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In this video, around 10:50 Minutes, Allan Savory claims that:

Now, if it does not decay biologically, it shifts to oxidation, which is a very slow process, and this smothers and kills grasses, leading to a shift to woody vegetation and bare soil, releasing carbon. To prevent that, we have traditionally used fire. But fire also leaves the soil bare, releasing carbon, and worse than that, burning one hectare of grassland gives off more, and more damaging, pollutants than 6,000 cars. And we are burning in Africa, every single year, more than one billion hectares of grasslands...

It is a bit unclear what exactly that sentence means, but I presume (and please correct me if I am wrong) that it means: the amount of CO2 emitted by burning 1 ha of grassland is greater than the amount of CO2 released by 6000 cars in one year.

Is this comparison (or rather, my understanding of it) accurate?

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    Pollution is about much more than just carbon dioxide. Carbon dioxide is bad for the climate but it does not really affect our health unless we get acute carbon dioxide poisoning. Particulate pollution and other combustion gasses from burning bio-matter however are damaging to our health. Maybe that is what they mean? – MichaelK Sep 18 '18 at 18:08
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    Welcome to Skeptics! I am concerned this quote is vague and meaningless - any answer would need to demonstrate (e.g. from Savory's other works) what he meant by it. Is @MichaelK right that it is particulate matter? Given he is referring to climate change and carbon, he is probably referring to greenhouse gases but a car releases carbon from oil (generally) whereas burning grass and growing it back has no net effect to carbon in the atmosphere. – Oddthinking Sep 18 '18 at 18:25
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    I'm pretty sure that burning 6000 cars would be worse. – Daniel R Hicks Sep 18 '18 at 19:57
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    The claim has a logical fallacy, in that it omits to answer "6000 cars doing what for how long?". Burning a hectare of grassland cannot possibly be more damaging that 6000 cars sitting still for 1 second. How about 6000 cars idling their engines for 100 years? The claim needs a timescale to be plausible. – DJClayworth Sep 18 '18 at 20:54
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    It should also be pointed out that grassland fires are a natural part of the environmental cycle and cars are not. – Joe W Sep 19 '18 at 14:53
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The African grass species Pennisetum purpureum was measured to produce 40 tons of dry biomass per hectare per year, of which about 44% is purely carbon. Does releasing the CO2 of 18 tons of carbon produce the equivalent CO2 as 6000 cars?

CO2 has an atomic weight of 44, compared with 12 for C. So 18 tons of C will become 44 X 18 / 12 = 65 tons of CO2. That CO2 emission is equivalent to 14 cars according to the EPA estimate of 4.6 metric tons / year, much less than 6000 cars.

But this is purely on the basis of CO2. The original claim was regarding "more damaging" pollution, which indicates emissions other than what's found abundantly in nature, like CO2. Therefore this claim cannot be evaluated on the basis of carbon alone.

Scientists in 2003 provided this study, which highlighted the need to evaluate many other compounds in evaluating pollution from burning grasslands, specifically oxygenated volatile organic compounds (OVOCs). By contrast, fuel combustion in cars is very clean in places where gasoline must be produced according to emissions regulations, such as the United States.

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    "carbon" isn't even a pollutant. The whole idea of just measuring CO2 produced by one process vs another and concluding which "pollutes more" is utter insanity. – jwenting Sep 24 '18 at 7:31
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    It's easy to switch out "greenhouse gas" and replace it with "pollutant" in today's dialogue. Y'know what another abundant greenhouse gas is? Water vapor. – elliot svensson Sep 24 '18 at 14:15
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    Except that you can't increase the amount of water vapor in the atmosphere by many multiples. It's naturally self-limiting. Precipitation. – PoloHoleSet Oct 4 '18 at 18:07
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    No, that's not the same, at all. Plants being unable to utilize CO2 beyond a certain amount would, in fact, allow even more CO2 concentrations in the atmosphere. H20 vapor being physically limited is pretty much the opposite of what you linked to. I'm pointing out that water vapor is not as problematic as CO2, because you can't increase H2O levels many times above baseline like you can with CO2. – PoloHoleSet Oct 5 '18 at 16:46
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    @elliotsvensson - and it's a nonsensical point that has no reason to be in a conversation about human factors on the current warming trends. It's a complete non sequitur, but if you want to include it, don't be snarky about us no longer talking about burning fossil fuels. You bring up water vapor, and then make a snide remark about whether we are talking about CO2 or not. – PoloHoleSet Oct 8 '18 at 14:45
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This answer skims the line on how much calculation is allowed and will only talk about CO2 re: your guess that the claim relates to the amount of CO2 released by 6000 cars in one year.

According to the EPA

A typical passenger vehicle emits about 4.6 metric tons of carbon dioxide per year.

As such 6,000 cars would produce about 27600 tons of CO2.

(of which about 7527 tons would be carbon)

1 hectare is 10,000 square meters.

So we might think about this as a smaller problem:

does burning 1 square meter of grassland produce 2.76 tons of CO2?

Looking up the total dry weight biomass of grass and and roots, I could only find numbers for grassland in Wales rather than Africa:

http://nora.nerc.ac.uk/id/eprint/516387/

The table gives these numbers for "Improved grassland".

Average standing above ground biomass (g dry mass m-2) : 806.5

Average total root biomass for 0-15 cm (g dry mass m-2) : 1106.3

Caveat: the amount could vary quite a bit for African grassland, I can find no numbers for that.

A follow on question: can burning about 2 kg of dry biomass produce 2.76 tons of CO2?

There seems to be a few orders of magnitude difference here

I suspect either the claim is incorrect or it may have perhaps been referring to the output of cars for a day rather than a year or something similar.

  • Apart from that. All this grass would have extraced the same amount of CO2 from the air to grow. – borjab Sep 20 '18 at 14:20
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    @borjab I didn't want to get into that so I decided to go with the simplest possible interpretation and whether it was vaguely plausible. The ideal would be a link to a research paper where someone looks a the impact of burning 1 ha of african grassland but I don't think that exists. – Murphy Sep 20 '18 at 14:29
  • In Wales we have this for "improved grassland"... ( publications.naturalengland.org.uk/file/4605375663833088 ) ... it's not clear what the plants' height aboveground is, but it clearly varies from a couple inches to 1 m. In Africa we have this ( ucmp.berkeley.edu/exhibits/biomes/grasslands.php ), and of the grasses of "temperate grassland" several are 1 m in height. There could easily be a factor of 10 difference, or a factor of 100 if the African grass happens to be pennisetum purpureum ( en.wikipedia.org/wiki/Pennisetum_purpureum ). – elliot svensson Sep 20 '18 at 14:57
  • @elliotsvensson that link still puts biomass production at 40 tons/ha/year which would be 4kg for a square meter, and that's with 4-6 harvests per year. That doesn't seem to be a factor of 100 difference. – Murphy Sep 20 '18 at 15:03
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His explanation of that number can partially be found in “Climate Change, Healthy Soils and Holistic Planned Grazing: A Restoration Story”:

Black Carbon, per unit of mass, can absorb a million times more solar energy than carbon dioxide. It is particulate matter, rather than a gas, formed by the incomplete combustion of fossil fuels, biofuels, and biomass, and is a major component of soot. Globally, it comes from biomass burning, including wildfires (36%),* domestic/residential sources (25%), transport and industry (19% each) and energy/power/other (1%)

* Globally, wildfires burn 350-450 million hectares each year. A further 1-2 billion hectares is burned annually via hunting, agricultural and fuel reduction burns.

It also speaks about nitrous oxide and methane each of which is released when burning biomass.

  • You're talking about soot, and soot settles out of the air fairly rapidly, especially in the presence of rain. CO2 does not. – Daniel R Hicks Dec 14 '18 at 12:57
  • @DanielRHicks OK sure, but that is part of his explanation. I didn't run the numbers, I don't know if or how black carbon calculates in, but there is also nitrous exide and methane listed there. – Realz Slaw Dec 14 '18 at 14:54

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