I've just posted a How To on carbon offsetting, using my own recent travels from LA to Philly to Tampa and back to LA as an example. All told, I traveled 7,905 miles and contributed a total of 3.873 tons of CO2 emissions; total cost to offset that with wind energy was $48.00, as evident in the receipt below:

For the How To, please visit the How To's section or use this direct link:
http://dianovo.com/how_to/emission_carbon_offsetting

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Incidentally: I wasn't in Tampa, FL for the Superbowl - football is one of my least favorite sports.