Carbon Dioxide Equivalent

[Geoff]

See Wikipedia: http://en.wikipedia.org/wiki/Carbon_dioxide_equivalent

This includes the effect of other gases including methane: http://en.wikipedia.org/wiki/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).