The PI has an article describing the wide differences in how the dozen most prominent websites calculate carbon footprints.  A lot of environmentally conscious folks and organizations like to calculate their footprint and then pay a "carbon offset" fee that goes to plant more trees or otherwise clean up the specific amount of carbon that we use. 

However, as many of us have suspected, there is no standard.  A PI reporter, Lisa Stiffler, and her husband calculated their transportation miles and electrical usage and other carbon-based usage and then plugged in the numbers to 10 widely used sites.  The results varied widely.  A UW Study checking out a typical family's carbon usage found a high of 75,795 pounds of carbon dioxide emitted, as calculated by the Bonneville Environmental Foundation, and a low of 27,029 pounds as calculated by the EPA. 

The writer of the article, Claire Trageser, says, "The variation suggests tallies of carbon emissions have been oversimplified to produce a "one-click" solution to an extremely complicated problem -- global warming."

Odds are we are at the beginning of our understanding of how to calculate our usage and how to offset it, not to mention how to reduce it.