A Semi-Annual Publication of Manomet

By: John Hagan

It was a frosty morning near the outpost of Kokadjo—not unusual for June in northern Maine. I was standing in the middle of an 80-acre clearcut. It was 1991; my introduction to industrial forestry. It looked like a war zone. But it smelled like Christmas. The fresh-cut stumps of fragrant balsam fir elicited happy holiday memories. The visual and olfactory input was hard to reconcile. That clearcut became a metaphor for what was about to unfold—a research career in forestry and ecology filled with one annoying paradox after another. The latest, and most complex, has to do with using wood for energy.

One of the first lessons I learned from foresters when I began studying birds in the vast timberlands of Maine was the notion of "annual allowable cut." At first it was just forestry jargon. But when I finally got it, it seemed like magic.

If you manage large tracts of forest for timber (sawtimber and pulpwood and other wood products), to stay in business for the long run you need to calculate an "annual allowable cut." That is, you calculate how much your forest will grow in the coming year and that amount becomes how much you allow yourself to cut in the coming year. To be sustainable over time, you cut only the new growth. It involves a lot of math and growth equations, but it's not rocket science.


ABOVE: The author working with timber companies on sustainable forestry practices.

Although individual trees come and go, the overall amount of timber volume on the landscape never really changes from year to year, even with harvesting. You have an in perpetuity supply of wood and paper. Sunlight and water fall from the sky. Trees grow. It's photosynthesis, but it seems like magic. Relative to the stock market, forests are far more predictable. They just grow, dependably.

The concept of annual allowable cut is critical because it forms the basis of the idea that using wood for energy is "carbon neutral." If you burn a tree (or trees) for energy, the CO2 emitted into the atmosphere is exactly offset by the new growth of all the other trees in your forest. You don't have to wait 50 years to grow a replacement tree because the thousands of other trees in the landscape that weren't cut are doing that for you. Each year they are increasing their collective diameters just enough to offset what was cut. Presto! A carbon neutral source of energy. It could not be more simple or elegant or easy to understand.

Or, that's what most everyone thought (including me) before we conducted the "Manomet Study" for Massachusetts Department of Energy Resources (DOER) in June, 2010. The study became controversial precisely because it challenged the commonly held belief that biomass energy is carbon neutral.

Massachusetts Sets Climate Change Goals

In 2008, Massachusetts passed its Global Warming Solutions Act. The Act set a goal of reducing greenhouse gas (GHG) emissions within the state to between 10 and 25 percent below 1990 levels by 2020, and 80 percent below by 2050. This is the level of reduction most scientists agree is needed, globally, to keep climate change to only about 4 ºF of warming this century. Massachusetts was intent on doing its fair share.

Burning wood for energy in place of fossil fuels like coal or natural gas figured prominently into achieving this ambitious emissions reduction goal in Massachusetts, because, unlike fossil fuels, wood was thought to be "carbon neutral," as explained above. You don't have to count the CO2 that goes into the atmosphere from burning wood for energy because new growth in a sustainably managed forest landscape pulls all of it right back out of the atmosphere. So, switching to wood energy made perfect sense as a strategy to reduce greenhouse gas emissions.

But some concerned citizens in Massachusetts raised questions about whether the carbon neutral assumption was true. There was almost nothing in the scientific literature on carbon accounting for woody biomass at the large scale of a state, even a small state like Massachusetts. Everybody knew it was carbon neutral. Why waste money to study something that was so obviously true?

Nevertheless, in the spring of 2009 the Massachusetts Department of Energy Resources (DOER) issued a request for proposals to study, among other things, the GHG emissions of increasing the state's reliance on woody biomass for energy. Is wood really carbon neutral? No other state in the U.S. had asked the question as a part of formulating science-based climate change policy.

In November of 2009, Manomet, and several of our sister institutions received the contract from DOER to do the study. We had just six months to answer the GHG riddle. DOER and Manomet jointly released the study on June 10, 2010. Within an hour of the release the Associated Press headline went viral—"Mass. Study: Wood Power Worse Polluter Than Coal." (AP's words, not Manomet's.)

Complexity Simplified (?)

How could wood not be carbon neutral, especially if the wood is harvested from a certified sustainable forest? Answer: CO2 accounting doesn't work like accounting for sawtimber and pulpwood growth. I'll try to explain what is a very complex paradox.

What the atmosphere will see (in terms of greenhouse gas levels) is the critical question when formulating policy to reduce climate change by reducing emissions. It turns out that just because forest carbon levels remain stable across the landscape year to year doesn't necessarily tell you what the atmosphere "sees" when society makes a switch from fossil fuels to wood for energy.

Let me start with what the team found, and then I'll come back and explain how they got there.

If a state like Massachusetts, or even a group of countries like the European Union, decides to switch some of its energy source from fossil fuels to woody biomass, there will be some period of time, from a decade to perhaps a century, when what the atmosphere sees in terms of GHG concentrations is actually greater than if you had just kept on using fossil fuels. The study team called this increase in atmospheric GHGs a "carbon debt." (Figure 1).

Figure 1 — 'Debt-Dividend' Model for GHG Emissions

Figure 1

FIGURE 1: In Year=0 a fixed amount wood biomass is used to generate electricity and the resulting CO2e emissions (tonnes) is graphed over time. For comparison, the amount of coal emissions to produce an equivalent amount of energy is graphed. Initially, wood biomass results in a 50% greater emission per unit of usable energy due to the lower embedded energy of wood. This is called the "carbon debt" relative to fossil fuel. However, forests recapture the carbon over time with new forest growth. CO2 emissions from coal cannot (affordably, practically) be recaptured at present. When the forest regrows to the point that the fossil fuel carbon debt is "paid off," then the atmosphere begins to accrue a "carbon dividend."

Over time, with forest growth , the carbon debt to the atmosphere can be "paid back" to the point where we begin to realize carbon "dividends" from wood energy. That is, at some point in the future there is less CO2 in the atmosphere as a result of using wood for energy instead of fossil fuels. Over time, these dividends can pile up, resulting in greater and greater reductions in atmospheric carbon relative to what would have been the case in a world without wood energy.

Unfortunately, the debt comes before the dividend. To comprehend the reasons for the initial spike (debt) in atmospheric CO2 and also why the atmosphere eventually sees significant CO2 reductions, it's critical to understand the answers to two questions.

First question: Why does the atmosphere see an initial spike in CO2 concentrations?

The reason for the initial spike in atmospheric GHGs is easy. Wood is less efficient than fossil fuels. To produce one "unit" of electricity or heat with wood will release about 50% more CO2 into the atmosphere than using coal. And producing a unit of energy with wood instead of natural gas (a much more efficient fuel) releases 300% more GHG into the atmosphere. It's physics.

On Day One, when you turn on the biomass power plant, more CO2 will move into the atmosphere than would have if you had produced the same amount of energy with fossil fuels. This is an important point, because if wood released less GHGs than fossil fuels per unit of energy, wood energy would benefit the climate right away.

Second question: Why doesn't the whole forest landscape fully offset emissions each year?

LoggingYou may be thinking… 'Why does the inefficiency (and excess emissions) of wood matter if all the carbon going into the atmosphere is just going to be recaptured by a growing forest in the same year?'

Whether you can offset that initial "debt" in emissions with new forest growth in the same year depends on whether the millions of trees in the rest of the landscape would have grown anyway, under a what's called a business-as-usual scenario (that is, a scenario where you didn't change your forest management practices to produce biomass and just kept on using fossil fuels).

Whether you are talking about a small state like Massachusetts, or a larger "working forest" state like Maine, the answer is—those trees would have likely grown anyway, at least over the next several decades. They would have pulled CO2 out of the atmosphere anyway, with or without the biomass industry. Technically (in strict terms of what the atmosphere sees), you're not offsetting the emissions, if the remaining trees would have grown anyway.

This is the key: to accurately estimate what the atmosphere will see as a result of switching to wood for energy you have to compare two future scenarios: one that involves switching to wood, and one where we continue to rely on fossil fuels and don't expand our reliance on forest biomass for energy.

To estimate what the atmosphere will see (and what policy makers need to know) the two scenarios have to be modeled, side-by-side, and all the GHG molecules have to be present and accounted for in both the forest and the atmosphere. It's a math problem. Of course, there are many other interesting scenarios, like increasing energy conservation, or switching to wind energy. But those other scenarios would not answer the question about the implications of switching to wood from fossil fuels, which was the team's charge from DOER.

The GHG accounting approach proposed by many critics of the Manomet Study is to just measure how much carbon is in the forest over time. They say that as long as carbon in the forest is holding steady (or even increasing as it is in some states), then biomass energy is carbon neutral. They say all you really need to know is whether forest carbon stocks are holding steady across the landscape over time.

I'll use a hypothetical two-scenario example to explain why this approach doesn't tell you what the atmosphere will see.

Let's say you own 100,000 acres of forest. The forest stocks of wood (i.e., carbon) never change from year to year, because, being a responsible landowner, you only harvest new growth each year.

ClearingNow let's say, just for argument's sake, 100% of your annual harvest goes to sawtimber. The harvested carbon goes into sawtimber for construction and stays mostly sequestered in furniture or flooring or construction lumber somewhere (that is, it does not go back into the atmosphere anytime soon). The forest keeps growing and you are able to get a whole new batch of sawtimber next year. The forest functions like a "conveyor belt" moving carbon from the atmosphere and turning it in to sawtimber— sticks of carbon. The forest system functions as a "sink"—removing more CO2 from the atmosphere than it releases each year.

Now, let's say, all of a sudden the price of fossil fuels goes through the roof and there is a new market for wood energy. You decide to switch from selling your wood to sawtimber markets and instead you sell your wood to the biomass market. When you make this switch, instead of your carbon going to sawtimber, the carbon instead goes into the atmosphere through combustion.

In both scenarios the forest landscape is sustainably managed and provides a steady, constant, sustainable stock of carbon sequestered in trees on the landscape. The stocks on the landscape are identical in both scenarios. But the two scenarios have vastly different implications for how much CO2 is moved into the atmosphere. Just because the forest stocks are staying stable on the landscape says nothing about what the atmosphere sees when you make the switch from sawtimber to biomass energy.

Figure 2 — Cumulative CO2 'debt'

Figure 1

FIGURE 2: Figure 1 shows the carbon debt-dividend curve over time for just the first year of emissions after switching to biomass for energy. In reality, that same pattern repeats each year you continue to use biomass. The multiple debt-dividend curves from each year "stack up." The result of that stacking is shown in the graph above. Under biomass scenarios (red lines) the emissions from biomass eventually level out as a result of forest regrowth. By contrast, the emissions from fossil fuels (black line) just keep rising because the emissions are not recovered or recaptured. Although emissions from wood are initially greater than for fossil fuels (coal, oil, or natural gas), over the long term wood emissions will be lower than for fossil fuels. The point in time when wood emissions equals fossil fuel emissions is depicted by points labeled "A." Using only logging debris as the fuel source results in shorter time to the carbon dividend than using green trees, which would have kept growing anyway.

Here's the annoying truth of the matter. To evaluate what the atmosphere sees from new wood biomass energy, it doesn't matter whether forest stocks are increasing, decreasing, or being sustainably managed. All that matters (for climate change) is how many molecules of CO2 would be residing in the atmosphere with and without biomass energy development. That's the question that has to be answered. And unfortunately, just counting the carbon in the forest won't tell you that.

You're thinking, "I get it! But now I wonder why the carbon debt ever turns into a dividend." I have an answer for that.

Now you know that when you switch to wood biomass you put a big "blob" of carbon into the atmosphere that you wouldn't have with fossil fuels. But trees do grow, and fossil fuels do not. You will have replaced non-renewable fossil fuels that keep piling up carbon in the atmosphere with a renewable source that is able to move carbon back and forth with the atmosphere.

Figure 1 traces effective emissions through time for just one year's worth of emissions. But in reality, there would be a new biomass curve for Year 2, another for Year 3, etc. etc., as long as you were using wood. If you do the math and overlay these repeating graphs, you get Figure 2. And the math shows that the forest landscape eventually comes into equilibrium with the new use of wood for energy. Because of regrowth, the emissions from wood asymptotes as more and more of the landscape moves into the "dividend" phase of Figure 1. When the wood emissions line crosses the fossil fuel line in Figure 2, wood has become a less-emitting fuel source than fossil fuels. Unfortunately, this point of crossing can take a couple of decades to a couple of centuries to happen, depending on the circumstances.

* * *

GIVEN THE ANSWERS TO THESE TWO QUESTIONS, the Manomet Study's primary conclusions are clear, and admittedly surprising. Using wood for energy is not immediately carbon neutral, even if the wood is harvested from a sustainably managed forest. But over longer timeframes, using wood for energy can reduce atmospheric carbon levels, perhaps very significantly if the conditions are right.

Assuming the Manomet Study got it right about the initial carbon debt (a half-dozen other studies have since come to much the same conclusion; see Further Reading), that does not automatically mean switching to wood for energy is bad climate policy. The debt-dividend curve in Figure 1 can be "pushed" to the left through new energy policy, thus speeding up, or shortening the time it takes to reap the GHG dividend of using wood for energy.

One way to get to the carbon dividend sooner is to use just logging debris for energy—the tops and branches left over after a logging operation—instead of live green trees. Most of this debris would decay and return to the atmosphere anyway over a decade or two. Using logging debris for wood energy may make a lot of sense from a GHG perspective (note: logging debris left in the forest has important habitat and nutrient functions too).

Still another way to get to the dividend period faster is to use wood for heat and electricity at the same time, instead of just electricity. Capturing all the heat energy of wood combustion can be a far more efficient use of wood than just electricity generation, and so the excess emissions of wood relative to fossil fuels are smaller. Policies that promote capturing the heat from wood will get to the climate dividend sooner.

It's not just about greenhouse gas emissions

The Manomet Study intentionally drew no conclusions about whether switching to wood was 'good' or 'bad.' That was not our job. While our science has huge implications for GHG accounting and therefore climate policy worldwide, there are many other values to consider besides GHG accounting in formulating energy policy. These include the benefits of greater U.S. energy independence, increased local jobs, new markets for landowners to encourage them to keep their forest as forest, or, on the other side, the disadvantages of increased local truck traffic, particulate pollution and possible human health effects, or the risk of bad forest management (through possible overharvesting), aesthetic impacts on the landscape, etc. (see Table 1)

Table 1 — Weighing the options

Some factors to consider before making a personal decision about whether switching to biomass energy makes sense.

How would you weigh the relative importance of the ten factors below? All these factors need to be integrated into a simple 'yes' or 'no' vote. The science is important, but in the end you have to make a judgement call about which ones are most important to you.

Weighing the Options

Despite all the serious science that went into the Manomet Study, what you as an individual (and society as a whole) decide to do with the science boils down to balancing values, not science. There are other values (Table 1) besides GHG emissions that need to be taken into account regarding the biomass issue. Indeed, climate change is a big one for society, as already determined by the Massachusetts legislature and governor. But there are others, and we humans are not very good at accommodating more than two values in our brains at one time, much less the ten listed in Table 1.

Do you feel lucky?

Recall that when the Manomet Study was done (2010), Massachusetts had already set a goal of reducing GHGs 10-25% below 1990 level by the year 2020. If Manomet is correct, then switching to wood would be like hitting the GHG accelerator on emissions when you really need to be hitting the brakes. Our study indicated that the state of Massachusetts couldn't meet its 2020 reductions goal by switching to large-scale electricity generation using wood biomass. In fact, it would actually increase emissions (and total atmospheric carbon) before 2020. Whether 2020 was the "right" year to pick is another question entirely (and more of a social one than scientific).

If climate change is an issue for you, you still have a tough call to make in light of our study. Virtually every way of switching to woody (forest) biomass energy increases GHGs over the next decade or so. (One notable exception is if the wood was grown specifically to be burned as energy and the wood would not have grown otherwise, such as a biomass plantation created in an old unproductive agricultural field). But many wood energy systems based on waste wood can produce GHG benefits in just 15 or 20 years. Even replacing natural gas with green (live) trees to produce electricity can eventually provide a carbon benefit, although it might take a century for this to happen.

So how should a citizen concerned about climate change decide whether to support wood biomass energy? Here's one way to formulate your answer. If you are concerned about crossing a climate tipping point, which some scientists say could occur in the next decade, then anything that increases GHGs in the atmosphere in the next 10 years is a bad choice for you. You can see the "climate cliff" through your front windshield.

If, on the other hand, you prefer to take the long view—say 100 years—then you may be comfortable with a near-term (e.g., two or three decade) increase in GHGs from wood in order to get the long-term benefit. Climate scientists are just not clear on exactly where the "cliff" might be. Is it 2017 or 2050, or somewhere in between. Or, will the effects just be slow and gradual?

As a society, we have to make an assessment of risk. And this means you have to do your own assessment of risk. You have to decide what you think about the notion of a climate tipping point, and when that might occur. It reminds me of the words of Inspector Callahan in Dirty Harry—"You gotta ask yourself one question. 'Do I feel lucky?'." Well, do you?

Whether we're untangling the challenges of climate change, sustainable agriculture, or shorebird declines, we've created a very complicated world for ourselves. These are hard, complex problems to solve. Science makes us smarter about an issue, like the GHG emissions that would result from switching to wood for energy. But any two of us, informed by the same science, might legitimately choose different courses of action depending on our individual perceptions of risk.

That's why science—i.e., knowledge—isn't enough by itself to solve big, complex challenges. Unless we pass science through the lens of our collective values, we don't know what to do with it. That's why Manomet emphasizes science and social process together. Only when combined with social process to evaluate risk (or opportunity) does science have a fighting chance.

Further Reading

Cherubini, F., Peters, G. P., Berntsen, T., Strømman, A. H., & Hertwich, E. (2011). CO2 emissions from biomass combustion for bioenergy: atmospheric decay and contribution to global warming. GCB Bioenergy, 3(5), 413-426. doi:10.1111/j.1757-1707.2011.01102.x

Gunn, J.S., D. Ganz, W.S. Keeton. (2011). Biogenic vs. geologic carbon emissions and forest biomass energy production. Global Change Biology – Bioenergy. Early View Article.

Hudiburg, T. W., Law, B. E., Wirth, C., & Luyssaert, S. (2011). Regional carbon dioxide implications of forest bioenergy production. Nature Climate Change, 1(11), 419-423. Nature Publishing Group. doi:10.1038/nclimate1264

McKechnie, J., Colombo, S., Chen, J., Mabee, W., & MacLean, H. L. (2011). Forest bioenergy or forest carbon? Assessing trade-offs in greenhouse gas mitigation with wood-based fuels. Environmental Science & Technology, 45(2), 789-95. doi:10.1021/ es1024004

Walker T, Cardellichio P, Colnes, A., Gunn, J., Kittler, B., Perschel, B., Recchia, C., and Saah, D. (2010). Biomass Sustainability and Carbon Policy Study. Report to the Commonwealth of Massachusetts Department of Energy Resources. Manomet Center for Conservation Sciences. NCI- 2010-03. NCI-2010-03. 189 pp.

Walker, T., P. Cardellichio, J.S. Gunn, D. Saah, J.M. Hagan. (2012). Carbon Accounting for Woody Biomass from Massachusetts (USA) Managed Forests: A Framework for Determining the Temporal Impacts of Wood Biomass Energy on Atmospheric Greenhouse Gas Levels. Journal of Sustainable Forestry. In Press.

Zanchi, G., Pena, N., & Bird, N. (2011). Is woody bioenergy carbon neutral? A comparative assessment of emissions from consumption of woody bioenergy and fossil fuel. GCB Bioenergy, Earl View, n/a-n/a. doi:10.1111/j.1757-1707.2011.01149.x

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