Carbon Dioxide Equivalent

Carbon dioxide equivalent (CO2e) is an internationally accepted measure, that expresses the amount of global warming of greenhouse gases (GHGs) in terms of the amount of carbon dioxide (CO2) that would have the same global warming potential (GWP). Examples of GHGs are methane, perfluorocarbons and nitrous oxide.

The CO2e of something is derived by multiplying the tonnes of the green house gasses emitted (in production,transport,consumption e.t.c) by the respective GWP. The GWP being a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by definition 1).

The Global warming potential is measured over a specified timescale (generally, 100 years) for emissions of current GWP for existing GHG concentrations. It is arguable however that this time scale should be dramatically decreased, to around 20 years, as the problem is happening now and needs attention in the short term, in 100 years time it will be too late to start worrying.

Reducing the time scale of course, increases the GWP and makes things much worse, but of course it is much worse!

Carbon Dioxide From Weighed Products

For products that can be weighed the Co2e produced can be expressed indepently of units of weight (e.g. 7 kg per kg is the same as 7 tonnes per tonne). To get the Co2e produced simply multiply by the factor given. (e.g. 100 gms of plastic packageing (where the Co2eFactor is 7 has produced about 7*100gm = 700 gm of Co2e in its manufacture).

See Wikipedia:

This includes the effect of other gases including methane:

But “The Third assessment report of the IPCC stated that when averaged over 100
years each kg of CH4 warms the Earth 25 times as much as the same mass of CO2. The
Fourth assessment report has updated this number to include indirect effects and states
that the relative impact of CH4 to CO2 averaged over 20 years is 72.[2].”

The 100 year figure for methane (25) has been the one most used hitherto. But
given the the presence of strong positive feedbacks in the Earth’s climate system, it may
be better to switch to the 20 year figure (75).