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If you think you're saving the environment by shopping from home in your pajamas, think again. The carbon footprint you leave behind when you buy holiday gifts online may be bigger than if you'd hopped into your car and driven to the mall.

Jerry Storch, CEO of brick-and-mortar retailer Toys R Us, calls shopping online "very un-green." He recently told the Financial Times, "[People are] just so enraptured how cool it is that they can order anything and get it brought to their home that they aren't thinking about the carbon footprint of that. But that will change."

Amazon.com sees things differently. It states flatly on its website that, "Online shopping is inherently more environmentally friendly than traditional retailing. The efficiencies of online shopping result in a greener shopping experience than traditional retailing."

So, who's right? Which form of shopping is more eco-friendly? The answer, say experts, depends on a host of variables.

A UPS or FedEx truck, for example, packed full of Amazon.com packages and following an efficient route, consumes far less fuel per package than you will if you drive to the mall and make a single purchase. Moreover, some package delivery companies have begun to convert their truck fleets to natural gas or other fuels more friendly to the environment than gasoline. That further shrinks the carbon footprint of ordering online.

Jason Mathers, a senior manager at the Environmental Defense Fund, agrees that's true in urban settings where delivery stops are close together, but it's less true if the deliveries are rural, with stops father apart. Delivering online purchases by truck in rural Connecticut, for example, he calls "hardly the greenest way to shop."

Mathers says the greenness of online shopping depends upon a wide variety of factors, including the type of vehicles used, the distances driven, the number of products per shipment, the rate of product returns, and the environmental cost of the packaging.

The Logistics Research Center at Heriot-Watt University did a carbon audit of conventional versus online shopping in 2009. They found that while "neither home delivery nor conventional shopping has an absolute advantage, on average home delivery is likely to generate less [carbon] than the typical shopping trip."

Transportation, notes Mathers, is only one component of a product's carbon cost. When Apple, he says, analyzed the carbon cost of an iPad, it found that transportation amounted to only 10 percent. So, he says, if saving carbon is your goal, the channel through which you buy-online or from a brick and mortar store-is relatively irrelevant.

Shoppers wanting to be eco-friendly, he says, should instead focus on other issues. Ask yourself: Do I really need to buy this, at all? If not, you've saved yourself and the planet heap of carbon right there. If yes, do you really need to buy it new? Buying a product used, he says, will save energy, no matter whether you buy online or in a store.