Context
It has been suggested that burning biomass, such as that derived from tree wood, may be similar, or worse than burning fossil fuels such as coal, from a CO2 "release" perspective. This is addressed in part here. What should be stated, in terms of what this post is not advocating for:- I am not advocating tree biomass burning as The Answer for carbon-neutral energy generation.
- I am not advocating using rainforest as biomass feedstock.
- I am not advocating the removal of trees without sustainable reforestation.
- In this post, I am not addressing impacts other than those associated with greenhouse gases and climate change.
Timescales
It has been suggested that burning of coal and biomass are in fact comparable. We know that coal is the energy-rich content which used to be living matter at the earth's surface. Biomass, such as wood chips or pellets, was also living matter at the earth's surface. If both energy sources were derived from dead plant matter, and those plants grew by fixing carbon out of the atmosphere while they were alive, what is the difference between the two, from a carbon balance standpoint?
Aren't both coal and biomass burning simply releasing CO2 back into the atmosphere that was there in the first place?
Yes, but the critical element here is time. Biomass was recently living plant matter, sequestering carbon from the atmosphere (generally, we're talking about tens of years here, or less). Coal was living plant matter millions of years ago. Why does this matter?
At any point in time, there exists carbon in the atmosphere (CO2) and sequestered carbon in the ground. Although there is some natural variation in the relative proportions of the two, over long time periods (thousands to millions of years), we have a rough equilibrium point between them. Atmospheric carbon is fixed by plants, which then die and after millions of years, become coal, and other fossil fuels.
When humans burn biomass, they have accelerated the flow of carbon back into the atmosphere by years. Burning coal reverses the flow of carbon back into the atmosphere, and accelerated by millions of years. The equilibrium point is significantly shifted when you accelerate one of the flows (sequestered to atmospheric) by orders of magnitude. This is not theoretical. We, of course, can observe this, as atmospheric CO2 has risen from a pre-industrial concentration of ~280ppm to over 400ppm. Unless we have a way to also accelerate the flow of atmospheric carbon back into sequestered carbon, accelerating the reverse flow changes the CO2 balance point.
As long as biomass is allowed/encouraged to regrow (see my caveat #3 above), the CO2 release of burned biomass is cancelled when a new plant regrows in the original one's place.
As many of the companies cutting timber are also involved in the replanting of trees (more to preserve their raw material base, and less out of ecological concern), and very few fossil fuel companies are actively replacing their raw material in kind, we have an important difference between coal and biomass.
Note that this same basic argument holds for biofuels in general, although biofuel feedstock also varies considerably in its ability to provide more usable energy than humans must input.
Carbon Sequestration Rate
All else being equal, it would be preferable to leave forests, and other plant life, as they were naturally. However, humans need both food and energy, so if plants are not going to be used (directly), any sensible analysis must compare to the alternatives.
Use of biomass should generally (†) only be used to displace usage of fossil fuels, including coal. If biomass is burned, but regrowth is promoted, there hasn't been any net sequestration effect. However, as opposed to fossil fuels, there also hasn't been a large net increase in atmospheric carbon.
When a plant product is harvested for biomass or biofuel, the plant (or replacement plant) is generally stimulated to regrow faster. This is an effect any backyard gardener will recognize: cutting stimulates growth, and growth is often fastest when a plant is young. This is a combination of the natural response by many plants (when the plant is not completely killed), and also the fact that the removed plant is no longer competing for resources (water, sun, soil). A tree removed lets sunshine in that was previously absorbed by its canopy.
We also know that plants do not grow indefinitely. Plants have a finite life, and a given species has a typical maximum size it achieves before its death. Forest density also eventually reaches a limit as available sunshine is all consumed. As a result of these factors, we see that as a forest matures, its net growth rate slows, and its ability to sequester new carbon slows. Every tree that dies may be replaced by a new young tree with a period of rapid growth. However, decomposition of the dead trees also releases CO2 as well as CH4. These factors buffer the net sequestration rate.
The Ecological Society of America (ESA) presents a treatment of this topic here. A basic diagram of carbon flows is shown on page 2 and 3. The largest flows of carbon into growing plant matter and out of dead matter are in balance. A relatively small amount of carbon (compared to total flows) is "permanently" sequestered in the soil.
On page 4, a chart shows the total carbon (trees, soil and dead wood) in a forest, as a function of time after a fire. A human harvesting operation (with proper soil maintenance) should have a similar effect on a given area of forest. Note that the rate at which carbon is accumulated is fastest when the forest is young (15 - 40 years old in the chart). After this time, the rate of carbon accumulation slows.
In this 2004 BioScience publication, the authors offer a model of carbon sequestration over time in crop soil. Figure 3 shows that the carbon sequestered does not simply increase indefinitely, but rather asymptotes to an equilibrium value. If we only leave forest, grassland, etc. untouched, the soil-sequestered carbon does not continue to increase forever. If we're looking to possible actions to take to counter our carbon flows into the atmosphere, it may actually be reasonable to harvest forest, or crops, on a timescale that keeps growth rates high, and carbon sequestration rates at their maximum. Old forests and grasslands do not do this.
Again, this is only offered as an alternative to fossil fuels, not as a preferred alternative to other energy solutions such as wind, solar or geothermal. † However, there are likely to always be applications where energy density of fuel is critical (e.g. airplanes or long-distance cars) and for that, biofuels may have a role to play. Sequestration of carbon is a critical byproduct here.
When a plant product is harvested for biomass or biofuel, the plant (or replacement plant) is generally stimulated to regrow faster. This is an effect any backyard gardener will recognize: cutting stimulates growth, and growth is often fastest when a plant is young. This is a combination of the natural response by many plants (when the plant is not completely killed), and also the fact that the removed plant is no longer competing for resources (water, sun, soil). A tree removed lets sunshine in that was previously absorbed by its canopy.
We also know that plants do not grow indefinitely. Plants have a finite life, and a given species has a typical maximum size it achieves before its death. Forest density also eventually reaches a limit as available sunshine is all consumed. As a result of these factors, we see that as a forest matures, its net growth rate slows, and its ability to sequester new carbon slows. Every tree that dies may be replaced by a new young tree with a period of rapid growth. However, decomposition of the dead trees also releases CO2 as well as CH4. These factors buffer the net sequestration rate.
The Ecological Society of America (ESA) presents a treatment of this topic here. A basic diagram of carbon flows is shown on page 2 and 3. The largest flows of carbon into growing plant matter and out of dead matter are in balance. A relatively small amount of carbon (compared to total flows) is "permanently" sequestered in the soil.
On page 4, a chart shows the total carbon (trees, soil and dead wood) in a forest, as a function of time after a fire. A human harvesting operation (with proper soil maintenance) should have a similar effect on a given area of forest. Note that the rate at which carbon is accumulated is fastest when the forest is young (15 - 40 years old in the chart). After this time, the rate of carbon accumulation slows.
In this 2004 BioScience publication, the authors offer a model of carbon sequestration over time in crop soil. Figure 3 shows that the carbon sequestered does not simply increase indefinitely, but rather asymptotes to an equilibrium value. If we only leave forest, grassland, etc. untouched, the soil-sequestered carbon does not continue to increase forever. If we're looking to possible actions to take to counter our carbon flows into the atmosphere, it may actually be reasonable to harvest forest, or crops, on a timescale that keeps growth rates high, and carbon sequestration rates at their maximum. Old forests and grasslands do not do this.
Again, this is only offered as an alternative to fossil fuels, not as a preferred alternative to other energy solutions such as wind, solar or geothermal. † However, there are likely to always be applications where energy density of fuel is critical (e.g. airplanes or long-distance cars) and for that, biofuels may have a role to play. Sequestration of carbon is a critical byproduct here.
