You haven't bought a thing for any of your 27 adorable nieces and nephews, and you dread the throngs of wild-eyed parents stalking Toys "R" Us this time of year, their sweaty desperation sweetly underscored by the schmaltzy tunes of "Jingle Bell Rocks" and "Grandma Got Run Over by a Reindeer." You're in holiday shopping denial. And you'll probably end up loading up your credit cards with overnight delivery charges at the last minute, buying out the Web's entire stash of 5-foot-tall plush Harry Potter goblets of fire on the afternoon of Dec. 23.

Not so fast, e-commerce Santa.

Consider this: How would your adorable little nieces and nephews feel if they knew your shameless procrastination contributes to the destruction of the planet, sacrificing their priceless futures for a few moments of greedy joy on Christmas morning? Don't you realize that all those air-shipped "next day" deliveries are five times as fuel-inefficient as gifts sent by plodding trucks?

OK, so the little buggers probably couldn't care less. But do you?

Now that we're fully in the throes of the ritualistic consumer frenzy that is the holiday shopping season, probably the last thing on most Net shoppers' minds is what impact all that clicking to buy has on the environment. The truth is, even policymakers, social scientists, environmentalists and engineers don't really know for sure. Researchers are only now beginning to study what e-commerce means for the Earth. The first major conference on the topic, the Joint Symposium on E-commerce and the Environment, in October in New York, brought together 100 researchers from the likes of Lawrence Berkeley National Laboratory and Ford Motor Co. to compare notes on everything from e-commerce and energy consumption to land use.

"Everyone is just starting to wake up and realize that e-commerce might have environmental effects that we aren't aware of," says H. Scott Matthews, a researcher with the Green Design Initiative, a faculty and student research group at Carnegie Mellon University that is conducting one of the few major studies of the issue.

But no one knows exactly what those effects might be. "Anyone who tells you that they've got this figured out is probably exaggerating," says David Rejeski, a former researcher at the Environmental Protection Agency who is now at the Woodrow Wilson School of Public and International Affairs at Princeton.

It's easy to imagine how good shopping online could be for the environment. After all, retail space with its hardwood floors, heating and cooling costs and huge parking lots can be a bigger drain on natural resources than warehouse space. And think of all the trips to the mall in the old gas-guzzling SUV that one efficient delivery truck speeding to and fro in a single neighborhood can eliminate. Plus, e-commerce cuts out entire steps in the distribution chain, presumably making the experience of buying online better for all. Click your way to a better world, baby!

But what such back-of-the-envelope analysis leaves out is the knotty vagaries of human behavior. We buy three shirts online only to ship back two that don't fit by air. We order virtually, but print out the receipts for our records. We buy five different books from Barnes & Noble online and have them sent in five different packages because we can't bother to wait for the total order to be filled. Wouldn't one trip to the bookstore have been more energy-efficient? And then there's the instant-gratification itch. It's the nagging impulse that says: "I want it now!" -- or, at the latest, tomorrow morning -- even if that means that an airplane will have to fly whatever it is to me from the other side of the country, so I won't have to bother to get off my couch.

Will shopping online bring new efficiency to our acquisitiveness or only give us one more way to clutter up the world with still more stuff? Should we be heartened by the news that the rate of growth in our energy consumption is slowing, or dismayed at how much easier it is than ever before to buy something we don't really need?

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