Any renewable shopping bag is better than a disposable bag. However, the production of cotton has numerous negative impacts. The World Wildlife Fund states that just 1kg of cotton requires 20,000L of water to produce. Other impacts from cotton farming include soil erosion, and soil/water pollution. Lighter cotton bags use less water to produce, however they still contribute to the pollution caused by chemical fertilisers and pesticides. Cotton tote bags are better than disposable bags, but they are not the best option for the environment.
Polyester bags are fractionally more environmentally friendly than polypropylene bags. Pablo Paster states that whilst the greenhouse gases emitted in making a 35g polyester bag are equivalent to the manufacturing of 7 disposable plastic bags, polypropylene bags come in slightly higher, at the equivalent of 11 disposable bags.
However, the most environmentally friendly option is to make your own bag out of old fabric. The environmental damage has already been done in producing the material, but upcycling it into a bag prevents it from entering landfill.
Cole, Lisa, 'What is the most eco friendly shopping bag money can't buy? A Morsbag!', Less Stuff (2017), <https://www.less-stuff.co.uk/eco-shopping-bag-morsbag/>
Paster, Pablo, 'Reusable Shopping Bags: Which is the Greenest of Them All?', Tree Hugger (March 2009), <https://www.treehugger.com/clean-technology/reusable-shopping-bags-which-is-the-greenest-of-them-all.html>
World Wildlife Fund, 'Cotton: Overview' (2020), <https://www.worldwildlife.org/industries/cotton>