For the "no such thing as a small decision" file . . . .
A new study in Sustainable Production and Consumption reports that one person eating a store-bought sandwich five days a week produces as much C02 as does a car that drives for 12 hours. Multiply by everyone eating a sandwich for lunch, and .... you get the idea.
Home-made sandwiches, on the other hand, are less harmful for the environment and as well as easier on the wallet.
Read a summary of the study here.