Having worked in carbon accounting, the methodology is dogshit.

Basically, they multiply amount spent by a financial emission factor (kg CO2e/ £), which usually has ~80% uncertainty, based on sector averages they match with the vendors name.

Say you spend £50 at Boots, well, they estimate your CO2 emissions as £50 x average CO2/£ of “pharmaceuticals, healthcare and beauty” products, independently of what you actually purchased.

It gets even more vague when the vendor is multi-sector, like a supermarket selling literally everything.

There’s no way they can get more accurate data unless merchants report product-level details of your purchases to the bank or government.

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