What are negative emissions? A Primer
4 min read

What are negative emissions? A Primer

Are you unfamiliar with negative emissions? Here is a short primer, intended to be a living document that is updated over time.

Why are we talking about negative emissions?

That's because negative emissions are fundamentally required to achieve net zero emissions (as shown in the sketch above), which is kind of a big deal in today's climate change conversation.

The concept of negative emissions is not well-understood, since it is a fairly new concept, compared to its cousin, 'regular old emissions.'

The concept of negative emissions is fraught with questions like, how much carbon does a tree store? How do we know when it is chopped down? How much of it will decompose over what time frame?

The same questions applied to soil carbon are even trickier - how do you prevent soil from being turned over the course of many years?

For manufactured negative emissions technologies, we use the tool called Life Cycle Assessment, or LCA. These assessments are pretty much unknown outside of the sustainability profession, but they are the best way we have to determine how much greenhouse gas a manufactured object releases. Interestingly, to my knowledge, LCAs commonly do not include end-of-life (disposal), in their carbon-release calculation.

Carbon Offsets and Credits

Despite the complications of negative emissions, the carbon offsets industry transfers millions of dollars every year from buyers who want to reduce their emissions into forest providers who plant and maintain trees, and, increasingly, into machines that scrub atmosphere for carbon. Carbon offsets also fund zero-emissions technologies like solar farms, but, for how that works, look out for another article. This "downward flux" of carbon-based gasses and particulate from the atmosphere into the land or ocean is called negative emissions. Measuring negative emissions with confidence is a big question in the carbon offsets space.  

Direct Air Capture (DAC)

Direct air capture (DAC) is the most certain type of atmospheric carbon removal technology, and also the most expensive. It is barely out of the lab, and there are only a handful of DAC deployments worldwide. However, it is very attractive among entrepreneurs and policymakers, because it can measure exactly how much CO2 is extracted from the air. However, at present, direct air capture is not a negative-emissions technology, as, in most cases, it releases more carbon into the air through its manufacture and operation than it captures due to its high energy requirements (see recent article published in Nature).

CarbFix in Iceland. Image courtesy of Wikipedia

CarbFix in Iceland is a rare fully-functioning direct air capture facility, running on abundant geothermal energy available there. Other direct air capture facilities, such as those in development by Carbon Engineering, use fossil fuels, such as natural gas, to generate the heat necessary to capture carbon dioxide from air. In the Permian Basin in Texas, Carbon Engineering is working with Occidental Petroleum to capture atmospheric carbon dioxide for a process called "Enhanced Oil Recovery," where the gas is injected into oil wells to remove more oil from the ground.

As I see it, mechanical direct air capture has at least four major flaws, shared below. Thanks to the AirMiners community for inputs.

(1) the dominant technology requires high heat (though some new technologies currently in the lab, called moisture-swing or electro-swing, don't require heat),

(2) a major potential storage reservoir, geology, is unproven at scale*, and the alternative commercial storage mechanism, aggregate for concrete, already has mature supply chains based on mined limestone, and is slow to change,

(3) 1,600 cubic meters of air need to be filtered for every one cubic meter of carbon dioxide, at least for atmospheric carbon removal (point source is 100-1,000 times more concentrated),

(4) the chemicals required are not a renewable resource, with their own life cycle emissions. Though advances in materials engineering are in the lab, within the field of metal organic frameworks.

*Re. geologic storage reservoir unproven at scale: Susan Dorwald from the AirMiners community comments that "Sleipner has been operating since 1996 and stores .9 Mt/yr. Petra Nova captured 1.7 Mt/yr. The ADM project in Illinois stores 1 Mt/yr." This is a legit comment. However, these are all point-source capture facilities, not direct-air-capture. What is unproven is that a direct-air-capture-to-geologic-storage system will generate true negative emissions through a life cycle assessment.

In Conclusion

Negative emissions, or the downward flux of carbon from the atmosphere into the terrestrial space deserves continued focus and development. This post did not even mention the oceans, where things get trickier: gaseous carbon-based gasses are absorbed by the ocean (it covers most of the planet, remember?), causing acidification and warming. However, aquaculture of sea-plants like kelp is often proposed as a carbon-removal approach (which has co-benefits! <3 <3), where the kelp or other sea-plant absorb atmospheric carbon-gasses through photosynthesis, to be harvested or simply fall to the bottom of the ocean. The big problem in the oceans is that the carbon-based gasses are overabundant at the surface layers of the ocean. If we could only dispersing carbon through the ocean, we'd increase our "carbon budget." But give me one way to effectively disperse the ocean and I'll give you a rainbow unicorn. Good luck. Some might say that we live on Planet Ocean, not Planet Earth... and how we evolve our relationship to the ocean is up to us. More on that later.

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