Science corner: why carbon sequestration isn’t any better than avoided emissions


Update: This post has engendered a fair amount of confusion, for which I am mostly to blame. So let me provide some necessary context. Tree-planting projects are a popular type of carbon offset, but they unfortunately suffer from a large number of quality problems and do not in fact make for very good offsets.

However, one recurring argument made in favor of tree-planting projects is that they perform carbon sequestration rather than preventing new emissions. That is, trees absorb carbon that is already in the air, which some see as a useful complement to reducing fossil fuel use.

This notion rests on a misconception. In reality, there is no meaningful distinction between carbon already in the air and carbon not yet emitted. You should instead use other metrics to gauge offset quality, such as measurability, permanence, timing, and additionality. None of these metrics tend to favor tree-planting projects.

OK, on to the original post…

* * *

I’m on a tree kick lately, in case you didn’t notice. Today I’m going to set aside the controversial stuff and instead deliver some straight-up knowledge. Specifically, I’m going to address the common misconception that offsets from carbon sequestration (e.g., tree-planting) are somehow different in kind than offsets from clean energy (e.g., wind).

The misconception stems from a seemingly sensible intuition that sequestration deals with carbon that’s already up in the air, whereas clean energy deals with carbon that hasn’t yet been emitted. It’s the difference between cleaning up a mess you’ve already made vs. preventing a new mess from occurring, right?

Well, kinda sorta, but the distinction isn’t environmentally meaningful. Assuming the offsets are of equal quality, the end point in both cases is exactly the same. There are a couple of ways of explaining why this is so. I’ll mention two.

The first is to consider the fact that clean energy can be thought of as a type of carbon sequestration. When a wind turbine spins, it is in effect sequestering carbon in the form of coal, which otherwise would have been burned for electricity. This is an admittedly strange way of thinking about what a wind farm does, but it’s perfectly valid. Trees sequester carbon in the form of wood. Wind turbines sequester it in the form of coal. It’s not particularly relevant whether the carbon was formerly blowing around in the air or buried beneath our feet. All that matters is that the carbon is kept out of the atmosphere.

The second way is to consider the bucket analogy. You’re filling a bucket with water. You want a drink. You can fill your cup by dipping it into the bucket or you can fill it with water flowing from the tap. The end result is the same: the bucket is less full by a cup.

For those who like numbers, the bucket analogy can be turned into a simple algebra problem. There are ten tons of carbon already in the atmosphere, and every year enough coal is burned to add one more ton. This year, you can either switch from coal to wind energy (catching one ton of carbon at the tap). Or you can continue burning coal and also plant a bunch of trees (scooping one ton of carbon from the bucket). In either scenario, you’ve got ten tons of carbon in the atmosphere at the end of the year. QED.

Incidentally, the same logic applies to the misconception that trees clean the air of pollutants but wind turbines don’t. Trees are often credited with absorbing or filtering pollutants such as nitric oxide, sulfur dioxide, and carbon monoxide. But where do you suppose these pollutants come from? Right: from burning fossil fuels. So wind farms — by displacing the burning of fossil fuels — have a similarly salutary effect on air quality.

And this logic further applies to energy efficiency measures, such as installing a compact fluorescent light bulb. To paraphrase Gertrude Stein (no relation), a ton is a ton is a ton of CO2, which is why carbon offsets from different project types can be freely traded.

Photo available from flickr user Peasap under Creative Commons license.

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  1. matt - August 31, 2007

    hmm, I must say I read your posting and I don’t think that it cleared anything up for me. Saying that I am not a fan of tree planting as an offset project is a giant understatement. What happens when the tree is cut down after a few years? How much carbon did it sequester? If the tree is burned down how much carbon did it sequester versus release into the environment from burning. There are a lot of scenerios I can paint that might not make trees the best choice for offsetting carbon.

  2. Tom Arnold - August 31, 2007

    Hi Matt:
    We’ve covered these issues on trees before — these reasons are one of the main reasons that that trees are currently not in our portfolio.
    But, I think the question Adam was attempting to answer is “Why are energy based offsets the right thing to do? Shouldn’t we be trying to absorb the CO2?”
    That’s where the bucket analogy comes in.

  3. Chad - September 5, 2007

    I agree with Adam. It is irrelevant whether we reduce atmospheric CO2 by releasing less or absorbing more. The result is the same. Equally true is the fact that it is irrelevant if I reduce CO2 directly by myself, or pay someone else to do it. Again, the result is the same.
    If paying someone to absorb CO2 is cheaper than cutting my own emissions, then that is what I should do. To do anything else is wasteful, and to believe either route is morally superior to the other is nothing but religion.
    One could argue that forest-based sequestration doesn’t WORK, but that is another matter entirely.

  4. John Grant - September 5, 2007

    An interesting point of view but it seems to me there is a flaw in the argument of sequestering being equal to displacement of Carbon. This is the issue of timescale, true carbon sequestered in trees is still carbon removed from the atmosphere so in the present and during the lifespan of the tree it can be treated as exactly the same effect of displacing the burning fossil fuels.

    However, fossilised carbon is locked away for possibly millions of years where as sequestered carbon is only “locked away” for the lifetime of the tree. Does not sequestration just hand over the problem to my grandchildren? They will have to continue sequestering ever increasing amounts of carbon rather than leaving it safely beneath our feet, as we use an alternative technology which has a very limited or zero carbon footprint.

  5. Adam Stein - September 5, 2007

    Hi all,
    It’s clear that I biffed part of this post pretty bad. Matt and John, you are correct that there are a variety of issues with tree-planting as a source of carbon offsets — including permanence and measurability — many of which I address here.
    The purpose of this post was to address a common defense of tree-planting projects, the notion that they are different in kind than renewable energy projects because they deal with carbon that is already “up there.” In fact, this defense is just an accounting trick with no environmental significance, which was the main point of this post.
    So our recommendation continues to be to focus on projects that offer the highest quality, regardless of whether they come from sequestration, renewables, or whatever. And for the time being, we will continue to avoid tree-planting projects.

  6. Chad - September 5, 2007

    Both Matt and John seemed focused on a “tree”, when I would argue that they need to see the forest. It is generally irrelevant what happens to the trees one plants (whether they are harvested, burned, or allowed to rot), as long as their seedlings are allowed to replace them. The end-of-life of a tree is carbon neutral as long as it is replaced.

  7. Jane - September 5, 2007

    How do we know clean energy generation is actually preventing the production of “dirty” energy? After all, demand responds to supply.
    Europe already has carbon trading. The kind of offsets I’d really like to see would involve buying up and retiring these emissions permits. Then you’d know exactly what your purchase accomplishes.

  8. Adam Stein - September 5, 2007

    Hi Jane,
    This is a pretty common question about offsets — does stimulating the production of clean energy displace dirty energy, or does it just increase energy consumption? It’s actually a tricky question to answer definitively, but the really short answer is that energy demand is fairly inelastic — the amount we consume doesn’t seem to be heavily dependent on the supply.
    Regarding Kyoto offsets, this is something we’d consider doing for Phase II of Kyoto. Phase I of Kyoto was marked by an overallocation of allowances, and our purchases wouldn’t have made a dent in carbon emissions. But Tom is the real expert here, so I’ll stop before I say something incorrect.
    – Adam

  9. Tom Arnold - September 5, 2007

    Hi Jane:
    We’ve thought pretty hard about that strategy, and if we get a good firm cap in the United States, it would be a good way to in essence lower the cap even further. This strategy relies on the cap being firm, and as Adam correctly points out the first European (ETS) phase of allowance trading was too lax and price dropped to zero.
    We’re not even there yet in the US, so if we want to build a carbon market its going to be based on project based reductions.

  10. Tony S - September 6, 2007

    Agree fully that article is misconceived – comparing the biosphere over 100s of years with geological-based time scales is scientifically dangerous.
    A tonne of carbon IS a tonne, but difference between sequestering it with trees and leaving it in the ground IS NOT the same. Selecting only specific issues such as air pollutants absorption, without balancing the fact that both N2O AND CH4 are emitted by the biosphere is unbalanced. I’m sure many know how academic papers have suggested tree planting in temperate zones can create an overall WARMING in the climate due to foliage and other factors. This may be the opposite in tropical zones, but again the science isn’t clear enough.
    As such, I drink from the bucket everytime.

  11. James - September 6, 2007

    I agree that offsetting carbon from the displacement of fossil fuel generation is just as effective as sequestration. However, in order for there to be real carbon reduction through renewable energy it is imperative that the money paid for carbon credits only goes to projects that are truly additional. It is also important to ensure that the CO2 savings are not sold twice (once as a REC and once as an offset). I fear most of the CO2 offsets sold these days are purchases from renewable energy plants AFTER they have already made the decision to construct, and therefore cannot be considered additional.

  12. Adam Stein - September 6, 2007

    Hi James,
    We’re on the same page on these topics. It certainly is the case that TerraPass carbon reductions are sold only once (and in fact, I think double-selling is more of a theoretical problem than an actual one in today’s market). And additionality is pretty much our topmost concern when we make any purchase.
    – Adam

  13. Adam Stein - September 6, 2007

    Tony —
    Don’t you mean you drink from the tap? It seems like you’re stating a preference for renewables over trees.

  14. Karen/ - September 6, 2007

    Do you have plans to discuss reusing/recycling in the near future? Do these types of efforts qualify for carbon offsets? Are they as valuable as the tree planting or clean energy production you mention here? Our business involves resharpening tools, such as carbide-tipped saw blades. Much less energy is used to recondition saw blades (and also knife and other tool blades) rather than producing new blades. Plus, keeping high quality steel out of the landfills for as long as possible seems to make sense. What are your thoughts?

  15. Adam Stein - September 6, 2007

    Hi Karen,
    My thought is that reuse and recycling is awesome, for the reasons you cite. However, I suspect that the activities your business engages in would not be suitable as a source of carbon offsets, for two reasons.
    The first is that it would be hard to measure the carbon reductions associated with reusing a saw blade vs. making a new one.
    The second (and bigger) problem is that your business is probably already profitable, and therefore the carbon offsets themselves wouldn’t be responsible for the emissions reductions.
    None of this takes away from the good work you’re doing. It’s just that there are a lot of technical requirements governing carbon offsets. Hopefully this won’t diminish the satisfaction of doing an environmental good deed.
    – Adam

  16. Ed - September 8, 2007

    If climate is changing because of human action (despite what FOX News says), and because of the carbon already in the atmosphere, then it would seem that finding ways to remove the carbon already there is our most urgent need. As more environmentally-friendly technologies are developed, carbon emissions per person should go down.
    In advanced countries there are people (and even politicians) who are trying to limit their carbon footprint. My impression, though, is that most are not. If a few use less, but most continue as they always have, and the carbon that’s already there hasn’t been removed, the point of no return is reached sooner.
    Pulling carbon out of the atmosphere now buys us some time.
    Go to (retreating arctic ice) for some scary graphics.

  17. Jason - September 9, 2007

    I think both this article and the previous concerning trees is a great commentary on a different look at being environmentally friendly.
    What most naysayers have negated to see is that, in both articles, you are not pushing for abolishment of tree planting as means for carbon offsetting, but rather putting perspective into its immediate advantages.
    I fully understand and agree with your argument.
    Shameless plug:

  18. Adam Stein - September 10, 2007

    Hi Ed,
    If climate is changing because of…the carbon already in the atmosphere,then it would seem that finding ways to remove the carbon already there is our most urgent need.
    This would be true if we had already stabilized the level of carbon in the atmosphere. We have not. Right now we’re at about 385 ppm CO2 in the atmosphere, and we’re probably going to hit at least 500 ppm before we stabilize. So it is not the case that removing carbon already in the atmosphere is our most urgent need. Please see the article above for more details.
    – Adam

  19. Anonymous - September 10, 2007

    Again, you state additionality is of utmost concern when purchasing. TerraPass is lucky the education of the voluntary market is nascent when it comes to what additionality is. In no way is the CCX additional, in no way are RECs additional. You know this and are still selling those assets as offsets. Is that fraud?

  20. Tom Arnold - September 10, 2007


    TerraPass has always adhered to the best market standards available in the US, and is constantly seeking to improve those standards. I think you’ll find that if you look at the appendix of the new renewable energy protocol from CRS, that it’s hard to imagine that wind is business is usual. We’ve asked them to strengthen that approach even more with the application of of a financial additionality test, or market survey. You can read our comments, or even add your own here:

    Secondly, many folks misunderstand how we work on the CCX — we work with project specific reductions that are registered on the CCX. That gives us a chance to review all the details and run additionality tests above and beyond that which the environmental audit committee imposes. One of these projects was challenged in the press, and we ran a complete outside review of it with leading academics and they judged it additional.

    Add on top of this the fact that TerraPass is annually audited by another third party organization, and I think you have a pretty good case that we are doing the best we can with available standards.

    I don’t dispute that the credibility of the market can grow even more. I do want TerraPass to lead that charge.