For much of the last century, buying stuff has meant driving to a store, at least for most Americans most of the time. According to the federal government, “shopping and errands” accounted for 35.4% of all household vehicle travel in 1990 – more than double that of 1969. But the trend is reversing: in 2009, shopping and errands accounted for just 30.7% of household vehicle travel. Other sources confirm that Americans are driving less to shop. The decline in leisure shopping is well-known. Less well-known is that Americans are taking fewer trips to the grocery store. Demographic change, revitalized urban centers and the “casualization of American life” all contribute to this trend. And then there’s the increasing popularity of online shopping.

Leaving aside a certain nostalgia for the in-person shopping experience, and the dilemma of what to do with all that empty real estate, and looking just at the net effect on CO2 emissions, I think the ascendancy of online shopping is a good thing.

With online shopping, vehicles, usually trucks, deliver purchases to many homes on a single route. On average, home delivery results in fewer emissions per item than get-it-yourself shopping.

The energy inefficiency of get-it-yourself shopping is partly offset when we buy a bunch of items in one trip. But it takes a whole lot of buying to neutralize the home delivery advantage: when customers buy fewer than 24 items per shopping trip, CO 2 emissions per item are still likely to be lower under home delivery.

Of course, some of us are able to walk, bike, or take public transportation to go shopping. But for most of us, and for most products, these are not reliable options. Plus, the larger stores have lower prices, and they tend to be in areas that favor cars, or at least would be time consuming and a big hassle to get to by other means. And that’s really important for people with limited time and budgets.

For instance, in one recent USDA study, the closest grocery store was an average of 2.0 miles away from low-income households, but the store primarily used for grocery shopping was, on average, 3.4 miles away. Value and not proximity predicted where these households shopped. A whopping 95% of those surveyed used a car to do grocery shopping.

Delivery trucks may also be more fuel-efficient than many cars, especially SUVs. UPS, for example, has invested millions of dollars in alternative fuel technologies, and its fleet includes thousands of low-emission, hydraulic, hydrogen fuel cell and electric vehicles. Fed Ex has a large hybrid fleet and is testing all-electric vehicles.

Delivery is only part of the emissions picture though. The environmental edge in home delivery may be lost, or at least minimized, by higher emissions at an earlier state of the product life cycle. For instance, e-commerce packaging practices – with all that shrink-wrapping, padding and boxing of individual items – is hardly a model of energy efficiency.

Next up we’ll zoom out and look at the entire product life cycle to better gauge the net emissions impact of online shopping.