Humans as carbon dioxide generators

Someone asked me how much we, as humans, contribute to the greenhouse effect, by breathing out carbon dioxide.

It's not a difficult calculation. We need the weight of food consumed per day and the carbon content. It has to be the dry mass - what the food would weigh if it was dehydrated - because water (the main constituent) contains no carbon.

Here are my estimates, wet weight:

Protein (meat, eggs, etc): 8oz.
Carbohydrate: 1 lb (bread, potatoes, other root veg, cakes)
Leafy vegetables don't weigh much so we can miss those out.
Fat - 2oz.

We won't be far out if we say 1.5lbs total.

The dry weight would be about a quarter to a third of this - say half a pound.

The approximate composition from an energy point of view would be CH2O. With masses C=12, O=16, H=1, carbon forms 12/30ths of the total, or two fifths. So we have a fifth (1/2 x 2/5) of a pound of carbon per day.

Multiply by 365 - we get 72 lb.

Multiply by about 3.5 for mass of CO2: 250lb.

In metric units this is around 125 kg, or 0.125 metric tons - an eighth of a ton.

This represents a thirtieth of the carbon dioxide "output" of a member of the UK.

Nigel Deacon / Habitat21 website

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