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Plastic Footprint Calculator

Carbon is not the axis plastic is usually judged on — persistence and litter are.

Clear
Over 52 periods6.9 kg CO2e0.007 tonnes
Emissions0.132 kg CO2e4 each of plastic bag
The same quantity of paper bag0.32 kg CO2eheavier to make, so it must be reused more
Difference0.188 kg CO2e2.42 times apart
Emission factor0.033 kg CO2e per each
Trees needed for a year to absorb it0.31at about 22 kg of CO2 per mature tree per year
As a share of a 5-tonne annual footprint0.14%roughly the global average per person
Transport is typically under a tenth of a food's footprint and sea shipping is extraordinarily efficient, so substituting what you eat beats sourcing it closer by a wide margin. Air-freighted produce is the real exception, and it is a small fraction of what is sold.

The formula

emissions = quantity * emission factor

Comparing is what these are for

An emission factor multiplied by a quantity gives a number that looks precise and is not. What it is good for is ranking: beef against chicken, flying against rail, a cotton tote against a plastic bag. Those comparisons are robust even when the absolute figures move by half.

The spread within a category dwarfs the difference between some categories

Poore and Nemecek surveyed nearly 40,000 farms and found the worst quartile of beef production emitting roughly six times the best. That is a wider range than the gap between the best beef and the worst pork. Where food comes from can matter as much as what it is — though not for beef against vegetables, where the gap is a hundredfold and nothing closes it.

Transport is usually a small share

The intuition that local food is low-carbon is mostly wrong: transport is typically under a tenth of a food's footprint, and shipping by sea is extraordinarily efficient. Air-freighted produce is the real exception, and it is a small fraction of what is sold. Substituting what you eat beats sourcing it closer, by a wide margin.

How much to trust a footprint

Life-cycle figures for the same product routinely differ by a factor of two between studies, because deciding what to count — the farm, the packaging, the shop, the drive home, the disposal — is a methodological choice rather than a measurement. The numbers here are central estimates from published meta-analyses. They are reliable for comparing one option against another, which is what people actually use them for, and not reliable to the second significant figure.

The other thing worth keeping in view is scale. Individual choices matter and they are not where the emissions are: the difference between the best and worst quartile of beef producers is larger than most people's entire dietary footprint, and that is a question about agriculture policy rather than about shopping.