Across the country, cities are joining the latest environmental trend – banning plastic grocery bags. Concerned about the amount of plastic that reaches our oceans and the impact on wildlife, communities have decided that banning the bags is a simple and environmentally responsible approach.

But is it? What does the science say?

The evidence points to the fact that banning the bags may actually be a net negative for the environment, yielding little benefit to wildlife while significantly increasing carbon emissions and other environmental impacts.

Advocates of banning plastic grocery bags often cite impacts on marine life and mammals, but they rarely attempt to quantify that impact. Unfortunately, many attempts to quantify those impacts are simply false or misleading. For example, one city council in Washington state was told “the ecological impacts of this plastic include over a million sea-birds and 100,000 marine mammals killed by either plastic ingestions or entanglement.”

In fact, the claim about harm to marine mammals and sea-birds has nothing to do with plastic bags. NOAA corrected the claim about seabirds on its web page saying, “We are so far unable to find a scientific reference for this figure.” The only study NOAA can find does not deal with plastic bags or even marine debris, but “active fishing gear bycatch,” in other words, fishing nets that are being used at sea, not discarded plastic bags.

The Times of London addressed this very issue in 2008, even quoting a Greenpeace biologist saying, “It’s very unlikely that many animals are killed by plastic bags. The evidence shows just the opposite. We are not going to solve the problem of waste by focusing on plastic bags.”

One of the most commonly heard claims is that plastic bags, and other plastic, have created the “Pacific Garbage Patch.” Some claim it is twice the size of Texas. This is simply false. Last year, Oregon State University reported that the actual amount is less than one percent the size of Texas. Oceanography professor Angel White sent out a release last year saying, “There is no doubt that the amount of plastic in the world’s oceans is troubling, but this kind of exaggeration undermines the credibility of scientists.”

Additionally, White notes that the amount of plastic in the ocean hasn’t been increasing. For example, the Wood’s Hole Oceanographic Institute found the amount of plastic in the Atlantic Ocean hasn’t increased since the 1980s.

This doesn’t mean plastic bags cause no impact. When determining the environmental costs and benefits, however, we need to be honest about the science.

Indeed, there are risks from banning plastic grocery bags.

The most significant environmental risk from banning plastic bags is the increase in energy use. Plastic bags are the most energy-efficient form of grocery bag. The U.K. Environment Agency compared energy use for plastic, paper and re-usable bags. It found the “global warming potential” of plastic grocery bags is one-fourth that of paper bags and 1/173rd that of a reusable cotton bag. In other words, consumers would have to use a reusable cotton bag 173 times before they broke even from an energy standpoint. Thus, even if consumers switched to reusable bags, it is not clear there would be a reduced environmental impact.

Ironically, many of the cities leading the charge against plastic bags are signatories to the U.S. Conference of Mayors Climate Protection Agreement. Yet, few of these cities even attempt to assess the climate impact of switching from the least energy-intensive grocery bag to bags that use far more energy to produce.

The U.K. Environment Agency study is echoed by other research as well, and the reason is simple – grocery stores began using plastic bags in part because they are cheaper to produce, in part because they use less energy to manufacture.

Finally, it should be noted that the benefit of banning plastic bags is mitigated by the fact that half of the bags are used for other purposes, like for garbage bags or for picking up after pets. Grocery shoppers will still have to buy other bags, likely plastic, for those purposes.

In the end, communities need to sincerely weigh these various environmental costs. Unfortunately, few do any analysis because the political symbolism of banning the bags is powerful. It is often easier to ignore the science that indicates such bans may actually harm the environment than make an honest effort to weigh these difficult issues.

This is why plastic bag bans have become more environmental fad than environmental benefit.