http://www.justfacts.com/pollution.asp#trash-plastic

In 2011, the United Kingdom’s Environment Agency released a study that evaluated nine categories of environmental impacts caused by different types of supermarket bags. The study found that paper bags have a worse effect on the environment than plastic bags in all nine impact categories, which include global warming potential, abiotic depletion, acidification, eutrophication, human toxicity, fresh water aquatic ecotoxicity, marine aquatic ecotoxicity, terrestrial ecotoxicity, and photochemical oxidation.

About those cloth bags? Disease spreading microbe incubators, dangerous to anyone silly enough to use them.