Paper, Plastic, Cloth: Which is climate-friendliest?

I'm taking a moment from the global warming blogging to feature this mini-movie, "The Bay vs The Bag."  It's been produced by a campaign to ban plastic bags in the San Francisco Bay area. This is great messaging for a great cause: I admire the creativity of the fun animation, and the balance between the amusing images and the serious message.

The "Pledge to Use Reusable Bags" started by member Peggy Van Dooren remains a perennial favorite on Change.org. Peggy sums up the issues concisely, "When these bags are discarded, they break down, and harm the environment by killing marine mammals and contaminating soil and water."

Come to think of it, this does give me the chance to bust some myths around the plastic-vs-paper bag debate, as it relates to climate change.

In a nutshell, what's in the bag matters a lot more towards stopping global warming than what the bag is made of -- around 186 times more, according to some 2007 data crunching by a blogger at The Daily Score. "This number was calculated using the concept of embodied energy," wrote Justin Brandt, "the energy used to produce, transport, and dispose of a product over its entire lifetime. For food this includes making fertilizers, processing, transportation, storage, and cooking."

Using data from the most on-point study I could find, I calculated the energy used to produce, process, transport, store, and cook four servings of two different diets: the first, a meat-based diet that included beef, potatoes, tropical fruit, and drinks such as soda; the second a vegetable-based diet composed of produce grown within the country where is was consumed and a soy-based protein source.

The first diet takes 113 MJ (megajoules) of energy to get to the table, while the second takes 24 MJ of energy. The difference between these two numbers is 89 MJ.

In contrast, it takes about 0.5 MJ, give or take, to produce and dispose of one plastic bag.

The energy saved by a family of four that chooses a vegetable-based diet for one day would be equivalent to the energy needed to produce 186 plastic bags, or drive the average U.S passenger car over 15 miles, Justin calculated. Further, it takes over 20 times more energy to produce a paper bag than a plastic one.

So, if you're standing in the checkout line, transfixed by the "paper versus plastic" question because your canvas grocery tote is hanging on a hook back at home (reusables handily beat both plastic and paper on embodied energy), and worrying about global warming in particular, your best option really is to put the beef roast back in the meat case, grab a couple bags of dried beans, and make a nice pot of vegetarian chili for dinner.

That said, here are the substantive reasons using the thermoplastic petro-sacks is a bad idea:

• They break down so slowly in the landfill that they're essentially indestructible.
• They're a menace to wildlife, which mistake them for food and then die of starvation, their stomachs too full of plastic bag to take in any more nourishment.
• And they're a bane of storm water runoff systems. Hundreds of Mumbaikers were killed by flooding in 2005 because plastic bags choked the drains.