Negative Carbon Emissions

A carbon dioxide scrubber is any sort of gadget that removes carbon dioxide from the air. There are various ways such gadgets can work, and various things we can do with them. For example, they’re already being used to clean the air in submarines and human-occupied spacecraft. I want to talk about carbon dioxide scrubbers as a way to reduce carbon emissions from burning fossil fuels, and a specific technology for doing this. But I don’t want to talk about those things today.

Why not? It turns out that if you start talking about the specifics of one particular approach to fighting global warming, people instantly want to start talking about other approaches they consider better. This makes some sense: it’s a big problem and we need to compare different approaches. But it’s also a bit frustrating: we need to study different approaches individually so we can know enough to compare them, or make progress on any one approach.

I mainly want to study the nitty-gritty details of various individual approaches, starting with one approach to carbon scrubbing. But if I don’t say anything about the bigger picture, people will be unsatisfied.

So, right now I want to say a bit about carbon dioxide scrubbers.

The first thing to realize—and this applies to all approaches to battling global warming—is the huge scale of the task. In 2018 we put 37.1 gigatonnes of CO2 into the atmosphere by burning fossil fuels and making cement.

That’s a lot! Let’s compare some of the other biggest human industries, in terms of the sheer mass being processed.

Cement production is big. Global cement production in 2017 was about 4.1 gigatonnes, with China making more than the rest of the world combined, and a large uncertainty in how much they made. But digging up and burning carbon is even bigger. For example, over 7 gigatonnes of coal is being mined per year. I can’t find figures on total agricultural production, but in 2004 we created about 5 gigatonnes of agricultural waste. Total grain production was just 2.53 gigatonnes in 2017. Total plastic production in 2017 was a mere 348 megatonnes.

So, to use technology to remove as much CO2 from the air as we’re currently putting in would require an industry that processes more mass than any other today.

I conclude that this won’t happen anytime soon. Indeed David McKay calls all methods of removing CO2 from air “the last thing we should talk about”. For now, he argues, we should focus on cutting carbon emissions. And I believe that to do that on a large enough scale requires economic incentives, for example a carbon tax.

But to keep global warming below 2°C over pre-industrial levels, it’s becoming increasingly likely that we’ll need negative carbon emissions:

Indeed, a lot of scenarios contemplated by policymakers involve net negative carbon emissions. Often they don’t realize just how hard these are to achieve! In his talk Mitigation on methadone: how negative emissions lock in our high-carbon addiction, Kevin Anderson has persuasively argued that policymakers are fooling themselves into thinking we can keep burning carbon as we like now and achieve the necessary negative emissions later. He’s not against negative carbon emissions. He’s against using vague fantasies of negative carbon emissions to put off confronting reality!

It is not well understood by policy makers, or indeed many academics, that IAMs [integrated assessment models] assume such a massive deployment of negative emission technologies. Yet when it comes to the more stringent Paris obligations, studies suggest that it is not possible to reach 1.5°C with a 50% chance without significant negative emissions. Even for 2°C, very few scenarios have explored mitigation without negative emissions, and contrary to common perception, negative emissions are also prevalent in higher stabilisation targets (Figure 2). Given such a pervasive and pivotal role of negative emissions in mitigation scenarios, their almost complete absence from climate policy discussions is disturbing and needs to be addressed urgently.

Read his whole article!

Pondering the difficulty of large-scale negative carbon emissions, but also their potential importance, I’m led to imagine scenarios like this:

In the 21st century we slowly wean ourselves of our addiction to burning carbon. By the end, we’re suffering a lot from global warming. It’s a real mess. But suppose our technological civilization survives, and we manage to develop a cheap source of clean energy. And once we switch to this, we don’t simply revert to our old bad habit of growing until we exhaust the available resources! We’ve learned our lesson—the hard way. We start trying to cleaning up the mess we made. Among other things, we start removing carbon dioxide from the atmosphere. We then spend a century—or two, or three—doing this. Thanks to various tipping points in the Earths’ climate system, we never get things back to the way they were. But we do, finally, make the Earth a beautiful place again.

If we’re aiming for some happy ending like this, it may pay to explore various ways to achieve negative carbon emissions even if we can’t scale them up fast enough to stop a big mess in the 21st century.

(Of course, I’m not suggesting this strategy as an alternative to cutting carbon emissions, or doing all sorts of other good things. We need a multi-pronged strategy, including some prongs that will only pay off in the long run, and only if we’re lucky.)

If we’re exploring various methods to achieve negative carbon emissions, a key aspect is figuring out economically viable pathways to scale up those methods. They’ll start small and they’ll inevitably be expensive at first. The ones that get big will get cheaper—per tonne of CO2 removed—as they grow.

This has various implications. For example, suppose someone builds a machine that sucks CO2 from the air and uses it to make carbonated soft drinks and to make plants grow better in greenhouses. As I mentioned, Climeworks is actually doing this!

In one sense, this is utterly pointless for fighting climate change, because these markets only use 6 megatonnes of CO2 annually—less than 0.02% of how much CO2 we’re dumping into the atmosphere!

But on the other hand, if this method of CO2 scrubbing can be scaled up and become cheaper and cheaper, it’s useful to start exploring the technology now. It could be the first step along some economically viable pathway.

I especially like the idea of CO2 scrubbing for coal-fired power plants. Of course to cut carbon emissions it would be better to ban coal-fired power plants. But this will take a while:

So, we can imagine an intermediate regime where regulations or a carbon tax make people sequester the CO2 from coal-fired power plants. And if this happens, there could be a big market for carbon dioxide scrubbers—for a while, at least.

I hope we can agree on at least one thing: the big picture is complicated. Next time I’ll zoom in and start talking about a specific technology for CO2 scrubbing.

33 Responses to Negative Carbon Emissions

  1. Ishi Crew says:

    I’ve recently heard or read some amazing stories about various ‘carbon scrubbing’ technologies. Maybe some will work–i don’t know. I also heard about some ideas for using algae grown in the sea as a renewable fuel source –not too different from wood grown on land (i used to cut wood — basically downed and dead trees to use for firewood in west virginia and alaska).

    Some other people are focusing on ‘reforestation’—planting trees.My area we don’t need that–trees will grow back but you have to stop cutting them down.

    The other local issue is ‘fracking’. including gas pipelines for the fracked gas. Maybe 30% of energy in my area is still nuclear powered, and some from coal–still may be 30%, though that is a declining but not extinct industry. WV still has alot of coal.

    I think behavioral changes are also relevant. Less commuting by car or plane (take a bus, walk, or ride a bike if you really have to go somewhere). Buy less of what you dont need (something i’m incapable of). We also do park cleaning, some tree planting, gardening around here. We also have reintroduced or protected some endangered plants, trees and animals. So we have a small herd of buffalos and dinosaurs here.

  2. John Baez says:

    Ishi wrote:

    I’ve recently heard or read some amazing stories about various ‘carbon scrubbing’ technologies. Maybe some will work–I don’t know.

    A lot of people writing these stories don’t check whether these technologies can become practical at large scale. This one is a bit better:

    • Jon Gerntner, The tiny Swiss company that thinks it can help stop climate change, New York Times Magazine, 12 February 2019.

    Some other people are focusing on ‘reforestation’—planting trees.My area we don’t need that–trees will grow back but you have to stop cutting them down.

    Preventing deforestation is important for fighting climate change. According to Wikipedia:

    Deforestation is the second largest anthropogenic source of carbon dioxide to the atmosphere, after fossil fuel combustion. Deforestation and forest degradation contribute to atmospheric greenhouse gas emissions through combustion of forest biomass and decomposition of remaining plant material and soil carbon. It used to account for more than 20% of carbon dioxide emissions, but it’s currently somewhere around the 10% mark. By 2008, deforestation was 12% of total CO2, or 15% if peatlands are included. These proportions are likely to have fallen since given the continued rise of fossil fuel use.

  3. Let’s achieve net-zero carbon dioxide emissions first, somehow, please!

    Someone I know looked into how many trees it would take to offset the carbon dioxide from their flights US-India, and it is way too many. Maybe carbon dioxide scrubbers are relevant to making aviation be carbon-neutral, because replacing fossil fuels in airplanes might be extremely difficult. That is, we would make it required to sequester as much carbon as is in whatever aviation fuel is produced, and that would be the use for the carbon dioxide scrubbers. I think we’d make the producers of aviation fuel responsible for that task, and the cost would be added to that of the fuel. (Yeah, in which socialist paradise is this going to happen? :) )

    We can hope that renewable electricity might power most, if not all, ground transport. I don’t know about shipping though.

    • John Baez says:

      Clearly our first main aim should be reducing carbon emissions. I just happen to be studying (and soon writing about) a technology for carbon dioxide scrubbing.

      Regarding trees, David McKay writes:

      The best plants in Europe capture carbon at a rate of roughly 10 tons of dry wood per hectare per year – equivalent to about 15 tons of CO2 per hectare per year—so to fix a European’s output of 11 tons of CO2 per year we need 7500 square metres of forest per person. [….]

      And then you’d have to find somewhere to permanently store 7.5 tons of wood per person per year! At a density of 500 kg per square meter, each person’s wood would occupy 15 cubic metres per year. A lifetime’s wood—which, remember, must be safely stored away and never burned – would occupy 1000 cubic metres. That’s five times the entire volume of a typical house. If anyone proposes using trees to undo climate change, they need to realise that country-sized facilities are required. I don’t see how it could ever work.

      Nonetheless, deforestation is something we should strongly avoid—not just because of the CO2 released, but because of the habitat loss. And afforestation is generally a good thing.

      The passage marked “[….]” is a nasty mistake in McKay’s book, where he writes:

      This required area of 7500 square metres per person is twice the area of Britain per person.

      Unless Britain has shrunk a lot since I last visited, I bet he means that 7500 square meters per person, times the population of Britain, is twice the area of Britain. Let’s see if that’s true.

      There are 272 people per square kilometer in the UK (which is not quite the same as Britain), and that’s 3680 square meters per person. So yes.

      • Mikko Kiviranta says:

        Where I live (Finland) there has been a suggestion of driving the annual growth of our forests into bogs and marshlands (which are plentiful here) where the wood would decompose to peat in oxygen-free conditions. The actors doing this would get emission permits in exchange, an element of the cap-and-trade system, which they could then sell to germans. This would make the effort profitable for those actors.

        Well, the european cap-and-trade system in its current form does not work that way, I think, and I don’t know offhand how scalable the approach would be. I recall having seen estimates where the energy content of the pre-existing peat deposits in Finlsnd were compared to the norwegian oil fields in the North Sea.

  4. Toby Bartels says:

    Scrubbing carbon for use in soft drinks seems particularly pointless, since that carbon will be emitted again shortly. But perhaps it’s an improvement over other sources. If most carbonation is synthesized from bound carbon, then it’s directly harmful and replacing it with neutrally-sourced carbonation is an improvement.

    But of course, the real value is the scrubbing technology, not the current use.

  5. domenico says:

    I am thinking that if it were possible calculate the emission of carbon dioxide for the production of a single product, including transportation at the point of sale, then it would be possible to include additional taxation proportional to the production of carbon dioxide (a label that contain the carbon dioxide emitted and the taxation): if any product is composed of certified component, then the calculation would be simple (component carbon dioxide emission+component transport emission).
    The overall effect would be the approaching of supply sources (the same product contain a reduced transport emission), and the approaching of the sources of production (making national production), reducing the cost of products (lower emission means lower consumption, and lower costs), and the overall effect should be a reduction of emissions (both for production and transport).

  6. Timber says:

    Seems to me that the best we could do at the moment for CO2 reduction is to promote large-scale reforestation. Perhaps using a civilian service corps, and tax incentives based on canopy cover?

    • John Baez says:

      Even more important than reforestation is preventing deforestation, which rather quickly releases lots of CO2. But they’re closely connected.

      The United Nations REDD+ program—“Reducing Emissions from Deforestation and Forest Degradation”—is doing lots of good things. People in various countries can hook up with them. Read some stories!.

      China has sent 60,000 soldiers to plant trees, and this is making the Earth noticeably greener as seen from satellites.

      But where is the United States’ version of Civilian Afforestation Corps? And yes, tax incentives.

      • Timber says:

        Slightly disagree – RE-forestation because it could significantly increase the proportion of YOUNG trees, could do more CO2 abatement than the equivalent amount of DE-forestation prevention. BTW, you have a truly awesome cousin – I do, too. The ESA ExoMars Rover was recently named after her!

      • John Baez says:

        I guess comparing “stopping deforestation” to “reforestation” is a bit like apples and oranges, so my remark was sort of silly. We could compare the total carbon released by deforesting a hectare of some sort of forest to the total carbon absorbed by planting a hectare of some sort of forest… and that would be interesting to do, but I haven’t seen that sort of data.

        On another note, this article is interesting:

        • Robson Fletcher, Canada’s forests actually emit more carbon than they absorb — despite what you’ve heard on Facebook, CBC, 12 Feburary 2019.

        It should be taken with a grain of salt, but it seems carefully written and sourced. It’s referring only to the ‘managed’ forests:

        meaning “all areas that are managed for harvesting, subject to fire or insect management, or protected as part of a park or other designation”.

        The basic idea is this: carbon emissions due to wildfires are exceeding the drawdown due to new growth. On the downside, lots of fires. For example:

        In 2015, largely due to raging wildfires, these forests kicked a whopping 237 more megatonnes of carbon dioxide into the atmosphere than they absorbed.

        On the upside, new growth:

        The “sink” effect is largely the result of new trees being planted and growing, after mature ones are cut down.

        The harvested lumber, meanwhile, releases its carbon to the atmosphere more slowly. The eventual decomposition of lumber products is actually included as part of our greenhouse-gas accounting, Blain said, with scientists estimating the related carbon emissions over a period of decades.

        On average, areas subject to forestry activity have been a net sink of roughly 26 megatonnes annually since 2001.

        But the upside has been smaller than the downside ever since 2009.

        • HI John and Timber

          For a sobering view on afforestation and reforestation you might find an article by Simon Lewis in April 4, 2019 issue of Nature. Yes, we need to reforest as much as we can, but can we?

          Anyway, it is a good discussion.



  7. Alex says:

    I’m particularly interested in what your research would show regarding Sustainable Energy Solutions (a company in Utah) approach of scrubbing coal plant flue gas. On one hand they focus on really concentrated sources of CO2, on the other hand they claim rather good energy efficiency for their approach. Can it be scaled to clean up regular air?

    • John Baez says:

      I’ll check them out. I’m actually more interested in technologies for scrubbing coal plant flue gas, since this sort of scrubbing is a lot more likely to be something people do in the next few decades than cleaning ordinary air. It’s easier to do effectively (since the CO2 is more concentrated), and it’s more likely to be something that regulations or a carbon tax or a cap-and-trade system will make happen. I think massive scrubbing of ordinary air is more likely to happen after we get much better technologies, like self-replicating systems of some sort.

      • Timber says:

        Trees and other plants are known to self-replicate :-)

      • John Baez says:

        Yes, they just don’t draw down as much carbon as we’d like, since that’s not their “goal”: they didn’t evolve to sequester carbon. We may develop self-replicating entities, either through biotechnology or nanotechnology or other means, that are optimized for turning carbon dioxide into harmless solids. They might be like plants or algae, or they might be quite different.

        (Now I’m talking about what could easily happen in 100 years, assuming our technology continues to advance. In my article, and most of what I’ll be writing in this series, I’ll be more focused on the next decade or two.)

  8. Steve Wenner says:

    I like the idea of using renewable energy to power carbon capture to make carbon fuels, which can be regarded as renewable energy storage or used as net-zero aviation fuel, etc. In particular, I was reading in the news recently about the company Carbon Engineering affiliated with Harvard. It seems they have a pilot plant at a paper mill in British Columbia (powered, I guess, by waste heat from the mill). Whether they have the right approach depends on the efficiency of the process, and I haven’t found any details.

  9. grlcowan says:

    Yes, getting a trillion tonnes of carbon dioxide out of the atmosphere is going to be a big job.

  10. zephirawt says:

    How much carbon dioxide emissions the carbon dioxide scrubbing would actually produce? The Climeworks experience could serve as a rational basis for such a calculation. How much iron and concrete their technology consumes during its life-time – and how much carbon dioxide it actually scrubs from atmosphere?

    • John Baez says:

      I don’t know the full accounting for any sort of carbon scrubbing technology. The Climeworks machinery is so experimental it’s not much like any large-scale industrial process… but any data would be better than none.

      Here’s an interesting article about this issue:

      • Sarang Supekar and Steve Skerlos, The latest bad news on carbon capture from coal power plants: higher costs.

      The basic idea is that if carbon capture and storage creates a rather obvious feedback effect, which people hadn’t been taking into consideration:

      The feedback loop arises because the excess fuel burnt to generate additional electricity and steam for CCS itself creates CO2 that must be captured and stored. Capturing this CO2 in turn requires more fuel, which again creates more CO2. Although previous studies had defined an energy penalty, they ignored this obvious feedback loop, with the result being a gross underestimation of CCS costs.

      Sounds like a geometric series is called for. Anyway, they redid the calculations.

      • Timber says:

        Easy to do if one knows the penalty factor. One could mandate that the penalty energy be supplied as a renewable offset.

  11. Paul Campbell says:

    One thing we can do is to stop recycling paper – effectively throwing that carbon into landfills (or maybe down old coal mines). That will increase the economic demand for trees and result in growing more and more carbon capture

    • Steve Cohn says:

      Throw it away, thats how we got into the current mess. The logic may work after maximizing the ability to recycle so that the fall back position isn’t destructive.

  12. Murray Hepple says:

    There is a proposal for carbon scrubbing which has been floating around for a while which does not involve excessive land usage or coal-fired power plants. Instead it involves large scale, ocean based, wave powered algae production. It is an interesting idea which I feel should be explored further given the current difficulties with atmospheric scrubbers and the urgency of the problem.

  13. domenico says:

    I heard of the use of cyanobacteria for the chemical synthesis of molecules, modifying the dna
    If I understood correctly, the chemical synthesis for chemical factory, using cyanobacteria instead of bacteria, is economically advantageous (for the air extraction of the carbon for the synthesis and the sunlight energy for the growth) and if it were possible to generalize the production of complex molecules, perhaps extending the bases of DNA of cyanobacteria, then from a natural source of energy (the photosynthesis) it may be possible to extract carbon dioxide from the air, not using petroleum for the chemical synthesis in the factories.
    I think that if the Great Oxygenation Event caused a global change in atmospheric composition (because of the cyanobacteria), then a controlled use of microorganisms could cause a Great Carbon Dioxide Removal Event (only the timing of the GOE event was long 100 megayears)
    the process could be accelerated optimizing the Calvin cycle, as it was done with tobacco.

    • John Baez says:

      I like the idea of harnessing cyanobacteria, possibly genetically engineered, to do good things. If they can suck CO2 out of the air, powered by sunlight, and turn it into some substance people like, it could in theory be commercially practical. And if it also sequesters carbon, that would be nice.

      For example, suppose we made all our plastics this way. Since we only used 348 megatonnes of plastic in 2017, that would only convert ~1% of our carbon emissions into useful material. But it would be lot better than the new ethane cracker plants people are planning in the Appalachians!

  14. Eugene says:

    Slightly off topic but interesting:

    Aditya Prajapati, Meenesh R. Singh. Assessment of Artificial Photosynthetic Systems for Integrated Carbon Capture and Conversion. ACS Sustainable Chemistry & Engineering, 2019; DOI: 10.1021/acssuschemeng.8b04969

  15. I’ve been talking about new technologies for fighting climate change, with an emphasis on negative carbon emissions. Now let’s begin looking at one technology in detail […]

  16. I usually join internet discussions because I am an amateur enthusiast on math and physics. But my “day-time job” happens to be gas separation engineering.

    Good idea John, to start talking about the CO2 problem.

    One crazy idea I had, was to scrub CO2 on Antarctica. On Antarctica, we have a lot of energy, especially wind. The katabatic winds. (
    We could use this energy to spray alkaline water in the air, capturing CO2. This would fall as CO2 rich snow on Antarctica. Most of Antarctica is actually a desert, so this snow will just stay there for thousands of years (after which, we might be in a next ice age).


    • John Baez says:

      Wow, what sort of gas separation engineering technologies do you use, or study?

      I like your idea. Of course it’s a bit crazy, but they all are. I wonder how alkaline the water would need to be, and how long it would need to remain liquid (or could it work when frozen?). It seems a bit hard to spray a lot of liquid water into the Antarctic air.

      Those katabatic winds are indeed impressive.

  17. Hi John

    Very glad to have found your blog. I love it. My take on DACCS is that the cost is not the important issue. It is an issue and very irresponsible of us to leave this cost to our children and grandchildren. But the important issue is energy costs. It takes between 0.15 and 0.3 Mtoe of energy to capture 1 MtCO2 from the atmosphere. We need to pull in about 730 Billion Tons total by 2100, of course we need to continue to do this after that. The rate of extraction at 2100 is about 18 billion tons. The energy required to do this is therefore, between 2.7 and 5.4 Billion tons of oi equivalent energy every year just to capture it. That does not include sequestering it. I have not found the energy cost of sequestration yet as nobody knows how to do it.

    My take here:

    best regards


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