Is burning 1ha of grassland more environmentally damaging than 6000 cars?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

I don't think that the carbon dioxide is the pollutant he is talking about, it is much more likely to be particulates (i.e. soot).

" 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."

This is irrelevant because it is seasonal grassland, which means that the carbon in the grass would have been released anyway, via natural processes (being eaten, digested and subsequently released by respiration or excretion). The CO2 would be taken up again when the grassland regrew the following year, so on a timescale greater than a year or so, it is carbon-neutral. The CO2 from cars is mostly fossil carbon and is new to the active carbon cycle and causes atmospheric CO2 levels to accumulate. This means the CO2 from fossil fuel emissions will lead to climate change, via the so-called "greenhouse effect", but the CO2 from burning seasonal grasslands will not.

I suspect the speaker knows this (or at least I would hope so) which would imply that the CO2 is not the pollutant in question, but the captions could imply otherwise (I have to say I am not that keen on TED talks, they may be interesting but they tend to cover more ground than can be supported by the time available, so lots of important background material is skipped, leading to misunderstandings like this one).