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, polyethylene) 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.
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. 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.
Some interesting facts around plastics:
The world-wide production of plastic is currently at 35 kilogramm per year per person. On average, it is increasing by 3% per year.
Most of the plastic is used for packaging (35%), followed by the construction sector and vehicle construction.
About 40% of the produced plastic is being disposed within 1 year (primarily packaging material). The average lifetime of plastic is 12 years. The main reason is the relatively long lifetime in the construction sector.
Recycling of plastic saves on average about 2.5 kg CO2 per kg of plastic. Thus recycled plastic produces about 3.5 kg CO2 compared to 6 kg of CO2 for new plastic (production and incineration).
About 6% of the world-wide oil consumption is used for the production of plastic (with increasing tendency).
Source: Pusch, Thema Umwelt, 1/2009, p. 3