One contributor to personal carbon footprint that we all city dwellers leave is the use of plastic bags while shopping.

Says a post in Time for Change…
“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.

The production of 1 kg of polyethylene (PET or LDPE), requires the equivalent of 2 kg of oil for energy and raw material (see here). Polyethylene PE ist the most commonly used plastic for plastic bags.
Burning 1 kg of oil creates about 3 kg of carbon dioxide (see e.g. our offline carbon footprint calculator). In other words: Per kg of plastic, about 6 kg carbon dioxide is created during production and incineration.
A plastic bag has a weight in the range of about 8 g to 60 g depending on size and thickness. For the further calculation, it now depends on which weight for a plastic bag you actually use. A common plastic carrying bag in our household had a weight between 25 g and 40 g. So I took the average of 32.5 g.
Take the above relation between kg plastics and kg of carbon dioxide, and you get about 200 g carbon dioxide for 32.5 g of plastic, which is the equivalent of the average plastic carrying bag in our household. Or in other words: For 5 plastic bags you get 1 kg of CO2.
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.”

http://timeforchange.org/plastic-bags-and-plastic-bottles-CO2-emissions

and another post talks about how this is being tackled in India.

http://blog.mywonderfulworld.org/2011/11/blog-a-thon-hey-plastic-youre-not-welcome-here.html