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I always thought, that burning wood, would be bad for the total CO2 in the atmosphere until I heard it being pitched as "renewable energy". Now I am slightly unsure.

The logic goes like this:

Wood has X amount of CO2 in it, which is released when burned. Since commercial forests are replaced or renewed, the same amount of CO2 is going to be trapped by the new tree. Let's assume that this is true in this particular case, and every burned tree is replaced by a new tree and the total amount of trees doesn't actually decrease (this is true in well-managed forests).

Of course, burning anything releases toxins into the air, but let's ignore that for a moment and focus just on the CO2.

Then there is the second argument, that letting a piece of wood biodegrade releases the same amount (or similar amount) of CO2 as burning it.

Do these two arguments have merit?

  • I believe the major difference will be in the timescale -- a growing tree accumulates CO2 slowly, and a biodegrading tree releases it slowly. But a burning tree releases CO2 quickly. – LShaver Dec 18 '18 at 16:33
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    I think a review of photosynthesis is in order. The CO2 plants take in are actually stored as sugars. Burning this will release it, but wood that decomposes naturally would have those sugars broken down by animals and fungi etc (who will release co2 via respiration) but will actually use much of that carbon to build their own bodies and keep the carbon within a naturall cycle. – flummingbird Dec 19 '18 at 21:55
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    @flummingbird I think that answers the part of the question about bio degradation. You should post this as an answer. – user1721135 Dec 20 '18 at 8:27
  • Don't forget methane from decaying wood. Depending on the environment the wood rots in that can push the CO2-equivalent emissions up a lot. – Code Abominator Dec 24 '18 at 3:03
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Conservation

It is a fundamental law of physics that matter/mass/energy (all being, ultimately, the same thing) cannot be created or destroyed — they just change form.

Under 'normal' conditions on Earth, any carbon atom that exists will continue to exist in perpetuity. It can and will form temporary bonds with other atoms to form distinct substances with distinct properties at various points in time, but the actual carbon atom itself will still exist, and the total number of carbon atoms in the world will not change.

(Yes, extraterrestrial carbon sources do exist, but that is beyond the scope of this topic.)

Photosynthesis

Growing trees obtain all of their carbon from photosynthesis. Carbon in one form (carbon dioxide, a gas) is converted to another form (carbohydrates, which are solids). Those carbohydrates then go on to change form and produce other solids (e.g. lignin and cellulose) that form the bulk of what we know as 'wood'.

During this entire process the number of carbon atoms has not changed. The only thing that has changed is what other atoms have attached to and detached from each carbon atom. No carbon atoms have been created. No carbon atoms have been destroyed.

Burning Wood

If wood burns completely and fully in the presence of oxygen, the only thing left behind is ash. Ash is inorganic. Ash does not contain carbon. All of the carbon atoms have — once again — been unbound from and rebound to other atoms. All of those molecules go up the chimney and into the atmosphere. That means all of the carbon atoms are also in the atmosphere. The same carbon atoms. The same atmosphere.

(Yes, a small amount of particulate matter (solid carbon) will become airborne, and some of the carbon will end up in the form of carbon monoxide instead of carbon dioxide, but since both of those actually serve to reduce atmospheric CO₂ levels, we can happily ignore them.)

Rotting Wood

A tree that 'biodegrades' is nothing more than a tree that is left to rot. Rotting is essentially the process whereby various types of fungi break down the molecules in wood (e.g. lignin and cellulose) into less complex forms. Those forms (and the fungi themselves) are part of a complex biological food web. Rotting wood is 'food' in the process of being 'eaten'.

Unless the tree is completely submerged in water, nearly all of the biological life that is associated with rotting wood relies on aerobic cellular respiration in order to survive. Aerobic respiration is essentially the reverse of photosynthesis (carbohydrates + oxygen » carbon dioxide + water + energy) so the cycle is, once again, complete. The same carbon atoms that entered the tree in gaseous form have been eaten, processed and released in gaseous form by various species of plants and animals.

So, under 'normal' conditions, both processes (burning and rotting) are carbon neutral. The same amount of carbon atoms go in and out, and they are in the same form (CO₂).

When 'Carbon Neutral' Does Not Hold

It is under 'abnormal' conditions that imbalances occur — in the short–medium term. An incomplete/interrupted burn will leave behind charcoal that will probably take centuries or millennia to return to gaseous form. A tree felled into a marsh/water will break down in anaerobic conditions and leave behind residues that may become fossil fuels and return to the atmosphere millions of years later.

There are so many variables that you can twist yourself into knots just thinking about them. To actually arrive at an answer you need to reduce the variables — your question needs to be more specific.

Firewood vs Rotting Wood

Wood heaters are specifically built to extract as much heat from firewood as possible. In recent years, with secondary and tertiary burn phases, they have gotten very good at that. Rocket mass heaters (mass heaters in general) are even better. In any case, 'complete combustion' is the goal. Negligible amounts of charcoal should be left behind (or emitted as particulate matter or carbon monoxide). Virtually all of the carbon ends up as CO₂.

In a semi-temperate climate, a tree that is felled and left on the ground, piled up, or placed into a hugelkultur mound, is in a less-controlled environment. It is almost certain that some parts of the tree will decay under anaerobic conditions and thus the embodied carbon will be converted to a more persistent form — one that will remain as a solid or liquid in the soil for (probably) tens to hundreds, but sometimes even thousands to millions of years.

On a time-scale of tens-of-millions of years, there is no difference between the two. On a time-scale of decades, letting the wood decay naturally will sequester a tiny, tiny bit more atmospheric CO₂. The large number of variables makes it hard to nail down "tiny, tiny bit" to anything more specific than "far less than 1%".

Ironically, it has been the 'improvements' in wood heater technology — the same ones that have made them 'more efficient' and 'less polluting' — that have made 'burning wood' less useful in sequestering carbon. Old heaters and open fireplaces tended to leave behind much larger amounts of charcoal which would then be incorporated into the garden or spread out into a field/yard — ultimately becoming buried (sequestering the carbon for hundreds to thousands of years). Particulate matter (solid carbon) from 'smoky' chimneys would also settle on the ground outside of homes and sequester the carbon — in the process increasing the organic fraction of the soil and improving fertility and crop yields for decades. Those old heaters and fireplaces were actually carbon-negative!

Wrapping it up...

Efficient wood heaters — thanks to near-perfect combustion — are very good at returning all of the carbon embodied in firewood back to the atmosphere very quickly. They are carbon-neutral and — provided the firewood comes from managed woodlots — completely sustainable.

Allowing wood to decay naturally in a semi-temperate climate will sequester a tiny amount of carbon for decades to centuries. The actual amount is so small and subject to such enormous variability that it makes for a poor and unreliable strategy for the reduction of atmospheric carbon levels over any time-frame.

Many strategies exist that can slow down and/or modify the decay rate of wood and allow it to sequester carbon in the short–medium term, but just leaving it to rot on the ground is not one of them.

tl;dr:
Yes. Burning wood is carbon-neutral. Letting it rot on the ground is also carbon-neutral. The (short–medium term) difference between the two is so small that it should not influence your thinking in any way. There is no difference between the two in the long term. If you want to sequester carbon (in the short–medium term) using fallen trees then you need to actively/artificially manage/control the decay process — it won't happen to any useful extent otherwise.

  • Section headings added to improve readability/navigation. – Tim Dec 22 '18 at 0:40
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I think a review of photosynthesis is in order. The CO2 plants take in are actually stored as sugars. Burning this will release it, but wood that decomposes naturally would have those sugars broken down by animals and fungi etc (who will release co2 via respiration) but will actually use much of that carbon to build their own bodies and keep the carbon within a naturall cycle.

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    What happens to those bodies? They die, decompose, get eaten. Life cycles don't remove appreciable amounts of atmospheric CO2. The exception is when some part of the life cycle gets sequested, for example as happens in the creation of coal, oil, or other fossil fuels. – Jean-Paul Calderone Dec 20 '18 at 21:14
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The simple answer is that burning it would be roughly carbon neutral (it releases what it absorbed during growth) versus letting it decompose where it would be a carbon sink. Fossil fuels in the ground came from this vegetative carbon (+ millions of years and pressure).

Burning renewable fuels is considered better than burning fossil fuels in the ground because the burning fossil fuels released carbon that was already captured and would have remained captured. Burning something renewable simply (roughly) re-releases what was captured.

Plants are significant carbon sinks (unburned), and decomposition doesn't release anywhere near what the plants absorbed from the air (this is why forests are carbon sinks). The carbon mostly goes into the soil, and the cellulose is broken down by bacteria, as well physical actions like weather and walking on it, e.g. herds are important for renewing and sustaining grasslands.

Obviously it's better not to burn a renewable source at all having captured the carbon, because unburned it's carbon negative instead of carbon neutral. And burning anything has other pollution issues beyond greenhouse gases, especially if the combustion is incomplete (e.g. insufficient oxygen during the process).

So, no, burning wood is nowhere near letting it biodegrade. Burning = neutral, biodegrade = negative, in terms of greenhouse gas emissions. Again, this is grossly simplified, but works as a general rule of thumb.

Nasa (2014) - Tropical forests absorb 1.4 billion metric tons of carbon dioxide

"When the trees are consumed by forest fires or cleared and burned to make way for pastureland, carbon that took decades to store is released back into the atmosphere near-instantaneously." Tropical Deforestation Is the Third-Biggest Carbon Emitter in the World

...and that's just tropical forests.

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Yes and no.

The answer to the question you asked is yes. Biodegradation produces carbon dioxide and heat. The heat of biodegradation will be wasted. It makes much more sense to usefully harvest the heat stored in wood, and thus, not letting it biodegrade but rather burn it in power plants.

However, you seem to think there are only two options: biodegradation and energy use. This is false. There is a third option: store the wood in a place where it won't biodegrade.

For example, you can use wood to construct houses. This is a very durable nearly permanent carbon sink (if the lumber is reused and/or buried after the house is dismantled, it could be argued the carbon sink will be permanent).

In theory, you can use wood to produce paper and cardboard, but unfortunately, the black liquor waste that is burned to energy in producing paper and cardboard releases some of the carbon dioxide in wood. Well, at least, you get the energy from burning it. So, making paper and cardboard usually doesn't make sense from a climate change perspective.

You can also bury wood.

Unfortunately, when wood is burned to energy, it releases more carbon dioxide emissions per unit of energy than coal, peat, natural gas or oil does. So, it actually makes sense to construct houses from the parts of wood that can be usefully used for construction purposes (sawlogs), and bury the rest. However, there are no economic incentives in place currently to encourage burying wood, so instead of wood burial, the rest is either burned to energy, or used to make paper and cardboard (and the resulting black liquor waste is burned to energy).

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