In This Issue
Spring Bridge Issue on Engineering and Climate Change
March 15, 2020 Volume 50 Issue 1
The seven articles in this issue cannot cover all engineering-related aspects of climate change, but they highlight several areas of concern.

The Giving Earth

Monday, March 16, 2020

Author: Jennifer Wilcox

People all over the world must take action to wean from an addiction to fossil fuels. Otherwise, it would seem that Earth is simply here for humans to consume.

In The Giving Tree by poet Shel Silverstein (1964), the tree tells the once playful boy now grown into old age, “I wish that I could give you something … but I have nothing left. I am just an old stump. I am sorry.… well, an old stump is good for sitting and resting. Come, Boy, sit down. Sit down and rest.” The story teaches the importance of giving and being selfless; the tree is an example of perfect altruism, while the boy who took everything from her is an example of perfect selfishness.

Modern society’s relationship with Earth reflects the one between the boy and the tree: People have taken what is beneficial to them but have not continued to care for the planet.

What if methods could be engineered to render fossil energy “green” by replacing the pore space in rocks depleted of oil and gas with sequestered CO2? This is possible,[1] but it does not seem a fair trade to Earth.


Earth has sufficient fossil fuel to sustain the world’s appetite for thousands of years to come.[2] But continuing down the path of Boy in the story and using up all this fuel would result in cumulative emissions of 50,000 billion tonnes of CO2 (GtCO2) (Wilcox et al. 2017). Since the Industrial Revolution roughly 1,800 GtCO2 have been emitted into the atmosphere (Allen et al. 2009). The burning of fossil fuels for energy production and resulting accumulation of CO2 in the atmosphere have created a world that is warming faster than at any other time in human history (Mann et al. 1999).

Alternatives based on the sun, water, and wind could meet global energy demands while preventing the reckless use of Earth’s fossil resources. But, realistically, their widespread implementation will require a mediated transition based on collective work toward a common goal. The transition may include similar jobs that exist today in addition to new jobs and will involve the reverse flow of carbon back into Earth where it originated. It will require removing emissions that have been accumulating since the Industrial Revolution while simultaneously advancing deep decarbonization efforts, with renewables being a critical component of the transition.

Nature removes roughly half of global emissions each year through terrestrial and ocean uptake. But this takes a toll on natural systems; ocean acidification is the most prominent side effect. In 2018 alone, after land and ocean uptake and the release of CO2 through land-use change (e.g., deforestation, forest fires), roughly 17 GtCO2 were added to the atmosphere (Le Quéré et al. 2018).

The Need for Negative Emissions Technologies (NETs)

Recent studies (IPCC 2018; NASEM 2019) indicate that negative emissions technologies (NETs) will be required in addition to unprecedented reductions in anthropogenic emissions. However, atmospheric CO2 concentrations will not be reduced until the combined uptake by natural carbon sinks and NETs outweigh anthropogenic emissions.

To illustrate, in 2018 fossil fuel and industrial emissions were 34.3 + 2.0 GtCO2/yr and land-use change accounted for 4.9 + 3.0 GtCO2/yr. But ocean uptake of CO2 was 8.7 + 2.0 GtCO2/yr and land uptake was 11.2 + 3.0 GtCO2/yr. In this example, NETs would have to remove more than 20 GtCO2 in just one year to reduce the accumulated atmospheric CO2 (figure 1; NASEM 2019). This underscores the importance of a portfolio approach, and that NETs should not be viewed as a substitute for reducing CO2 emissions in the first place.

Figure 1 

Negative emissions technologies include planting biomass that may be harvested for energy production with emissions scrubbed of CO2, storing carbon in soils, increasing CO2 uptake from air through afforestation and reforestation, and reaction of CO2 with alkaline-containing minerals in the earth to form carbonates (NASEM 2019).

Direct Air Capture as Part of a Broader Portfolio

A method called direct air capture (DAC) uses a chemical approach to capture CO2 from air, not too different from how a forest does. The chemical approaches involved in DAC have a significantly greater efficiency and, unlike a forest, do not compete with food production for arable land. For example, a deciduous forest with an average tree density of 200 trees per acre requires roughly 390 km2 for the net uptake of 1 MtCO2. In contrast, a DAC facility that captures 1 MtCO2 per year may require up to 7 km2, or just short of 2 percent of the amount of land required by deciduous trees to meet the same target (NASEM 2019).

In a DAC plant, CO2-selective chemicals are housed in large contactors with fans that push air through them to capture CO2. As the air passes through the contactor, the CO2 in it collides and reacts with the chemicals while the primary components of air (nitrogen and oxygen) continue to move through. The contactors contain structured materials that moderate the air flow. If the air moves too fast, there may not be adequate time for reaction to take place, while air moving too slowly unnecessarily extends the time needed for the process. The captured CO2 is recovered in pure form from the chemicals using heat, condensing it for transportation in a truck or pipeline (e.g., for storage).

The concentration of CO2 in the atmosphere is roughly 300 times more concentrated than the CO2 in the exhaust of more concentrated sources such as -power plants. So why would one even consider capturing CO2 from air at just 410 ppm? Why not first focus on natural gas- and coal-fired power plants, which produce roughly 100 to 300 times more CO2 in their emissions streams? After all, natural gas and coal still represent 23 percent and 27 percent of the global energy resource mix, respectively (EIA 2019a). The minimum amount of thermodynamic energy required to capture CO2 from air is 3 times greater than that required to remove it from the exhaust of a coal-fired power plant. The greater dilution of CO2 in air translates to DAC requiring 300 times more contact area than coal-fired power plants to capture the equivalent CO2 (Wilcox 2012). These differences translate directly to larger energy and capital costs for DAC compared to more concentrated sources.

Unfortunately, even aggressive efforts to limit emissions show that amounts of CO2 will still be too high in the atmosphere, and for many greenhouse gas sources there simply is no method available to eliminate them. Earlier efforts might have supported the option of just mitigating emissions; now it is essential to both mitigate and remove emissions.

There is no silver bullet that will solve the climate crisis. The solution is difficult, and this difficulty is likely one of the reasons for general stalling on actions to avoid emissions on a significant scale. Efforts to achieve climate goals must include every tool available, including DAC.

Comparison of Commercial-Scale DAC Technologies

Leading DAC methods include two gas separation technologies: solid sorbents and liquid solvents. In both cases, air moves through a contactor to interact with chemicals that remove CO2.

Solid Sorbents

With solid sorbents, the chemicals are bound to micro- and mesoporous materials embedded in the structured material that contains larger air transport channels (on the order of millimeters) so that the air can move through easily without requiring significant fan power.

The micro- and mesoporous materials have very high surface areas to maximize the number of chemicals on the surface for chemically binding CO2. As an example, a microporous activated carbon may have a surface area up to 2,200 m2/g, while in a single gram of material a metal organic framework sorbent may have a surface area up to 6,000 m2/g—just over the size of a football field (Wilcox 2012).

The solid sorbents are embedded in a honeycomb-like framework, not too unlike the catalytic converter in an automobile; just as exhaust from automobiles moves through the catalytic converter, the air moves through the channels in these materials.

Liquid Solvents

Instead of being bound on porous solid materials, CO2-reacting chemicals may be dissolved in a liquid to form a solvent that is pumped over a structure with a high surface area so that the air interacts with the solvent quickly. This structure is called “packing material.” First-generation packing materials for absorption processes were invented in the 1940s. The packing material allows the solvent to uniformly and thinly distribute to maximize the surface area between the gas containing CO2 and the chemical in the solvent, similar to the solid sorbent method. An advantage of the solvent approach is that the solvent is inexpensive and easy to make in large quantities.


The technologies differ in their cost breakdowns. The capital costs of the solvent-based systems are dominated by large chemical process equipment (NASEM 2019), which is also a benefit since it leads to economies of scale (i.e., cost-effective, large-scale deployment -projects). One such project involves a partnership with a commissioning date of 2023, to be located in Texas (Carbon Engineering 2019). This first major DAC -project is designed to capture 1 MtCO2/yr.

Solid sorbent capital costs are dominated by the costs of manufacturing the necessary micro- and mesoporous materials (NASEM 2019), which do not benefit from economies of scale. Efforts are underway to reduce costs and increase the rate of materials production.

Power for DAC Plants

The energy required to carry out DAC on a scale of millions of tonnes of removal per year should not be underestimated. Depending on the energy resource, capturing 1 MtCO2 per year requires 300–500 MW of power. Therefore, the design of a DAC plant must also include the design of a power plant coupled to it, to maximize the net removal[3] of CO2 from air.

For instance, with conventional natural gas -power as the energy resource, for every 2 tonnes of CO2 removed roughly 1 tonne would be emitted back into the -atmosphere. Care should be taken to ensure that CO2 is not emitted by the power source, which means that either renewable power or natural gas power with full capture should be used. Either option would be a significant component of the cost of the DAC plant.

Thus, to maximize the potential of DAC requires coupling the capture plants with carbon-free power, but one must be cautious that these valuable resources are not first more suitable for decarbonizing fossil-based sectors.

Costs of DAC and Anticipated Reductions

What is the true cost of DAC deployment today? Estimates in the literature range broadly and most are based on lab or demonstration-scale investigations. Only one company, Climeworks, has demonstrated through multiple deployments that the current cost of DAC is $600/tCO2 (Evans 2017; Gertner 2019). Since the power source coupled to the DAC plants operated by -Climeworks is very low- to zero-carbon, the cost of removal roughly equates to the net removed cost. The company has publicly stated that it has plans to decrease these costs to $200–300/tCO2 within the next 5 years (Gertner 2019).

A number of studies estimate nth-of-a-kind plants on the order of $100/tCO2 (e.g., Keith et al. 2018; NASEM 2019) that separate CO2 from air to high purity (i.e., >98 percent) suitable for transportation and storage. If the deployment of DAC can increase from the current thousands of tonnes per year (ktCO2/yr) removal, as demonstrated by Climeworks, to millions of tonnes per year over the next decade or two, as anticipated, the lower costs may be realized.

Other technologies that are still in development—such as electrochemical approaches (Bandi et al. 1995; Eisaman et al. 2009; Voskian and Hatton 2019) and the use of concentrated solar power for aqueous-phase absorption and crystalline-phase release (Brethome et al. 2018)—may be demonstrated as R&D in this field accelerates.

Paying for Large-Scale CO2 Removal:
The Role of Policy

In the United States, DAC qualifies for two policy incentives. The federal 45Q tax credit provides up to $35/tCO2 for utilization and up to $50/tCO2 for geologic storage (Christensen 2019). In addition, California has a low-carbon fuel standard (LCFS) that places a cap on the maximum carbon intensity (CI) of transportation fuels sold in California and grants credits for fuels below the CI requirement (CARB 2018). The credit is currently traded at $150–$200/tCO2.

An entity that operates DAC coupled to geologic storage anywhere in the world may qualify for LCFS. At current costs of DAC, these incentives are still unable to close the economic gap without reliance on today’s small CO2 market, such as enhanced oil recovery (EOR) (roughly 85 percent) and the food and beverage industries (roughly 10 percent).

It is important to recognize that the demonstrated costs of DAC are not a limiting factor for its deployment. Rather, the lack of policy that puts a price on the permanent removal of CO2 is limiting progress in both conventional carbon capture and storage (CCS)[4] and DAC. The storage of gigatonnes of CO2 per year in the Earth’s subsurface will be essential to meet climate goals. Permanent storage of CO2 will be required for capturing CO2 at point sources such as power plants[5] in addition to CO2 removal strategies from air, such as bioenergy with CCS and DAC. Without permanent storage, neither bioenergy nor DAC result in negative emissions. 

Emerging CO2 Markets and the Transition Away from Fossil Fuels

Utilization and geologic storage of CO2 should not be viewed as an either-or option but rather as a continuum toward achieving climate goals. Beyond the current small CO2 market, there are emerging markets for use of CO2 as a feedstock, for example in synthetic -aggregates[6] for construction and road building and in synthetic fuels.[7] These markets have the potential to use CO2 as a feedstock on the scale of gigatonnes -globally, with the first leading to permanent storage in the form of -carbonate. With synthetic fuels, the approach is at best carbon neutral, assuming that the liquid fuel will be used for the transportation sector and reemitted into the air in a distributed fashion. With synthetic fuels using CO2 and H2 as reactants, both the source of H2 and the DAC power must have minimal or zero associated carbon emissions to have the greatest CO2 removal impact.

Today CO2-EOR is the largest CO2 market in the United States.[8] Although most CO2 for EOR is sourced naturally, it is anticipated that with regulations such as California’s LCFS and the federal 45Q tax credit, there will be greater incentive to use anthropogenic CO2 and even CO2 from air.

Ultimately, CO2 should be overused in the EOR process such that more CO2 stays underground than the produced oil would create. This would require coupling projects suitable for both dedicated storage and EOR since the density of the carbon atoms in compressed CO2 at the temperature and pressure conditions of the earth would never be greater than the density of carbon atoms in the oil to begin with. Projects that couple EOR with dedicated storage would be appropriate through a transition phase toward completely weaning away from the need to recover any oil. Perhaps policies would shift from subsidizing both EOR and storage projects to subsidizing only storage projects, allowing operators to gain experience in CO2 storage while transitioning their business away from EOR.

Globally, roughly 30 MtCO2/yr is stored through CO2-EOR, with an additional 10 MtCO2/yr stored through dedicated sequestration projects (Global CCS Institute 2019). To meet climate goals, the geologic storage of CO2 must increase at least a hundredfold by midcentury.

Who Will Build and Operate the Facilities for Gigatonne Recovery? Workforce Impacts

The oil and gas industries support roughly 164,000 US jobs, just under 2 percent of total US employment (BLS 2019b).[9] A recent article reveals that 20 fossil fuel companies have contributed to 35 percent of all energy-related CO2 and methane emissions globally, totaling 480 GtCO2eq[10] since 1965 (Taylor and Watts 2019). This group might naturally be expected to take significant steps toward the solution.

A short list of the positions that will be needed to transition away from being a global society addicted to fossil fuels includes key job categories in the oil and gas industries: geologists, geophysicists, geochemists, drilling engineers, mining engineers, petroleum engineers, chemical engineers, and surveyors. There will also be new jobs created at DAC facilities, perhaps jobs similar to those at utilities such as power plants.

To increase from MtCO2/yr capture and storage to GtCO2/yr will largely require talent that is already in the workforce. Drilling wells, understanding fluid transport in subsurface porous media, and advancing -catalysis for fuel synthesis are areas of expertise that overlap between current oil and gas (fossil) energy and a fossil-free energy future.


Humans must recognize the toll of our selfishness on the giving Earth. We must minimize our carbon emissions, create infrastructure for geological storage, facilitate a transition to renewable energy, and develop negative emissions technologies to combat rising atmospheric CO2 concentrations and their detrimental effects. Most of all, we must ensure that there never comes a day when the Earth has nothing left to give.


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[1]  Recent studies show that the use of CO2 from air for enhanced oil recovery (e.g., from a partially depleted oil reservoir) may result in the equivalent or even more CO2 stored in the Earth than created from production, transport, refining, and oxidation of the fuel (Núñez-López et al. 2019).


[2]  This assumes combined reserves and resources for oil, natural gas (not including clathrates), and coal of 6.8 trillion -barrels, 194,000 trillion ft3, and 15,570 billion tonnes, respectively (-Rogner et al. 2012).


[3]  Depending on the carbon intensity of the energy and material input, and the CO2 transportation to sink, there may be emissions into the atmosphere, which will reduce the plant’s net removal of CO2 from air. Emissions embodied in the materials or energy required to operate the DAC plant lead to an increase in the cost of CO2 capture from air on a net removed basis.


[4]  Conventional CCS is the capture of CO2 from a point source, followed by compression for trucking or pipeline conditions, for transport to a geologic site where it can be injected and permanently stored in the Earth’s subsurface.


[5]  The electric power sector represented 33 percent of US energy-related CO2 emissions in 2018 (EIA 2019a).


[6]  Projections from 2014 estimated aggregate demand of 53.2 Gt/yr, composed of crushed stone, sand, and gravel (Grand View Research 2019).


[7]  The global liquid fuel market today is 11 M barrels/day. Assuming that CO2 + H2 are a feedstock to synthetic fuel (density of 900 kg/m3) equates to a CO2 demand of roughly 5 MtCO2/day (EIA 2019b).


[8]  International CO2-EOR opportunities also exist, with potential on the Gt-scale in Saudi Arabia, Russia, China, India, and Oman (Ward et al. 2018).


[9]  By comparison, the coal industry was responsible for employing 52,700 people in 2019 and 51,700 in 2018 (BLS 2019a).


[10]  Because of possible methane leakage from natural gas processing and its transportation, GtCO2eq includes both CO2 and methane emissions associated with the oil and gas industry.


About the Author:Jennifer Wilcox is the James H. Manning Professor of Chemical Engineering at Worcester Polytechnic Institute.