Plastic bags are made from ethane (C2H6) which is a byproduct from production of natural gas and oil. Ethane has a global warming potential 5.5 times that of carbon dioxide according to the IPCC.
Ethane has a high heat capacity and makes natural gas too unstable to be used in industrial processes and home heating. Therefore ethane is removed and usually burned off.
A new study from the University of Colorado Boulder (see press release) finds that a steady decline of global ethane emissions following a peak in about 1970 ended between 2005 and 2010 in most of the Northern Hemisphere and has since reversed. The decline in ethane emissions after 1970 coincides with the growing widespread use of plastic bags.
The recent increase in ethane emission could be attributed to the recent spectacular increase in natural gas production from fracking. However, another factor could be the decreasing production of plastic bags due to the banning of such use by various municipalities and other governments. California has a statewide ban on use of plastic bags. (See my wryheat article “Tilting at plastic bags” and the ADI article “Plastic Patrol: Sunday’s Comic”)
Less production of plastic bags may mean that more ethane will be burned or simply released into the atmosphere.
The University of Colorado researchers say, “Chemical models by the team show that the increase in ethane and other associated hydrocarbons will likely cause additional ground-based ozone production, particularly in the summer months.”
“There are really good things about plastic bags—they produce less greenhouse gas, they use less water and they use far fewer chemicals compared to paper or cotton. The carbon footprint— that is, the amount of greenhouse gas that is produced during the life cycle of a plastic bag—is less than that of a paper bag or a cotton tote bag. If the most important environmental impact you wanted to alleviate was global warming, then you would go with plastic.” (Source)
The eco-fad of discouraging or banning use of plastic bags may have the unintended consequences of increasing global warming and producing more ozone smog. It seems that “saving the environment” is complicated.