As a climate economist who has conducted research in the area for more than 40 years, I’ve enjoyed Paul and Cynthia Stancioff’s occasional articles on climate change including the Aug. 26 article on carbon pricing. They have done a great job!

In the same edition, Les Otten had a letter suggesting that in an earlier article they demonstrated a misunderstanding of the science of wood. Also, Rob Riley, William Bell, and Dana Doran weigh in on biomass as a climate solution. Otten’s letter is full of much accurate information regarding carbon and biomass including the fact that, whether biomass (wood) waste is burned to provide heat or left to decompose, the carbon dioxide will end up in the atmosphere.

That is very true, and indeed many supposed experts come to Les’s conclusion—that means it is “climate neutral.” However, “climate neutrality” does not necessarily follow from the fact that in either case the carbon will end up in the atmosphere as carbon dioxide. A simple example will illustrate.

Suppose that every year a truckload of wood waste is produced (from e.g. trimming trees to prevent power outages) and is dumped in a pile and left to decompose. The pile will grow ever bigger, but, as decomposition sets in, some part of the pile will disappear and the carbon will end up as carbon dioxide in the atmosphere. With decay is at a constant proportion of the amount of biomass, the waste pile will continue to grow until the amount lost equals the amount that is deposited each year by our hypothetical truck.

At that point, some 20 or 30 years down the road, we will have reached an “equilibrium-sized” pile of wood waste (carbon stock). Now suppose we change our practice, and decide to turn the wood waste into wood pellets, which are burned to heat our homes (and the whole year’s production of pellets is burned that year).

The pile of waste that had accumulated will continue to decompose, and after a few decades will be totally gone, and all the carbon that had been stored in it will now be in the atmosphere. By turning the waste into wood pellets, over time, we lose to the atmosphere carbon dioxide equal to the amount stored in the waste pile as carbon. The wood pellet practice is not climate neutral in this example.


What if the pile of waste never existed in the first place and we have a new annual “flow” of waste? Well, the same conclusion holds as the flow of waste provides an “opportunity” to create a stock of carbon that is not in the atmosphere, and by burning it we lose that opportunity. (We economists are always thinking about “opportunity costs” as in sitting and reading this article is not without cost because you might have been helping your kids with homework instead.)

Is that the end of the story? Not necessarily. If the wood pellets are used to replace fossil fuels, then one might compare the amount of carbon not stored in the pile (and ending up in the atmosphere) over its now finite lifetime to the amount of fossil fuel emissions avoided over that time period. (Or over any finite period – the shorter the period the worse it looks for biomass.)

Depending on heating efficiencies, possible fossil energy used to produce and transport wood pellets, type of fossil fuel use avoided and lifetime of the hypothetical pile of waste, the carbon offset of using biomass could be positive or negative for the climate over that period. But then what happens after that?

Once the waste pile is gone, the annual flow of wood waste provides a renewable supply of energy that can be used indefinitely to offset fossil fuel use. In a Science article in 2009, I and colleagues termed this initial negative effect on the climate of biomass use a “carbon debt.” Continued biomass use to offset fossil fuel will eventually “pay off” that debt, and be carbon neutral indefinitely (although we may need to be concerned about nitrous oxide emissions linked to the carbon cycle.)

This simple example applies more generally to biomass use. In the Mid-west, the holy grail of bioenergy has long been the use of corn stover (left over corn stalks after the grain is harvested) to produce ethanol as a substitute for gasoline with the idea that this biomass waste is going to decompose anyway.

However, here, again, if left on the field it only gradually decays, spending a good amount of time as soil organic carbon, where it has other soil benefits, but also maintaining and maybe growing the stock of carbon in the soil (and keeping it out of the atmosphere). If the soil carbon stock is not maintained by leaving crop residue in the field than like our wood waste pile, decomposition will lead to a net increase of atmospheric carbon.


There are possible ways to grow biomass, harvest much of the above ground crop, and leave roots and some residue behind that would not deplete or may even increase soil carbon. However, even there, are we then missing an opportunity to store even more carbon? If we have no other option for energy, then rather than freeze to death, we might well be willing to tolerate this initial carbon debt.

However, if we have other no-carbon options for providing heat, then we might not want to pass up the opportunity we have to store carbon in biomass (living, as residue, and as soil organic carbon.)

What are the planetary-scale implications, and should we incur a carbon debt? Our terming this as a “carbon debt” has been controversial in the environmental community as the implication is that we may be willing to accept some damage to the climate as a trade-off to achieve long-term sustainability at, albeit, a slightly warmer climate than if we achieved this without incurring the climate debt.

Incurring debt, if using the money wisely, can be a good thing to do and so for those absolutely opposed to biomass energy our “carbon debt” term is dangerous because it does not automatically criminalize the practice. At the planetary scale, if we have no other reasonable form of energy and would thus need to exist in a much poorer world (no cars, no lights, no heat, no AC), the tradeoff between a slightly warmer but wealthy world, and a much poorer, unwarmed world might be one we would be prepared to accept.

But there are ways out of this that don’t imply a necessarily warmer world, reducing the problem to one of economic cost. For one, the world is going to accumulate more carbon debt regardless of whether we use biomass or not because we are clearly not prepared to turn off the lights and car ignitions tomorrow.

If we followed Paul and Cynthia’s carbon pricing recommendation, pricing all flows of carbon via a global cap on carbon emissions that gradually fell to zero, biomass energy (even paying for the cost of carbon not stored in our waste pile) may make economic sense, but that would mean steeper reductions in use of fossil fuels elsewhere to remain under the carbon cap.


For example, the only reasonable option we (currently) have to replace jet fuel is a synthetic jet fuel from biomass. Rather than give up air travel, we may be willing to use biomass for that option, and go to electric cars and fossil fuel-free electricity a little faster than we would otherwise, or find other ways to enhance carbon storage in soils and vegetation.

We might also hold out hopes for free air carbon capture that could later remove carbon from the atmosphere, or maybe some other carbon-free rapid transportation option, allowing us to then replenish depleted soil and vegetation carbon stocks.

As with most everything in economics, there are few if any absolutes, it is a matter of tradeoffs, taking into account the economics (market and non-market effects) of various technological options as we seek to “maximize our utility” – or put more simply, make the best of what we have for ourselves, our children and our community. So, wood pellets and climate change are not so simple.

I should note for the record, I have a (very efficient) wood burning fireplace which I use because I enjoy looking at a fire but it also serves as a supplementary heating source.

John Reilly of Newry has a Ph.D. in economics and has contributed research on climate change for over 40 years, stepping down in 2020 as Co-Director of MIT’s Joint Program on the Science and Policy of Global Change (, authoring over 100 peer-reviewed articles, most related to climate change, serving as an author on the Nobel-prize winning Intergovernmental Panel on Climate Change and in various advisory roles to the US government. He also spent a couple of decades earlier in his career in the US National Energy Laboratories and the US Department of Agriculture. More recently, he co-authored a policy paper on how agriculture could contribute to solving climate change ( )


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