The true climate cost of a thing

A life-cycle model that shows its working: every step, and what each step is made of.

A tomato has a number attached to it. Somewhere on the internet you can find out that it is 0.4 kilograms of carbon dioxide equivalent, or 2.1, or 1.4, and none of those sources will tell you why they disagree. They disagree because a footprint is not a measurement. It is the end of a long argument about what to count and who to charge, and publishing only the answer hides the argument.

This model runs the argument in the open. Give it three things: an item, where it was produced, and where it is eaten. It returns the chain as it actually happens: a spine of stages from the field to the bin, with a branching tree under each stage showing where that stage's emissions came from. The branches have branches. A tomato needs fertiliser¹, the fertiliser needs ammonia, the ammonia needs natural gas, and getting that gas out of the ground leaks methane². Four levels down, and every level is a real emission somewhere.

Allocation, which is where the disagreement lives

Every branch carries a multiplier saying what share of that process belongs to the product in the model. A slaughterhouse also makes leather and tallow. A dairy cow also becomes beef. A coffee mill also sells pulp. Charging the whole burden to the one output you happen to care about is a choice, and in almost every published footprint it is an invisible one.

Two careful studies of the same cheese can differ twofold without either being wrong, because one split the cow's methane between milk and meat by economic value and the other by protein mass. The allocation³ factor is a judgement, it changes the answer, and it should be visible. Here it sits on every edge, and it compounds down the chain, so a process four levels deep may contribute only a third of what it emits.

Lifetime, which is allocation wearing a different hat

A car’s factory does not belong to any one journey. Neither does an airframe. What a per-kilometre figure actually charges you is one part in a lifetime — the vehicle divided by the distance it will cover before it is scrapped — and that lifetime is an assumption rather than a measurement. It is also the assumption those figures are most sensitive to, and the one most often buried in a constant.

So for the vehicles it is a control. Move it and the manufacturing share moves inversely: a petrol car retired at a hundred thousand kilometres carries roughly twice the factory per kilometre as the same car driven to four hundred thousand. The effect is larger for an electric car, which carries more of its burden in manufacture and less in fuel, which is exactly why the lifetime question matters more there. For a flight the same control is the airframe’s life in passenger-kilometres — seats times load factor times distance flown — so what the model charges you is your share of building the aircraft, divided the same way your share of the fuel is. Every default reproduces the published figure exactly; the control exists so the assumption can be argued with rather than inherited.

What the model says

Three groups, one axis. The point of putting a flight, a pair of jeans and a tomato in the same model is that they can then be asked the same question, and the answers turn out to be commensurable in ways that are hard to see when each lives in its own report. A cotton t-shirt worn for three years costs about what thirty-three kilometres of driving costs. A pair of trainers costs about half a beefsteak dinner. Whether that is a useful comparison is a separate argument; whether it is an available one is not.

Two of the entries exist mainly to make an argument about method. The electric car is a machine for turning a grid into motion, so it is the one item whose destination matters more than its origin. The same car is a different object in France and in Poland. Production electricity and consumption electricity are charged separately, so the model already distinguishes them. The leather shoe is the allocation argument in its purest form. A hide is a co-product of a beef animal, and how much of that animal's methane belongs to a shoe is a judgement rather than a measurement. At 2.2% by economic value the shoes come in under four kilograms; by mass allocation, at about 7%, they would triple. Nothing about the shoe changes.

The two tomatoes are the point of the whole exercise. Same plant, same distance, five times the emissions, and the entire difference is whether the greenhouse was heated. Food miles are the thing people reach for and they are usually the wrong lever: a banana crosses eight thousand kilometres by sea and still comes in at under a kilogram, because sea freight costs almost nothing per tonne-kilometre⁴. Switch that banana to air and it goes up more than tenfold. Distance matters enormously when it goes by air and barely at all when it does not.

Beef is unmoved by anything downstream. Enteric fermentation and land use happen before the animal leaves the farm, so the route is noise against them. No packaging decision and no shipping choice touches a number that is three-quarters methane and cleared forest.

How it is checked

Every total is compared against the ranges in the published literature⁵ rather than against a single figure, because these are distributions across thousands of farms and a point estimate would be false precision. All nine items land inside their bands. The tree is checked to sum at every node, allocation is checked to compound correctly down each chain, and the levers are checked to move the answer in the direction physics says they should.

One test exists because of a bug that hid well. Production-side electricity was being charged at the consuming region's grid intensity, so a French dairy looked dirty when someone in India ate the cheese. It is invisible in any single run the number just looks slightly high, and only shows up when you change the destination and something moves that has no business moving. There is now a test that walks every production stage and fails if any of them draws power from the wrong grid.

Sources

Numbered markers in the text above point here. Emission factors, cost ranges and lifespan figures are representative values from these sources, not measurements made for this project.

1. IPCC, 2019 Refinement to the 2006 Guidelines for National Greenhouse Gas Inventories, Volume 4 Chapter 11 - direct and indirect nitrous oxide from managed soils.The emission factor for nitrogen applied to soils.
2. IPCC, Sixth Assessment Report, Working Group I Chapter 7, Table 7.15 - global warming potentials.Methane at 27 and nitrous oxide at 273 over a century.
3. ISO 14044:2006, Environmental management - Life cycle assessment - Requirements and guidelines, clause 4.3.4.The allocation hierarchy: avoid, then physical, then economic.
4. UK Department for Energy Security and Net Zero, Greenhouse gas reporting: conversion factors, 2023.Freight emission factors by mode.
5. Poore & Nemecek, Science 360:987, 2018 - reducing food’s environmental impacts through producers and consumers.Per-kilogram footprints and the spread within each food.