A number of people have asked about the implications of using plastic bags on the personal carbon footprint as well as on the environment in general. There are some comparisons between paper bags and plastic bags available which clearly show that it all depends on how many times these plastic or paper bags are being used.
Littering is probably the severest problem related to plastic bags. Nevertheless let's now have a look at the carbon dioxide (CO2) emissions for the production and incineration of plastic bags.
The carbon footprint of plastic (LDPE or PET, poyethylene) is about 6 kg CO2 per kg of plastic. If you know the weight of your plastic bags, you can multiply it with the number of plastic bag you are using per year. Then you can easily calculate the carbon dioxide emitted by your own usage of plastic bags. See below for some background information.
Of course you'll find different figures on the Internet. The main factors are the weight of the plastic bag and whether the grey energy (energy used for production and disposal) is taken into account.