Are plastic bags better for the environment than their alternatives?

An increasing number of states in USA and other countries are enforcing a ban on the use of thin film plastic bags. As per an article by the authors in National Centre for Policy Analysis, plastic bags are better for the environment than reusable or paper bags, and there are no economic or environmental reasons for banning or taxing plastic bags. For an equivalent amount of groceries, production of paper bags requires three times more total energy and recovers only 1% of that energy through combustion. Paper bags also produce substantially more landfill waste. For an equivalent amount of groceries, single-use plastic bags produce 15.5 lbs of waste while paper bags produce nearly 75 lbs of waste. Paper bags also produce more greenhouse gases. Plastic bags generate 68% fewer greenhouse gases than composted paper bags, and consume 71% less energy during production than paper bags. Reusable bags may be the worst of all - requiring to be used 104 times to be less polluting than plastic bags. However, such bags are used only 52 times on average. Proponents of plastic bag bans primarily argue that such bans reduce the amount of waste entering landfills, lessen litter problems, help protect the environment and reduce petroleum consumption. However, none of these claims is supported by facts. Banning plastic bans is harmful to local economies and is not environmentally justified. The main reason policymakers give for banning thin-film plastic bags is the impact of the bags on the environment. However, the environmental effects of plastic bags are negligible - and in number of ways plastic bags are environmentally preferable to the alternatives.

Energy and Water Consumption:
Producing plastic, paper and other types of bags requires energy, but some of that energy can be recovered if bags are recycled through combustion:

Traditional plastic bags require only 182,361.4 kcal of energy to produce, but some 2,581.3 kcal of energy can be recovered through combustion.
Compostable plastic bags made of starch and other materials, require more than twice as much energy (494,741.9 kcal) to produce, but only 3,477.5 kcal can be recovered through combustion.
Paper bags fare the worst, with more than three times as much energy consumption as plastic bags (626,672.9 kcal), whereas only 6,859.5 kcal can be recovered through combustion.

Landfill Waste:
The same EPA study compared the weight of material entering the municipal waste stream, net of the material consumed by the combustion process, per 1,000 paper bags, 1,500 plastic bags and 1,500 compostable bags (for equivalent carry capacity):

The production, use and disposal of plastic bags produces a net 15.51 lbs of municipal solid waste.
Compostable plastic bags produce 42.32 lbs of municipal solid waste.
Paper bags produced the most municipal waste, nearly 75 lbs.

Thus, traditional plastic bags recover the largest percentage of energy. They also leave behind the smallest amount of municipal solid waste.

Water Use:
A study of Australian shopping bags found that of various alternatives - single-use plastic bags, compostable plastic bags, paper bags and reusable bags - paper bags had the worst energy and environmental impact with respect to global warming, land use, water use and solid waste. The study measured environmental impacts for the equivalent number of different types of bags - based on a functional unit of 520 paper, single-use plastic or compostable plastic bags, or 4.1 cloth bags. Production and use of plastic and compostable plastic bags consumed about 13.7 quarts of water (net), whereas cloth bags consumed about 52.8 quarts. The study found that single-use bags contributed 5.95 lbs of solid waste, whereas compostable plastic bags contributed only 1.83 lbs of solid waste. But reusable cloth bags contributed the most solid waste: 7.24 lbs.

Plastic Bags vs Paper Bags:
Plastic bags are significantly more environmentally friendly than paper bags. According to Use Less Stuff, an environmental advocacy group, plastic bags generate 39% less greenhouse gas emissions than uncomposted paper bags and 68% less greenhouse gas emissions than composted paper bags. Additionally, plastic bags consume less than 6% of the water needed to make paper bags. More than 16 plastic bags can be created for every one paper bag using the same amount of water. Plastic bags consume 71% less energy during production than paper bags. Using paper bags instead of plastic bags generates almost five times more solid waste.
The United Kingdom’s Environmental Agency evaluated nine categories of environmental impacts of paper and plastic bags. Paper bags were more environmentally harmful than plastic bags in every category: global warming potential, abiotic depletion, acidification, eutrophication, human toxicity, fresh water aquatic ecotoxicity, marine aquatic ecotoxicity, terrestrial ecotoxicity and photochemical oxidation.

Plastic Bags versus Cloth Bags:
Plastic bags are also noticeably more environmentally friendly than reusable cloth bags.
As reusable bags are made from cotton and other sources that require substantial amounts of farmland to produce, the production of cloth bags leads to destruction of forests in cotton producing regions. These farms can also increase erosion and lead to pesticides in drinking water. Cloth bags are much more challenging to recycle since they contain a combination of materials including metal, cotton and other fabrics. The United Kingdom’s Environmental Agency determined that though cotton bags have to be used 104 times before their environmental performance surpasses that of plastic bags, the average cotton bag is only used 52 times. As a result, cloth bags have twice the negative environmental impact of plastic bags.

Litter:
Studies show that plastic bags represent a tiny portion of litter and that banning them has not reduced the amount. Nationwide studies show that plastic bags constitute no more than 1-2% of all litter, on average. According to the Keep America Beautiful campaign, plastic bags are not one of the top 10 sources of litter nationwide.

Recycling:
A much larger percentage of plastic bags are recycled today than 10 years ago. According to a survey conducted by Moore Recycling Associates, the number of bags recovered increased 27% between 2009 and 2010. These bags made up approximately 13% of the total film and bag material recovered in 2010. This amounted to approximately 127 mln lbs of plastic bags recycled in 2010, compared with 100 mln lbs in 2009.
Importantly, there are no environmental benefits to banning plastic bags - but there is potential harm. Compared to cloth bags, plastic bags require less energy to produce and less energy to recycle and produce less municipal waste. Plastic bags generate fewer greenhouse gas emissions and require less water to produce than paper bags.