Since the industrial revolution, people have been burning an increasing amount of stuff. From little fires in cars to big fires in power plants, we are putting so much of these gases into the sky that we are changing the thermal properties of our atmosphere, trapping ever more heat. This is the greenhouse effect, which is the main contributor to climate change.

Beyond power and transportation, there are less obvious emissions sources as well:

cement production
steel production
agricultural emissions (soil nitrous oxide, animal methane)
industrial and permafrost leaks, etc.
You can explore the full range here.
Some sectors have clear paths to reduce emissions: electric vehicles, solar/wind/nuclear, however some will be slower or more complex to decarbonize:
air travel
industrial emissions
Some agricultural emissions, methane.

Figure 1. A ton of CO2

Recent data shows progress is too slow. To keep below two degrees, we will need to dramatically reduce current emissions and simultaneously remove 10-15 gigatons of CO₂/yr from the atmosphere by 2050 and scale that to about 20+ gigatons annually by 2100. Depending on how quickly we reduce emissions, the amount we need to remove from the atmosphere scales proportionally.

Greenhouse gas emissions are described in units of tons. It’s hard to think about how much “a ton of gas” really is -- this is how big, at surface temperature and pressure. Here’s an animated video visualizing a bunch of these one-ton balls in New York City.

You may also hear people talk about “400 parts per million CO₂” or similar. This maps directly to the amount of gas emitted: when emissions mix into the atmosphere, we reference concentrations of the emitted gas as a portion of the atmosphere. This is like stirring sugar into a cup of coffee. For every million parts atmosphere there are 400 parts CO₂ this proportion has been rising steadily exactly since the industrial revolution...

Other greenhouse gases

Greenhouse gases are released when stuff is burnt as well as the product of other chemical reactions in industry. Generally, we’re talking about mostly carbon dioxide (CO₂), methane (natural gas) (CH₄), and nitrous oxide (N₂O). N₂O and CH₄ are more potent greenhouse gases, but they occur in an order of magnitude less quantity than CO₂, so removing them is generally much harder. Methane also has a much shorter half life in the atmosphere than CO₂.

Do we really need to remove CO₂?

In the 90s, negative emissions were not mainstream apart from early research by David Keith, Klaus Lackner, and others.

There was some combination of optimism that the problem wasn’t so bad, that the world would decarbonize at a sufficient rate that negative emissions wouldn’t be necessary, and that a policy framework would force action (large scale carbon price/cap and trade, which still doesn’t exist worldwide). This was all combined with fears that negative emissions present a moral hazard by giving ourselves an “out” for crucial emissions reduction work.

In an attempt to hit a 2 degree warming target, the Paris agreement calls on countries to set “Nationally Determined Contributions”, or NDCs - commitments to a specific amount of emissions reduction over the coming decades. But these commitments are not even close to enough:

There are two complementary scary things about this chart:

All existing Paris commitments (“NDCs” in the above figure) don’t get us even close to a 2 degree trajectory
Countries are not even close to on track to hit even these commitments. This is an incredible collective action problem — each individual country faces minimal/zero “official” consequences for failing to do so.

To illustrate this:

Compounding the urgency, that research into negative emissions technologies (NETs) have been dramatically underfunded in proportion to their importance. To quote the National Academies (emphasis mine):

“...the federal government spent more than $22 billion on renewable energy research and development from 1978 to 2013. NETs have not received comparable public investment despite expectations that they might provide ~30 percent of the net emissions reductions required this century (i.e., maxima of 20 Gt/y CO₂ of negative emissions and 50 Gt/y CO₂ of mitigation in Figure S.1). NETs are essential to offset greenhouse gas emissions that cannot be eliminated, such as a large fraction of agricultural nitrous oxide and methane emissions.”

How to take CO₂ out of the sky

For a more comprehensive but still accessible overview, I recommend Adam Marblestone’s Climate Technology Primer</a> and the Summary section of the National Academies Report.

Briefly: there are plant-based, mineral-based, and chemical options.

Figure 3. The different methods of CO₂ capture.

Plant based solutions leverage a plant’s capacity to capture carbon via photosynthesis and the energy of the sun. Solutions in this category include (re)forestation, soil carbon sequestration, algae/kelp farming, and bio-energy with carbon capture and storage (often referred to as BECCS, this is basically a biomass power plant that burns wood and then is fitted with a carbon capture device to handle the smoke aka “flue gas").

Figure 4. Carbon storage techniques.

Mineral-based solutions include speeding the weathering of naturally occurring rocks, e.g. Olivine.

Chemical solutions include Direct Air Capture (typically coupled with geologic storage, the capture aspect is often referred to as DAC), where a big machine sucks CO₂ out of the air; providing gaseous concentrated CO₂ that can then be injected underground.

These solutions all have different technical tradeoffs, potentials for scale, costs, adoption curves, and more.

Overview of approaches

Trees and forests

Trees and plants are incredible at carbon capture, doing so for “free” via the energy of the sun despite how dilute CO₂ is in the atmosphere. The CO₂ trees absorb doesn’t magically go away. Instead, the C literally becomes part of the biomass of the tree. When a tree burns down, the carbon it absorbed is released right back into the air. For permanent sequestration, we’ll need to do something with the biomass of the forest. This could include making it into wooden building materials, biochar fertilizer, or dropping the biomass in the ocean. Or, if the forest is well managed, it can grow for centuries; and we can consider the sequestration “durable” as long as that management persists and is monitored.

To understand how much carbon is captured, scientists build models that relate easy-to-measure physical properties like diameter and height to the actual above-ground dry weight of a tree, about 50% of which is carbon. These are known as theallometric equations. More modern techniques leveraging remote sensing and LIDAR are being explored to do the same thing with fewer ground measurements.

Forests take up land that may be more economical if used for other things. Forests are dark colored and may actually decrease albedo(light-colored-ness -> heat reflectance from the sun), somewhat decreasing their utility (paper is <a href="https://journals.ametsoc.org/doi/10.1175/JCLI-D-18-0812.1">here and here’s</a> a lab working on albedo). There may also be impacts on the biodiversity of the ecosystem the land would otherwise be used for.

Direct air capture (DAC) + sequestration

Traditional direct air capture facilities pass atmospheric air through a “sorbent” that absorbs, concentrates, and then controls release of CO2. For a detailed but still accessible overview of the field, see The Role of Direct Air Capture in Mitigation of Anthropogenic Greenhouse Gas Emissions.

Companies currently working on this include Climeworks, Carbon Engineering, and Global Thermostat. Because CO₂ is so dilute (about 0.04% in atmospheric air), this process is energy intensive (22kJ/mol). Hence, Lifecycle Analysis is very important. For an example methodology, see Keith 2018 which describes Carbon Engineerings technical approach.

There are also some new DAC approaches, currently in the research phase, e.g. electro-swing adsorption (news article with a good summary. These are promising but quite early!

In any case, the result of DAC is pure gaseous CO₂, which can be either utilized or sequestered.

Sequestration

Sequestering it means permanently removing the CO₂ from the carbon cycle, and literally reducing the concentration of CO₂ in the atmosphere. This can be done by injecting the CO₂ injected underground into an oil well, salt dome, or saline aquifer.

It’s fairly straightforward to measure how much CO₂ is being pumped into geologic storage, and there are generally accepted practices in the oil industry to demonstrate secure storage. See the Geologic sequestration - "in situ" subsection of the Enhanced weathering and carbon mineralization section later down this blog post for more details.

Utilization

Alternatively, this CO₂ can also be utilized. Some utilization mechanisms can be considered negative emissions/long-term sequestration, eg using the CO₂ to cure cement. Other use cases include making fuels and chemicals. The fuels case is interesting, in that the emissions from the fuel wouldn’t cause an absolute increase in atmospheric CO₂ like it would if you pulled the fuel out of the ground. In a sense, you’re burning and “unburning” the fuel via capture, concentration, and fuel reformation.

Soils

Soil carbon sequestration methods aim to encourage modified agricultural techniques that increase the carbon sequestered in soil without reducing or otherwise affecting agricultural yield.

These regenerative mechanisms include no-till farming, using cover crops, and reducing usage of nitrogen fertilizers can enable storage in soil of some of the plant’s carbon volume.

Measuring soil carbon can be complex and has high variance: there’s the carbon in the biomass of the plant roots, and then the carbon expelled by the roots that can become stable in the soil as Soil Organic Carbon. Ground measurements are expensive (you take a core sample of the soil and stick it in a spectrometer, which is not a thing that farmers have lying around), and it’s still very early for soil carbon remote sensing.

Remote sensing is tough with soil because you do not just care about the carbon content of the topsoil, you care about the distribution of carbon vertically down the soil column for a couple feet – generally the deeper the carbon is, the more durably it is stored.

Doing things with biomass: Bio energy with carbon capture and storage (BECCS) and Biochar

BECCS

The idea of biomass power in general is that instead of burning coal or natural gas to turn a turbine, you burn biomass (which, because it is biomass, is made of carbon that used to be in the atmosphere).

Hence BECCS, as generally proposed, takes the “get the capture for free!” idea of plant-based carbon capture and combines it with "sequester the flue gas CO₂" idea that’s often applied to the CO₂ resulting from DAC.

BECCS strengths and weaknesses are similar to that of forests, where trees and grasses continue to be amazing at “free” carbon capture. The land use needed may be quite large, and likely more agriculturally intense. BECCS requires a very high volume of biomass (and hence has high sequestration potential, but is only currently deployed at very limited scale).

Depending on how you have combusted or <a href="https://en.wikipedia.org/wiki/Pyrolysis">pyrolyzed</a> biomass, you either end up with a stream of flue gas from which you can concentrate the CO₂ and do geologic storage/utilization/anything else you would do with flue gas, or you are able to keep the C in the biomass via pyrolysis and output Biochar (see below) – this is what some new BECCS companies like do.

Biochar

Another option is taking biomass and making biochar, generally via pyrolysis. The idea here is that you heat biomass to a high temperature in a low-oxygen environment; producing biochar: a chalky black material that is nearly pure carbon.

Biochar is used as a fertilizer and soil additive: it seems like it might improve fertility, increase soil organic carbon (SOC), and reduce soil heavy metal concentrations. For a more detailed overview, see the "Biochar Additions" section of Soil C Sequestration as a Biological Negative Emission Strategy.

Properly produced biochar has the potential to remain durable (keep the C sequestered) in soil for an extended period; however its durability is subject to similar environmental conditions as Soil Organic Carbon, particularly precipitation and soil water content as it relates to microbial decay. It may also have positive effects by reducing soil N₂O emissions (another potent greenhouse gas).

Enhanced weathering and carbon mineralization

For a more technical overview of this topic, I highly recommend <a href="https://www.frontiersin.org/articles/10.3389/fclim.2019.00009/full">An Overview of the Status and Challenges of CO</a>_{<a href="https://www.frontiersin.org/articles/10.3389/fclim.2019.00009/full">2</a>}<a href="https://www.frontiersin.org/articles/10.3389/fclim.2019.00009/full">Storage in Minerals and Geological Formations</a>.

Enhanced weathering – "ex situ"

<a href="https://en.wikipedia.org/wiki/Ultramafic_rock">Common rocks</a> form carbonate minerals when exposed to CO₂; often even at atmospheric concentrations; permanently binding the C as a mineral (eg CaCO₃ calcium carbonate).

On geologic timescales, <a href="https://en.wikipedia.org/wiki/Carbonate–silicate_cycle">this process</a> could restore earth to pre-industrial CO₂ levels. The idea behind "enhanced weathering" is to dramatically accelerate this natural process. <a href="http://projectvesta.org">Project Vestas</a> website provides an accessible walkthrough to this idea. This is sometimes referred to as "ex situ" mineralization because you are mining rocks and putting them through a sped-up carbon mineralization process aboveground.

<figcaption><a href="https://www.earth-syst-dynam-discuss.net/2/551/2011/esdd-2-551-2011-print.pdf">Figures Source</a></figcaption></figure>

Rocks such as <a href="https://en.wikipedia.org/wiki/Olivine">olivine</a> and <a href="https://en.wikipedia.org/wiki/Serpentinite">serpentinite</a> are particularly effective at CO₂ binding. The amount of carbon captured is generally proportional to the amount of surface area exposed to air, so we want a low-energy way to grind them to the right particle size to optimally bind carbon. <a href="https://www.frontiersin.org/articles/10.3389/fclim.2019.00007/full">More research is needed into the exact kinetics of the mineralization process, other risks (heavy metal release; ecosystem impacts of distributing vast quantities of small rock particles on land or in oceans; etc)</a>; but it is a really interesting idea.

Geologic sequestration - "in situ"

<figure class="kg-card kg-image-card kg-card-hascaption"><img src="https://www.orbuch.com/content/images/2020/02/borkjarni_0.jpg" class="kg-image"><figcaption>Carbonate mineral formed on basalt from Carbfixs geothemal injection site in Iceland</figcaption></figure>

You can think of "in-situ" mineralization as the geologic sequestration component that is crucial to DAC systems.

It happens when you inject a stream of CO₂ into the right sorts of geologic formations, and <a href="https://science.sciencemag.org/content/352/6291/1312">permanently stores the CO₂ in a minera</a>l. Its called "in-situ" because you are injecting CO₂ to where the rock already is, instead of bringing the rock above ground and exposing it to atmospheric air.

Lifecycle analysis and tradeoffs of carbon removal approaches

It’s important to interpret all carbon removal solutions through a lens of a full lifecycle analysis, which can help answer the question: is the entire operation of a given negative emissions solution net-negative?

For BECCS, for example, this analysis would include the greenhouse gas emissions to grow the biomass, the emissions of building the BECCS plant, the emissions of transporting the biomass to the plant, any loss of CO₂ to inefficiencies in the carbon capture system, and the emissions associated with the energy to run the big machine to pump the CO₂ underground.

All of this would need to add up to less than the volume of CO₂ sequestered -- ideally much less.

Alongside lifecycle analysis, there are a number of dimensions to evaluate carbon removal solutions. Many of these are an oversimplification; but may be helpful to think about the problem space (it’s extremely difficult or sometimes impossible to be sure about many of these questions):

Efficiency of capture

Does the energy to run a carbon capture machine emit more CO₂ than the amount we’re capturing (carbon negative lifecycle)?

How might this change over the coming decades, and how does that affect the cost?

Permanence of storage

Once it’s captured, what do we need to do to keep it from re-entering the atmosphere? For how long? How much does it cost to do so?

Ease of measurability/verifiability

Are we sure about exactly how much we’re sequestering?

Are we sure it won’t leak, or we’ll know right away if it does?

Who and how will we penalize if this happens?

Opportunity cost / moral hazard

What would we use this land for if we didn’t plant a forest on it?

How do we know the next land developer won’t burn it down?

Might this technology slow emissions reduction?

Potential for scale

What’s the upper bound of how much a CO₂ removal technology could sequester? What are the error bars on that estimate?

How much does a given technology cost today? Is there a path for it to be way cheaper in the coming decades?

How might innovation and adoption curves inflect over time; either for this technology directly or in its supply chain?

Other constraints or tail risks (often risks to permanence or issues exposed in lifecycle analysis)

Does this area have the right geology to support pumping CO₂ into the ground?

What happens to this forest if there’s a regime change or land use regulation change?

What if the currency in country X crashes, and farmers need more land?

What if drought/floods caused by a changing climate make this land no longer suitable for the thing we were hoping to do?

What if X technology doesn’t get cheaper as expected/at the expected rate?

Interesting and relevant things I did not discuss in this post

Here’s a small sample of topics relevant/adjacent to concepts discussed here that I’d like to learn more about. If any of these are particularly interesting to you, <a href="http://twitter.com/orbuch">tweet</a> (@orbuch) or email (ryan dot orbuch at gmail) me! You can also think of these as requests for blog posts from people with much more expertise in this field:

Carbon capture and utilization -- point-source capture/traditional CCS, “carbon-to-value”, “air-to-fuels” and more
Solar radiation management -- stratospheric sulfate aerosol injection and/or marine cloud brightening as a strategy to buy us more time for emissions reduction
Traditional carbon offsets; their strengths and weaknesses; what we should learn from them to scale negative emissions purchasing
Ocean fertilization, ocean liming, direct ocean carbon capture, other ocean stuff
Unit economics and cost/adoption curves of these carbon sequestration options in detail
The policy and subsidy landscape around carbon sequestration: 45Q; the California LCFS; the European Emissions Trading System, etc.
Project finance and how it works for negative emissions and energy projects, how it fits with and differs from other funding sources (VC, private equity, tax equity etc)
Bioengineering approaches: modifying plants to make them better at capturing/storing carbon; engineering proteins for use in DAC sorbents, etc

Conclusion

Here’s what I hope you remember from this piece:

10-gigaton-scale negative emissions are necessary in essentially every emissions reduction scenario. We have no choice but to fund, research, and deploy them if we’re serious about keeping warming to 2 degrees; or close to it. We are not even close to on track.
Negative emissions have been dramatically underfunded in proportion to their importance. This needs to be fixed if we’re going to have a shot at reducing the cost enough to make 10-gigaton-scale deployment possible by midcentury. It will take likely take years or decades for basic research and pilot projects to scale and get cheap enough; so we need to start right now.
It’s very unlikely any one category of technology, or any one natural approach, will scale enough. We should think of a portfolio across all the approaches outlined here, as well as more I didn’t discuss or have yet to be discovered.
We face the defining problem of our generation; of the entire human project thus far. Climate spans physics, chemistry, ecology, geology, policy, technology, land use, human rights, and more. It’s time we take this seriously as a gigantic opportunity for human progress, and rally to solve it!

Many people provided feedback on this post, and many more spent time with me over the last few months to help me learn about this field, check my assumptions, and frame my thinking. Thanks in particular to Jeremy Freeman, Christian Anderson, Clay Dumas, Sarah Sclarsic, Peter Reinhardt, Adam Marblestone, Maddie Hall, Klaus Lackner, Nat Keohane, Jennifer Wilcox, Jane Zelikova, Jason Jacobs, April Underwood, Celine Halioua, Florent Crivello, Steve Pacala, Brian Heligman, Michael Nielsen, Jose Luis Ricon, Steve Hamburg, Erika Reinhardt, Alexey Guzey, Ramez Naam, Raylene Yung, Noah Deich, Andrew Bergman, Phil Renforth, Greg Dipple, Giana Amandour, Mason Hartman, Landon Brand, Julio Freedman, Zara Heureux, Jeremy Büttner, Nan Ransohoff, Tamara Winter, and many many more than I could mention here!

One of the reasons I’ve had so much fun learning about climate is because the people working on it are really great!

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$*&^choose one wisely@*

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CO2

Contents

Other greenhouse gases

Do we really need to remove CO₂?

There are two complementary scary things about this chart:

To illustrate this:

How to take CO₂ out of the sky

Briefly: there are plant-based, mineral-based, and chemical options.

Overview of approaches

Trees and forests

Direct air capture (DAC) + sequestration

Sequestration

Utilization

Soils

Doing things with biomass: Bio energy with carbon capture and storage (BECCS) and Biochar

BECCS

Biochar

Enhanced weathering and carbon mineralization

Enhanced weathering – "ex situ"

Geologic sequestration - "in situ"

Lifecycle analysis and tradeoffs of carbon removal approaches

Interesting and relevant things I did not discuss in this post

Conclusion

Share your opinion

Other greenhouse gases

Do we really need to remove CO2?

There are two complementary scary things about this chart:

To illustrate this:

How to take CO2 out of the sky

Briefly: there are plant-based, mineral-based, and chemical options.

Overview of approaches

Trees and forests

Direct air capture (DAC) + sequestration

Sequestration

Utilization

Soils

Doing things with biomass: Bio energy with carbon capture and storage (BECCS) and Biochar

BECCS

Biochar

Enhanced weathering and carbon mineralization

Enhanced weathering – "ex situ"

Geologic sequestration - "in situ"

Lifecycle analysis and tradeoffs of carbon removal approaches

Interesting and relevant things I did not discuss in this post

Conclusion

Share your opinion

Do we really need to remove CO₂?

How to take CO₂ out of the sky