Q: Is online shopping likely to save on carbon emissions or not? How do the carbon emissions compare between me driving a 15-mile round-trip to the bookstore versus the carbon emissions for Amazon to ship the book through their warehouse and UPS to me, including the extra packaging?

Asked by Dan Scales, MS ’86, PhD ’96, Mountain View, Calif.

Not such a long time ago, the idea of purchasing goods online was somewhat exotic. But online shopping has become firmly entrenched in American consumer culture. Ninety-four percent of Americans have made an online purchase, and nearly 30 percent of holiday shopping is done entirely online. I’m a big believer that shopping on the World Wide Web is generally secure and convenient, and enables consumers to find bargain prices on everything from books and electronics to groceries and clothing. I even bought my mattress online last year, and had it delivered right to my apartment.

Books are by far the most popular items purchased through the Internet. In just the past two years, the number of consumers buying books online rose by nearly 10 percent. Most patronize book “e-tailers” because of lower prices, but done right, online bookselling also has a smaller carbon footprint.

Like any good novel, the story of how a bookworm gets her book has a beginning, a middle and an end. A book destined for a brick-and-mortar store is printed, packed in bulk, transported by heavy-duty truck to a publisher’s warehouse, transferred to an intermediate warehouse or two, and delivered to the bookstore. Customers might then drive 15 or more miles round trip to purchase the exciting new title. A book sold online has a slightly different plot line: after arrival at the publisher’s warehouse, air or freight travel to a sorting center and individual repackaging, its dramatic finale is home delivery by light-duty truck.

Transportation is the biggest contributor to carbon emissions in both retail and e-tail product pathways. When purchasing a book from a bookstore, each household drives separately, but delivery trucks take purchases to many customers on a single route. There’s also a decent chance that the delivery truck is more fuel-efficient than your family sedan. UPS, for example, has invested millions of dollars in alternative fuel technologies, and as of 2008, its fleet included more than 10,000 low-emission, hydraulic, hydrogen fuel cell and electric vehicles.

When it comes to packaging, however, brick-and-mortar bookshops generally claim the environmental edge. Shrink-wrapping, padding and boxing each individual novella, as e-tailers do, is hardly going to maximize materials efficiency and minimize waste. (Walking to a used bookstore, or downloading an ebook, will do exactly that—but we haven’t been asked about those options yet!)

Both online and brick-and-mortar booksellers operate climate-controlled storage warehouses, but retailers usually own or lease additional storage and distribution facilities. Likewise, the energy consumed to browse and purchase books online is much less than that needed to build, light, heat, and cool physical bookstores. By streamlining the purchase and delivery process, e-tailers minimize the need for buildings and their associated energy usage.

But we haven’t turned the final page of our carbon emissions whodunit quite yet. Human behavior can provide a plot twist that strongly affects the environmental impact of online and real-world book purchases alike. Common sense applies: buying multiple books from the same seller results in fewer emissions than ordering one at a time, and of course walking or biking to the bookstore, or combining book purchases with other errands in the area, can draw down the carbon footprint of a bookstore visit. And while it may be tempting to overnight the latest Twilight installment, express shipping means air transport, and that could consume as much as five times the fuel used in ground delivery.

Ready for the postscript? Even the most technologically advanced of the literati might be satisfied with the contemporary public library. With many now equipped with Internet catalogs and special-order capabilities, libraries provide the ultimate low-emissions setting for reading happily ever after.

Anna Argyridou received her MS in environmental engineering and science in June 2009.