CHAMPAIGN, Ill. — If you have a relative who is worried about global warming, it's hard to know what to buy him this Christmas.

Ipods, since they use less packaging, are greener than CDs, but they need regular recharging. Omaha Steaks are out, since cattle emit methane that creates greenhouse gases. A book from Amazon? If you are trying to avoid increasing Uncle Al's carbon footprint, you've got to worry about how much carbon went into making and delivering it.

Lots of people, however, are ready to take advantage of your concerns and sell you "carbon offsets." Most offer to take an action like planting trees or investing in wind farms that will reduce carbon dioxide emissions and so offset the carbon dioxide produced by your activities.

For example, using an online calculator from Terrapass, one of the leading offset sellers, I calculated that to offset my daughter's holiday flight home, I would need to offset 668 pounds of CO2. The cost: a mere $9.95.

There are three reasons to be suspicious about these offers. First, critics from British green activist George Monbiot to the free-market Competitive Enterprise Institute have pointed out that buying carbon offsets is awfully close to the medieval practice of buying indulgences for sins.

They're right. Carbon offsets don't actually change anyone's behavior — Al Gore still lives in a mansion that uses 20 times the electricity of the average home and flies all over the globe by private jet.

If you want to change your carbon footprint in the long run, you need to change your lifestyle. And if fighting global warming is a "compelling moral purpose," as Gore argues, buying one's way out of carbon sin is hardly appropriate.

Second, it's not clear that buying offsets actually reduces carbon emissions. One problem is that many offsets come with no guarantees that the promised activity actually takes place. Some companies, like Terrapass, tout their certification by third-party organizations such as the Center for Resource Solutions. But most Americans have never heard of CRS and have no reason to trust it; there's no role in the certification process for well-recognized, established third parties like the Better Business Bureau.

Even if vendors are actually spending the money to reduce carbon emissions, there's no way to know whether the contributions from the purchased offsets actually have an impact.

We don't know if more wind farms are being built as a result of the purchase of carbon offsets even if we can be certain that some wind farms are built. Business Week recently reported that a number of the businesses selling carbon offsets to Terrapass said that they would have done their projects even without Terrapass' investments.

Third, the carbon offset business has some circular aspects. Investor's Business Daily reported that Al Gore gets carbon offsets for his Nashville home from the private investment firm he co-founded and chairs, Generation Investment Management LLP.

The company buys the offsets for its employees, including Gore, as a benefit. Gore refused to comment on his financial relationship with the company to reporters investigating the offsets, but it is clear that Gore's climate activism promotes interests closely aligned to those of his company and that there is potential for Gore to profit by expanding the offset business.

How to have a greener holiday season? Throw carbon caution to the winds and order Bjorn Lomborg's book "Cool It" from your favorite bookseller.

The skeptical Danish professor brings some much needed perspective to the climate change debate, looking at the benefits today of carbon offsets compared to other charities. He does the math and it turns out that sending $5 to one of the organizations fighting HIV and malaria does $150-$200 worth of good; while the same $5 spent on carbon offsets provides about $2 in benefits.

Now there's a way to make people around the world have a truly happier holiday!

Andrew P. Morriss is the H. Ross & Helen Workman Professor of Law at the University of Illinois College of Law. This column was distributed by McClatchy-Tribune News Services.

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