According to 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 bags you are using per year. Then you can easily calculate the carbon dioxide emitted by your own usage of plastic bags. An average plastic grocery bag weighs 5.5 grams therefore 1 kg of plastic contains approximately 180 bags. Thus one bag generates about 33 grams of CO2. A household that uses 5 grocery bags per week would use approximately 250 bags per year; this represents 8.25 kilograms of CO2.
Use of plastic bags dropped by 53 per cent – or 215 million bags – in the three years after the fee was implemented, according to a 2012 City of Toronto report. According to the report, before the fee was introduced there were about 405 million bags used i.e. 2,250,000 grams of CO2 or 2.25 million tonnes (Mt CO2). This represents approximately 0.31% of Canada’s total annual emissions – just for Toronto.