans, with a residence time of one to 10 years, and in the intermediate and deep sea, with a residence time of 200 to 1,000 years. Thus, the ocean’s capacity to absorb CO2 depends on its ability to transport carbon in the surface waters to the intermediate and deep sea.
  • Land stores about 9 percent of the Earth’s carbon, predominantly in tropical and boreal regions. Despite soils and permafrost storing an equivalent of about 13,000 GtCO2, the 2,000 GtCO2 in vegetation are responsible for the biggest fluxes (about 451.4 GtCO2 per year). Although models suggest the land carbon uptake has been growing 20 percent per decade, these models are uncertain, and climate change is expected to diminish the land’s ability to store carbon.
  • The atmosphere holds about 2 percent of the Earth’s carbon. However, it plays a central role by interconnecting all other reservoirs.

With the continuous rise of atmospheric carbon of about 19 GtCO2 per year, land and oceanic uptakes do not counterbalance the anthropogenic CO2 emissions. Since CO2 represents about 75 percent of yearly anthropogenic greenhouse gas emissions, it is the focus of solutions policies (see figure 2). Under the current emission rate of 40 GtCO2 per year, the carbon budget of 420 Gt could be exhausted in about 10 years. Without negative emission technologies (NETs), limiting warming to 1.5° C—the “safe” upper limit for global warming—would require extreme and immediate measures. Carbon-removing NETs, which could capture 3.1 to 14.9 GtCO2 per year on average until 2100, would allow for a more phased transition.

A variety of technologies are being developed to either remove CO2 from the atmosphere (NETs) or develop neutral energy technologies (zero- or low-emission technologies).

Atmospheric CO2 management solutions consist of NETs (their use contribute to reducing the quantity of CO2 into the atmosphere) and zero- or low-emission technologies (their use have neutral or limited impact on the CO2 quantity in the atmosphere). NETs can be further classified as either natural processes enhancers or engineered processes (see figure 3).

Forest development results in positive net uptake of CO2 resulting from the photosynthesis (++) and respiration (–) of plants. This process could be enhanced by afforestation and reforestation, which could remove an additional 0.5 to 3.6 GtCO2 per year. Although they have a relatively low cost of $5 to $50 per tCO2, afforestation and reforestation are limited by land and water use in competition with food supply and bioenergy.

Oceans capture carbon in several ways. First, storing biomass, such as crop waste, underwater can be a cheap and effective way to prevent reemission from burning or decomposition. Another solution, boosting phytoplankton activity by adding nutrients in oceans, offers massive potential: from 2.6 to 6.2 GtCO2 per year at a limited cost of between $23 and $111 per tCO2. However, this process, called ocean fertilization, carries potential risks for ecosystems and ocean property and remains poorly studied and therefore very speculative. Third, artificial upwelling could offer another possibility by pumping nutrient-rich water from the subsurface ocean to the surface, but this remains theoretical and risky because of a lack of knowledge. Finally, ocean alkalinity enhancement consists of adding alkaline materials to sea water to fight ocean acidification. The technologies involving oceans have great potential, but all of them are in incipient stages.

Soils also offer possibilities to capture more carbon. Weathering is the natural process of rock decomposition via chemical and physical processes in which CO2 is spontaneously consumed. This process can be enhanced by augmenting the surface area of the rock exposed, and the most pragmatic approach is spreading fine-grained rock dust over croplands, which also has co-benefits for agriculture. However, this comes at a relatively high cost because of the required intense mining. Another solution is to increase soil carbon sequestration: several soil management practices could have a 20-year potential from 2.3 to 5 GtCO2 per year. Because of agricultural benefits such as reducing erosion and improving soil fertility, costs could even be negative, ranging from –$45 to $100 per tCO2.

Engineered NETs could help meet the IPCC target, but they still have to prove their scalability.

Bioenergy is usually considered carbon neutral, but it can be carbon negative when coupled with biochar stored in the soil or with carbon capture and storage (CCS). Although biochar allows capturing carbon and producing energy at the same time, it is constrained by cost penalties because of pyrolysis efficiency and avoided energy production with charcoal burning. This could be partly balanced by the biochar fertilizer effect. CCS used in the framework of bioenergy is already operable and has a significant potential of 0.5 to 5 GtCO2 per year. Although the technology is mature and promising, cost penalties are curbing its progress.

Direct air capture (DAC) technologies capture CO2 from ambient air and then store it or use it (see figure 4). DAC avoids or at least reduces transport fees because it can be built on the CO2 utilization or storage site. However, the low concentration of CO2 in ambient air makes it less efficient and costlier than CCS. Few companies have launched pilot plants or small commercial plants, which already capture up to 1 MtCO2 per year. DAC is very efficient in terms of land use—about 100 to 400 times more than forests. Despite the cost, DAC has massive potential and could have a crucial impact on carbon concentration with more advanced technologies. In fact, DAC is theoretically only constrained by geological storage capacity.

With large afforestation and reforestation projects having been deployed in the past for building ships, houses, and other wood products, this is by far the most mature technology. If bioenergy with CCS comes next, all the remaining technologies are still in preliminary stages. The ocean-based solutions are mostly in the lab stage, while the direct air capture technologies are in various stages between development and demonstration. Considering the relatively low maturity of NETs, their global potential remains speculative, and several estimates can be found (see figures 5 and 6).

Some NETs could offer huge new business opportunities, but they would require strong policy support and technology improvements.

In the COP21, signatories of the Paris Agreement agreed to follow a set of National Determined Contributions (NDCs). Although countries’ NDCs refer to net emission reductions, only afforestation and reforestation are mentioned so far—and in a way that is difficult to quantify globally. In 2017, the United Nations set global forestry goals that could capture between 1.6 and 3.8 GtCO2 per year. These values are not only speculative, but also subject to reversal, and even if they become true, they would only partially meet IPCC’s pathway P1 necessary carbon capture values (see figure 7). Multibillion-dollar reforestation and afforestation initiatives in Africa, South America, China, and India will be responsible for capturing between 1.74 and 2.6 GtCO2 per year, yet there are many challenges to the efficacy and transparency of all these programs.

Investments in other NETs have been minor because most projects lack a solid business plan. Investors are mainly the public sector, philanthropists, and oil and gas companies. However, they assert that current investments will not be enough to meet the IPCC target and that these businesses require government policy encouragement to take off. Many local, national, and international initiatives about carbon pricing could provide a solution. However, some do not consider NETs, and even if included in the trading schemes, carbon prices are generally too low to justify the commercial use of NETs.

Alternately, manufacturing valuable products by using carbon as feedstock could generate a new market. The New Carbon Economy Consortium estimates this market opportunity at $6,000 billion: $4,000 billion for zero- or low-emission solutions, mainly by making synthetic fuels, and $2,000 billion for negative emission solutions, principally by making building materials and plastics.
The development of NETs will require significant government support. Policies should target the following:

  • Support research, development, and demonstration to reduce uncertainties as well as move technologies such as ocean fertilization further along the maturity curve.
  • Help finance the development of short-term opportunities (such as pilots) to build experience and identify potential issues.
  • Integrate NETs in carbon emissions accounting to provide a strong signal to investors.
  • Build system flexibility to enable the rapid development of technologies and avoid the lock-out as they emerge.

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