Reforestation in southern Oregon, U.S.A. Source:

Negative emission technologies (NETs), also known as carbon dioxide removal, refer to processes by which carbon emissions are sequestered to lower emissions in an effort to a) prevent additional accumulation of greenhouse gases in the atmosphere, and/or b) remove greenhouse gases that have already accumulated in the atmosphere. In practice, carbon emissions would be sequestered at higher rates than they’re produced, lowering the overall greenhouse gas concentration in the atmosphere (i.e. negative emissions). 

CO2 makes up less than 1% of the atmosphere by volume. Avoiding putting more molecules of CO2 into the air from the global energy system is simpler than the effort to capture that molecule back out and keep it out permanently, or, sequester it.  Therefore, the Intergovernmental Panel on Climate Change (IPCC), the National Academies of Sciences, and many peer-reviewed publications stress the importance of peaking global emissions as soon as possible followed by rapid deployment of clean energy technologies to displace fossil fuels, as opposed to relying on NETs to lower atmospheric emissions. Delays increase the need for NET approaches in order to stay below desired global temperature thresholds. 

Negative emissions can be achieved biologically by increasing natural carbon sinks or through chemical engineering processes. All NET approaches have their own set of impacts, costs and limitations (such as the permanence of the carbon stored) (Fuss 2018). Negative emissions approaches include (EASAC, 2018):

A taxonomy of negative emissions technologies (NETs). NETs are distinguished by carbon capture approach, earth system and storage medium. Major implementation options are distinguished for each NET. Source: Minx et al. 2018.

The scale of NETs needed is dictated by estimates of the remaining “carbon budget.” In this context the remaining carbon budget refers to the cumulative amount of allowable CO2 emissions associated with a reasonable chance (66%) of staying below a certain temperature limit. Analyses suggest that to meet the 2°C goal of the Paris agreement, there is a remaining carbon budget of 810 GtCO2. (Realmonte, 2019). To achieve the 1.5ºC aspiration, by the same analysis, the budget shrinks to only 220 GtCO2. For comparison, annual greenhouse gas emissions have now reached approximately 50 GtCO2e per year.

While some argue that the need for NETs can be reduced significantly by aggressive deployment of efficiency, clean energy, lifestyle changes, and adequate incentives such as a price on carbon (e.g. van Vuuren 2018), the need for negative emissions is not entirely eliminated in most scenarios.

The IPCC 1.5°C report provided four scenarios (P1-P4) with increasing roles for negative emissions. All include carbon removal in the form of Agriculture, Forestry, and Other Land Use (AFOLU) scenario P1, and P2-4 also consider varying levels of Bioenergy with Carbon Capture and Sequestration (BECCS). The amount of CO2  removal to achieve the 1.5°C goal is greatly affected by how late fossil emissions peak and how steep they decline. The amount of carbon capture in P4 is about 20 GtCO2 per year in 2100 – a massive and costly undertaking.  (IPCC 2018).
Women with seedlings for reforestation in Tanzania. Image copyrights: USAID Africa Bureau – Uploaded by Elitre, Public Domain,

Afforestation and Reforestation

Deforestation is abundant around the world, especially in rich tropical forests such as the Amazon. Massive deforestation is carried out in pursuit of industrialization and development goals. Given this reality and the urgency of climate change, large-scale afforestation, reforestation, and proforestation are among critical negative emissions strategies. Afforestation refers to the planting of trees in areas that forests have not existed in recent history (they may have been previously converted for agriculture, mining, or other purposes). Reforestation refers to the replanting of trees in recently cleared forested lands. And proforestation refers to the practice of intentionally letting middle-aged and mature forests grow. Plants and trees are natural carbon sinks. Afforestation, reforestation, and proforestation work on the principle of carbon dioxide sequestration to aid plant and tree growth.

Afforestation/reforestation (AR) is not technologically demanding and relies on well-established principles allowing for immediate and easy deployment. Global estimates for sequestration through AR ranges from 0.73 to 5.5 Gt CO2/ year. Most estimates place the cost of sequestration through AR between 10 and 50 $/tCO2 (Minx et al, 2018). However, to achieve this level of sequestration, a large amount of land would have to be reclaimed or reforested, with estimates ranging from 320 to 970 million hectares (790 to 2400 million acres) of new forested land. 

Such massive AR would present challenges such as large nutrient requirements. Excessive application of fertilizers could pose the risk of nutrient runoffs and additional  emissions of greenhouse gas nitrogen oxides (with 300x the global warming potential of carbon dioxide). AR could run into political roadblocks as the continued storage of carbon could be disrupted through decisions to clear-cut forests in favor of other land use activities such as farming. The potential could be further limited depending on the fertility of reclaimed soil which could be significantly lowered owing to multiple cycles of deforestation or intensive cultivation.

Despite these challenges, afforestation and reforestation are integral to negative emission strategies that would be required to meet climate goals.

Land use competition between the petroleum industry and agriculture, near Denver City, Texas, U.S.A. The economy of this region is almost completely dependent on its underground resources of petroleum and water. Both resources result in distinctive land use patterns visible from space. Source: National Aeronautics and Space Administration.

Land and Soil Management

Soil is a large repository of carbon. Effective management of land can help increase the influx of carbon while minimizing the loss of carbon from soils. Examples of effective soil management practices include no-till and regenerative agriculture, regenerative grazing, using organic supplements such as manure, planting cover crops, and maintenance of wetlands. Effective land and soil management practices can result in carbon sequestration of 2-5 GtCO2/yr for costs ranging from $0 to $100/tCO2 (Minx et al, 2018). Many options are mature practices and relatively inexpensive, requiring limited technological support. Long term potential of carbon storage from these practices relies, however, on continued management that decreases soil disturbance. For example, tilling grasslands for conventional agriculture would quickly release much of the stored carbon back into the air (Silveira et al, 2018). 

Soil carbon sequestration can be intensified through the use of biochar – organic matter burned with limited oxygen through a process known as pyrolysis. Biochar increases agricultural productivity through reducing nutrient input and increasing water retention capability. Widespread application of biochar is estimated to have an annual sequestration potential of 0.5 to 2 Gt CO2 with an average cost ranging from $30 to $120/t CO2 (Minx et al, 2018). Biochar is being researched and piloted widely to help identify further benefits and to lower costs through successful use cases.

Carbon capture technology used at a coal mine in 2014. Source: Peabody Energy, Inc., CC BY-SA 4.0 , via Wikimedia Commons

Carbon Capture and Storage (CCS)

Carbon Capture and Storage (CCS) is a method to capture point source carbon emissions from industrial facilities or power generation stations thereby preventing them entering the atmosphere. It can be combined with other technologies to result in negative emissions. CCS allows for reducing the GHG emissions from electric power generation while renewable energy resource integration ramps up. In the industrial sector where decarbonization through the integration of non-fossil energy resources is currently technologically and economically challenging, CCS may provide a bridge in the decarbonization path (C2ES).

CCS is achieved through 1) capturing carbon at point sources, 2) transporting the captured carbon, and 3) storing the captured carbon over the long-term in sub-terrestrial reservoirs (CCS Association). Carbon capture can be achieved through post-combustion capture, pre-combustion capture or through oxy-fuel combustion. The captured carbon is transported via pipelines or freight ships to carbon storage sinks. Captured carbon must then be stored permanently in one of three ways: in underground rock formations (“geo-sequetration”), deep oceans, or as mineral carbonates (IPCC, 2005). 

Captured carbon dioxide can also be repurposed towards carbon-based products, also known as carbon dioxide utilization or CO2U (ICEF, 2017). Captured carbon dioxide could be used as the base chemical component or additive in the manufacturing of building materials, fuels, carbon-based chemicals such as methanol, ethanol etc., polymers and carbon reinforced composite materials. The International Cool Earth Forum elaborates on various CO2U techniques in their 2017 report.

There are a few notable barriers to the sustainable adoption of CCS at scale (Budinis et al., 2018):

BECCS requires plants and trees to be grown, cut down, and combusted to generate electricity. The associated carbon emissions are captured and sequestrated underground. Source: NASA.

Bioenergy with Carbon Capture and Storage (BECCS)

BECCS is an increasingly cited and researched negative emission strategy. BECCS has a simple working principle – carbon absorbed from the atmosphere is used by plants and trees to grow organic matter; this organic matter can be combusted to generate bio-energy (thermal energy). Carbon from the combustion process is captured rather than allowed to escape to the atmosphere, and is sequestered underground through CCS, resulting in a carbon negative process.

Combusting biomass to generate utility-scale electricity or heat are well-developed processes, but are currently achieved largely through co-firing with fossil fuels (Reid et al. 2020). Modifications can be made to the firing system and boilers to accelerate biomass combustion. A large amount of fossil generation would have to be replaced by biomass generation (with CCS) to meet carbon sequestration goals. 

The IPCC Special Report on Climate Change and Land (2019) shows three of the Shared Socioeconomic Pathways (SSPs) used by integrated assessment models to explore possible futures, with a notable decrease in pasture and increase in land for bioenergy and forest. Each scenario is accompanied by a brief description of the pathway and how it alters land use for cropland, pasture, land dedicated to bioenergy, forest, and natural land. Source: (IPCC 2019 Summary for Policymakers)

In the IPCC Special Report on Climate Change and Land (IPCC 2019), several Shared Socioeconomic Pathway (SSP) scenarios model changes in cropland area based on assumptions about intensive agriculture, human population, diet, and other factors. All the scenarios significantly increase land set aside for bioenergy compared to present day, ranging from about 3 to 7 million km2 (300 – 700 million hectares). But critics of bioenergy often flag the implications of such expansion for agriculture, biodiversity, and conservation. 

Estimates of the amount of land required for BECCS biomass production depend on land availability and productivity assumptions and characterizations of what is considered sustainable. The medium confidence estimate of potential for BECCS is 0.4 to 11.3 GtCO2 removed per year – up to about a quarter of present day annual emissions. However, when further sustainability constraints are in place, the estimates are reduced (IPCC 2019). 

Cost estimates vary and are uncertain for all NETs primarily due to the lack of operations at scale, unproven technologies, and particularly for BECCS, unknown land productivity at higher temperatures due to climate change, land degradation, and other land use priorities. Fuss and colleagues estimate the cost of BECCS at 100-200 $/tCO2. The full range of BECCS costs in the literature is 15-400 $/tCO2, with extensive research underway on how to lower costs and develop successful use cases. The EU has several successful BECCS pilots including the Drax Carbon Negative Power Plant in the UK

Banks of fans blow air through a carbon dioxide capturing solution in this rendering of a direct air capture plant. Source: Carbon Engineering

Direct Air Capture (DAC)

Carbon emissions can be produced by point sources (e.g. fossil fuel power plants) and nonpoint sources (e.g. automobiles). While point source emissions can be sequestered through CCS, non-point emissions are not suitable for this. Direct Air Capture (DAC) helps fill this gap. DAC separates carbon dioxide directly from ambient air through physical or chemical processes. It is a negative emissions strategy that may play a role in mitigating climate change, particularly in the mid to latter part of the 21st century. An increased demand for DAC may also emerge from the need to ameliorate significant air pollution in large cities such as Beijing and New Delhi. 

DAC has an estimated sequestration potential of 2-11 GtCO2/year. Cost estimates per ton of CO2 removed have dropped from about $600 (Socolow et al. 2011) by a factor of three or more. One method by the company Carbon Engineering, gives a range at $94-235 per tCO2 (Keith 2018), and others project that the cost of DACs could drop to around $55/ton CO2 by 2040 (Fasihi et al, 2019). That said, even at these lower costs, without an adequate price on carbon, a market to motivate deployment of DAC at the scale needed will not develop. 

DAC can be carried out through two processes: 1) Absorption and 2) Adsorption. Absorption is the process of removing CO2 from the atmosphere using a basic solvent (such as sodium hydroxide). Absorption has a major drawback in that it requires high quality, high temperature heat to aid the chemical process. Adsorption is the process through which a solid sorbent which has a high affinity for carbon dioxide is used. Carbon dioxide occupies the pores on the solid sorbent which is then later removed through an increase in temperature or a decrease in pressure. Adsorption requires lower quality heat at lower temperatures and allows for more wide-spread installation. 

A recent study (Realmonte et al. 2019) explored scenarios with both technologies and found that the absorption process lends itself to larger plant size where 30,000 facilities would be required to remove 30 GtCO2/year by 2100, while the adsorption process would require 30 million units to capture the same amount. The authors point to other massive production numbers such as the 73 million cars manufactured in 2017, but also offered several cautions. On top of the massive amount of expected pollution from the sorbents needed, DAC at scale would use one quarter of the total global energy demand by 2100. They also stress the importance of near term mitigation efforts as opposed to reliance on the future potential of DAC.  

Many hurdles must be overcome for DAC to reach technical maturity such as improving methods of capture, new regulatory environments, cost, energy inputs; and the resource requirements of energy, equipment, and chemicals. In addition, sufficient siting of facilities at scale is needed to capture and process the CO2, whether the carbon is sequestered underground or repurposed.

Perhaps the ultimate promise of DAC is the potential to dial down the global average temperature beyond what may be achieved by massive deployment of the efficient use of clean energy. For example, if future climate change of even below 2°C is determined to have too many negative impacts, DAC in combination with other NETs such as afforestation, land management and Bioenergy with Carbon Capture and Storage, could be used to drawdown the concentration of carbon dioxide to a desired concentration and a more favorable climate.

Fog clearing from the turn basin as the morning sky turns blue. Source: National Aeronautics and Space Administration.

Paths Forward

Many negative emissions technologies are in early stages of experimentation, demonstration, or adoption. As such, it is difficult to fully assess their various performances, costs, and impacts. Technological development and policy priorities are critical to the success of NETs. Challenges can be overcome by more aggressively scaling up pilot efforts, promoting international agreements that accelerate mitigation, addressing the weakness of market forces to fund deployment of CCS and viable NETs, and allocating resources needed to meet the 2°C goal of the Paris Accord. This MIT Energy Initiative Podcast sheds additional light on negative emissions strategies and how they can be implemented at scale. 

One summary of viability, costs, and environmental considerations of known NETs today (EASAC, 2018).

Delay in deployment of conventional mitigation exacerbates the magnitude of the task as CO2 concentrations increase. Researchers caution against thinking of NETs as a safety net for the future, as this can drive rationalization of continued fossil fuel use in the present, and valuable time investing in alternatives may be lost (Smith et al. 2016). What if NETs don’t meet expectations on technical, economic or environmental criteria? The window for action is still open to achieve the Paris goal, but it requires massive reductions in greenhouse gases and wise land use practices that gain increasing traction from the present moving forward.