21/11/2022

Would it really be better to let hides rot than turn them into leather? No…

In the Waterbear film ‘SLAY’, it is claimed that would be better for the environment if hides and skins were thrown away and allowed to rot than to process them to leather. The claim is supported by reference to a calculation by Circumfauna, an initiative of Collective Fashion Justice. The problem is that the calculation and the rationale for it are nonsense.

Circumfauna assert that because the emissions from letting hides rot would be less than those from processing them to leather, which will eventually be discarded or incinerated, that throwing the hides away is the best option. They allege that this is the case even if the leather that would have been made is replaced by synthetic leather.

The problem with this is argument is that it would also apply to the oil from which synthetic leather is made, which would produce far fewer emissions if left in the ground. And synthetic leather will also eventually be discarded or incinerated so by that logic, it would be better to not make it either. In fact, there are no raw materials that can be processed to final products without increasing emissions; iron ore has no emissions but the emissions from steel production are huge. The idea that raw materials should not be processed because it will increase their carbon footprint is bluntly, ridiculous.

There are also significant issues with the calculation itself. Circumfauna acknowledge that if leather was not made, synthetic leather could be used instead but claim that this would still be less impactful than making leather. However, using their numbers, it is clear that this not the case.

  • Emissions for leather per m2 = Emissions from livestock rearing (93 kg CO2e/m2) + Emissions from leather production (17 kg CO2e/m2) = 110 kg CO2e/m2 leather
  • Emissions from rotting hides = 1.152 kg CO2e/m2 leather
  • Emissions from synthetic leather production = 15.8 kg CO2e/m2 synthetic leather

So, if hides are left to rot and the potential leather is replaced with synthetic leather, the emissions would be:

  • Emissions from livestock rearing (93 kg CO2e/m2) + Emissions from rotting hides (1.152 kg CO2e/m2 leather) + Emissions from synthetic leather production (15.8 kg CO2e/m2 synthetic leather)

Which equals 109.95 kg CO2e/m2 total emissions.

The net gain is almost zero. This an insignificant difference even before considering the other issues around plastic use and the fact that plastic products will be replaced more frequently that leather products, necessitating greater volumes of material and therefore, more emissions.

However, there is a more fundamental flaw in the calculation. The start point for the calculation are the CO2e emissions from 1 tonne of raw hide (624 kg CO2/tonne). In order to compare the emissions of leather and synthetic leather in square metres, Circumfauna calculate the number of hides per tonne of raw hide and then the multiply that by the square metres of leather from one hide. To do this this, they use an average hide weight of 6.1 kg taken from a UNESCO report. On this basis, they calculate that a tonne of raw hides would amount to 163.93 hides

The problem here is that the value in the UNESCO report refers to finished leather and not to raw hides. As such, the 6.1 kg figure is completely wrong. In fact, according to FAO statistics , a raw hide weighs on average about 26.7 kg. This is confirmed by the EU PEFCR for leather which gives an input flow of 7.06 kg of raw hide per m2 of leather. As such and using the Circumfauna figure of 4.41 m2 of leather per hide, a raw hide would weigh 31.14kg

  • 7.06 kg raw hide per m2 leather x 4.41 m2 leather = 31.14 kg per hide

If we take an average between the FAO and EU figures, the average weight of hide would be 28.9 kg. On this basis, the number of hides per tonne is:

  • 1000 kg hides / 28.9 kg per hide = 34.6 hides / tonne of raw hide

This significantly changes the calculations of CO2 emissions per m2 of rotting hides:

  • 34.6 hides x 4.41 m2 per hide = 152.59 m2 per tonne of raw hide
  • 624 kg CO2 emissions per tonne of raw hide / 152.59 m2 per tonne of raw hide
  • = 4.08 kg CO2e/m2 raw hide

Using the correct weight for raw hides, emissions per m2 rise by a factor of over 3.5. If this corrected value is used in the initial calculation:

  • Emissions from livestock rearing (93 kg CO2e/m2) + Emissions from rotting hides (4.08 kg CO2e/m2 leather) + Emissions from synthetic leather production (15.8 kg CO2e/m2 synthetic leather)

The emissions from allowing hide to rot and replacing the potential leather with synthetic leather rise to 112.88 kg CO2e/m2 total emissions.

This is 2.88 kg CO2e/m2 higher than the emissions figure for leather production given by Circumfauna. Factor that by around 2 billion square metres of leather production per year and the result would be an additional 5.76 million tonnes of CO2 emissions arising from substituting leather with plastic.

In other words, using Circumfauna’s own calculations but with the correct value for the number of hides in a tonne, it is clear that turning hides, that will be produced whether they are used or not, into leather is a better choice than substituting leather with plastic synthetic leather.

For no apparent reason, the calculation also includes a reference to carbon to methane conversions. Given the figures used by Circumfauna this is completely unnecessary as they are all presented as CO2e and therefore, directly comparable, and no figures for methane are actually given. Furthermore, there is no conversion factor given from carbon or methane to carbon dioxide, which would be needed given that all the data is presented as CO2e. All of which speaks to the unscientific nature of the calculation.

However, it raises an interesting question about methane. It is probable that under the anaerobic conditions of a landfill, hides would decompose to methane, which would ultimately oxidise to carbon dioxide. The figure for CO2 emissions from rotting hides is presented in absolute terms, i.e. 1 tonne of hides produces 624kg of CO2. This is irrelevant for the purposes of the previous calculations as the ratio of CO2 to CO2e is 1:1, but it provides a basis to calculate the likely methane emissions.

So what are the implications of rotting hides producing methane?

The molecular weight of CO2 is 44 and for methane (CH4), 16. As such, the mass of methane required to produce 624kg of CO2 is:

  • 624 x (16/44) = 226.9kg CH4 - (CH4+2O2 → CO2+2H2O)

So assuming complete degradation of 1 tonne of raw hides in anaerobic conditions, 226.9kg of CH4 would be produced. CH4 has global warming potential 28 times greater than CO2 over a 100 year timeframe, as used to calculate CO2e. As such, the CO2e emissions of 1 tonne of rotting hide could be:

  • 226.9kg CH4 x 28 = 6353.4 kg CO2e per tonne of raw hide

If this figure is used in Circumfauna’s calculation, and even if we accept the erroneous figure of 163.9 hides per tonne, the emissions of allowing 1 tonne of hides to rot amount to 118.78 kg CO2e per square metre of leather replaced by synthetic leather:

  • 6353.4 kg CO2e emissions per tonne of rotting raw hide/ (163.9 hides per tonne x 4.41m2 per hide) = 8.78 kg CO2e per m2
  • Livestock emissions (93kg CO2e per m2) + rotting hide emissions (8.78 kg CO2e per m2) + synthetic leather emissions (17 kg CO2e per m2) = 118 kg CO2e per m2

Using the correct hide number (34.6 hides per tonne), the methane emissions per square metre of rotting hide plus producing synthetic leather rise to 41.63 kg CO2e per m2, a total of 150 kg CO2e per m2, far in excess of the emissions of leather production calculated by Circumfauna. This is all based on Circumfauna’s assumptions but even if CH4 constituted only 10% of the hide emissions, it would still be better to convert them to leather than to let them rot and replace leather with plastic.

The calculation by Circumfauna is demonstrably false. Substituting leather with synthetic leather is not an environmentally sound choice even before consideration of the issues around using plastic, the durability of leather, end of life and so on.

In summary, the premise is deluded, the calculation fatally flawed and the conclusion the exact opposite of what it should be.

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