Wildfires are getting more and more common due to dryer and hotter summer seasons.

I'm interested in the impact of wildfires in terms of CO2 contribution in the atmosphere. I'd like to get a gross estimate, considering average-size trees and a relatively high density, like what you would observe in wild areas with relatively frequent rains.

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    How old/what species are the trees, until you tell us things like that it's next to impossible for us to answer the question accurately. – a coder Mar 10 '19 at 10:00
  • I understand this is a very general question. But it looks that Tim's answer cover a range of possibilities. I was particularly interested in rain forests in southern Patagonia (fitzroya, nothofagus, etc) which are dense, old and--I assume--store a high amount of carbon, but a general answer could be useful for more people. – vaz Mar 11 '19 at 10:24
  • @vaz South Patagonia's got a range of climates thanks mainly to the mountains. Got a specific area in mind? If we go with cool temperate moist, then the 18 forests covered in the study averaged 642 tC/ha (~2,350 tCO₂/ha) with a standard deviation covering 46% of that. In the absence of specifics, you could thus consider a rough lower bound to be 1,270 tCO₂/ha and a rough upper bound to be 3,430 tCO₂/ha. Or 1.3–3.4 x10³ tCO₂/ha. – Tim Mar 12 '19 at 11:31
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    @Tim yes, the question came to me after 10,000 ha got burnt in Aysén, which is on the west side of the Andes (on the east side there are barely any woods). Thanks for your detailed answer! – vaz Mar 13 '19 at 10:14

Warm temperate moist forests store ~500 tC/ha (tons of carbon per hectare). If that fully burns it gets converted to ~1,835 tCO₂/ha.

So, a wildfire can emit up to ~1,835 tons of CO₂ — returning it back to the atmosphere from whence it originally came — for every hectare that is burnt.

For anyone interested:

  • Cool temperate moist forests store/release ~25% more.
  • Cool temperate dry forests store/release ~44% less.
  • Tropical forests store/release ~50% less.
  • Boreal forests store/release ~80% less.

...assuming you can get them to burn.

Note: The above figures are averages from an Australian National University paper published in Proceedings of the National Academy of Sciences (Re-evaluation of forest biomass carbon stocks and lessons from the world’s most carbon-dense forests) and subject to large deviation depending on the dominant species. Eucalyptus regnans ("mountain ash") forests, for example, top the studied list at ~1,900 tC/ha (3.8x the example above). Aussie bushfires and Californian wildfires — which are roughly equivalent — can often be fierce beasts as a result of the large amount of available fuel.

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    Answer updated. Here's the raw link: pnas.org/content/pnas/106/28/11635.full.pdf – Tim Mar 10 '19 at 6:35
  • You are way overestimating the ability of boreal forests to emit CO2. 20% of 1835 tonnes per hectare is 367 tonnes per hectare, or equivalently, 367 square meters of wood per hectare. Where I live, practically no managed forest will ever reach that density. – juhist Mar 11 '19 at 11:13
  • @juhist Luckily, I am not estimating anything. Qualified university researchers studied 52 boreal forests and measured them. The paper containing the data is named and linked. If you think your anecdote and sample size of 1 is more accurate than their study, feel free to take it up with them. Further, you are comparing 1,835 tons of CO₂ to tons of wood — which is a fail. Use 20% of the ~500 tC/ha figure if you want a legitimate basis for comparison. Also, remember what the term 'average' means. – Tim Mar 11 '19 at 13:10
  • I'm using the entire forests of Finland. Yes, the sample size is 1 if you consider one country one unit of sample, but the total area for the sample is an entire country with lots of forests! Besides, 1 cubic meter of wood (sorry, should have said m3 not m2) is 500 kg, only part of which is carbon atoms, other important atoms being hydrogen and oxygen. So, 1 m3 of wood is not 1 tonne of carbon, because it's only 500 kg, only part of which is carbon. – juhist Mar 11 '19 at 14:08

The typical rule of thumb in forestry is that one cubic meter of wood emits (roughly) one tonne of carbon dioxide. The unit of interest in forestry is the volume of wood in the forest. Thus, what you're essentially asking is: how many cubic meters of wood does one hectare of forest have?

This, of course, varies hugely. Forests in warm regions reach a higher density than boreal forests.

Also, you should take into account that lots of forests are managed for maximum productivity. This means the forest won't ever be let to reach its steady-state natural density, because then the growth slows down and the forest becomes uneconomical.

I'll provide an answer for one country: Finland. According to a source, Finland's forests have 2400 million cubic meters of wood per 22.8 million hectares. This means 105 cubic meters of wood per hectare.

So, if the forest fire hits an average forest in Finland, it will release 105 tonnes of carbon dioxide per hectare. Very young forests with practically no trees won't burn (0 tonnes of carbon dioxide per hectare), and old forests ready for renewal will release twice the average, i.e. 210 tonnes of carbon dioxide per hectare.

Edit: the source apparently considers only growing stock, i.e. wood that is above the ground and is usable for commercial purposes. There may be some other biomass as well that will burn. So, treat the 105 tonnes of carbon dioxide per hectare (average forest) or 210 tonnes of carbon dioxide per hectare (old forest) as a lower bound. I still believe the 367 tonnes of carbon dioxide per hectare from Tim's answer is a bit too high for boreal taiga forest.

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  • Does the 2400 figure include just what's harvestable, or everything that would be burnt (bark, leaves, underbrush, etc)? – LShaver Mar 11 '19 at 13:30
  • @LShaver That's actually a good question. I assume the volume of leaves is minimal compared to volume of solid useful wood. But, the answer is: I'm not certain. – juhist Mar 11 '19 at 14:03
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    The 2400 figure quoted is actually growing stock: "Volume of all living trees in a given area of forest or wooded land that have more than a certain diameter at breast height. It is usually measured in solid cubic metres (m³). It includes the stem from ground level or stump height up to a given top diameter, and may also include branches above a certain diameter." In short, it's only the amount of commercially viable harvestable wood, and is therefore far, far less that what burns in a bushfire. – Tim Mar 11 '19 at 15:27
  • The ANU data in the research paper includes above-ground living biomass, above-ground dead biomass, and also below-ground living biomass. That is: It includes anything that can and usually will burn — unless humans or weather interfere. – Tim Mar 11 '19 at 15:32
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    FWIW this article mentions "170 tonnes of carbon dioxide equivalents per hectare" which is in line with your calculated results. It is for "forest fires in the boreal plains" of Canada. I expect the climate and forest density there to be somewhat similar to that of Finland, but I doubt it's similar to the region the OP is interested in (rain forests in southern Patagonia) – THelper Mar 12 '19 at 8:45

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