climate change

The challenge of estimating emissions

A few years ago I was researching the carbon footprint of meat, and found a whole range of figures for how damaging it was. One of the most frequently cited studies, from the FAO in 2006, suggests that lifestock has a bigger climate impact than transport at 18% of global emissions. High enough, but the highest calculation I came across suggested livestock accounts for a grand 51% of greenhouse gas emissions.

At the other end of the scale, an estimate of lifestock’s contribution to US emissions assigned it just 2.8%. One person was even prepared to claim that cattle farming could actually be carbon positive.

You can get these wildly differing estimates depending on where you draw the lines. Do you just include the emissions from raising beef on the farm, or from the whole supply chain through to consumption, factoring in processing, waste and transport? The higher estimates include deforestation and land clearance for ranching. Just how far should you push it – include the energy needed to run the fridges in the supermarket meat aisle? The energy used in cooking meat once consumers get it home?

Terms can often be used inter-changeably or get missed in reporting. Are we talking CO2 or greenhouse gas emissions? You can make the impact of cattle farming lower by focusing on CO2 rather than greenhouse gases, since it’s methane that’s the real problem with cows. And of course we could be dealing with just beef, the meat industry, meat and dairy, to lifestock generally.

I was reminded of this while reading Sustainable Materials the other week. The authors show a similar problem with steel production, calculating that steel is responsible for 4.5%, 5.7%, 7.4%, 8%, 9.3%, 13%, 20%, 25% or even 35% of global emissions depending on what you include. They point out that all these calculations are a simple ratio. The complication comes in choosing your numerator and your denominator, but essentially all those answers are correct.

What are we to make of this problem? It means that working out the carbon footprint of anything is very difficult, and it’s best to work to generalisations rather than split things too precisely. It’s also worth asking how numbers are being used – are they being used to find solutions, or to shift blame?


  1. this is really interesting. I am a politician. It might be one of the reasons of the heated debates in politics: anyone is right, it just depends on what you include. So how to defend fierce measures?

  2. It is equally tricky when trying to work out how much carbon to offset from travel or just ordinary living. Neither is it clear how much one should ‘pay’ per tonne of carbon emitted.

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