Welcome to your Personal Commitment Summary, from here you'll be able to edit your previous choices and see the relevant changes they might have.
Here's all the juicy good stuff of how we calculate the numbers.
Greenhouse gases, based on 1 mile of travel
Data from 2012 Guidelines to Defra/DECC's GHG Conversion Factors
Number of days associated with frequency
Amount of people involved in journey
Data from TfGM and Wikipedia
Average weights of items used in comparisions
Data from Wikipedia