Historically(averaging over the past 400,000 years

Historically(averaging over the past 400,000 years) CO2 is around 240ppmv, and now it is around 385ppmv(60% higher). Methane is historically(over past 1,000 years) around 700ppb and now at 1700ppb(140% higher). The % for CO2 is smaller if we start from the warm period average of around 280ppmv.

While the sheer volume of human released carbon dioxide and its warming affects probably are far greater than that of human released methane, humanity seems to have changed the concentration of atmospheric methane much more than that of carbon dioxide.

The fact is that changes in agriculture and diet are the easiest way for an individual to lesson his or her environmental impact. Local, organic, and vegetarian diets are a simple highly effective remediation strategy.

Disclosure: I am a vegetarian and so strongly biased on this particular issue, but this point cannot be ignored.

sources:
http://en.wikipedia.org/wiki/Carbon_dioxide_i...atmosphere
http://ecen.com/eee55/eee55e/growth_of%20meth...sphere.htm

Methane intially reacts with ozone in a 'chain' reaction that ultimately produces CO2 and water vapour.

You could summarise the reactions into:

(3)CH4 + (4)O3 = (3)CO2 + (6)H2O

Oxidation of methane is the main source of water vapor in the upper stratosphere