Guest Column: What’s your real ‘carbon footprint’?

What’s your carbon footprint? If you’ve tried to find out what that means, chances are you’ve only gotten half the story.

The notion of a personal “carbon footprint” was created by global warming alarmists to foster a sense of individual accountability with respect to the consumption of energy produced by the burning of coal, oil and natural gas. The idea is that the more energy from these sources you use, the more carbon dioxide (CO2) you cause to be released into the atmosphere, thereby contributing to global warming.

British oil company BP is running television ads featuring hapless man-on-the-street interviewees perplexed by the question, “What’s your carbon footprint?” You can calculate your carbon footprint at BP’s Web site or at a host of other Web sites (results are typically expressed in metric tons of carbon dioxide).

The more entrepreneurial of these Web sites then try to capitalize (literally) on any feelings of guilt you may have by offering to relieve your conscience (and wallet) through the purchase of so-called “carbon offsets.” For prices ranging from $5.50 to $30 per metric ton of CO2 (mt-CO2) emitted annually, you can purchase “off-sets” from sellers who claim they will use your money to neutralize or offset your personal CO2 emissions—by planting trees or supporting wind/solar energy projects, for example. The goal of becoming “carbon neutral”—that is, not contributing to atmospheric CO2 on a net basis—is supposed to erase your footprint and, thereby, alleviate your guilt.

We won’t quibble with the precision of the various estimates of CO2 emissions that you might get from the different carbon footprint calculators. They’re extremely crude, and likely overestimate or underestimate your actual “footprint,” but they’re a useful starting point for considering the question of your personal impact on global temperature.

You’ll still want to hold on to your wallets, though. Those who seek to profit from the carbon footprint concept would have you believe calculating your CO2 emissions is the first—and last—step in discovering your impact on global temperature. But there’s much more to it than that.

At, we’ve constructed “The Real Carbon Footprint Calculator” (—an online tool enabling you to input your estimated CO2 emissions and to find out your potential impact on global temperature. You can also discover how much it would cost to avoid a single degree of warming based on the price of a carbon offset you might be considering purchasing.

Let’s run though a sample calculation.

The average “carbon footprint” for a U.S. household is 19 metric tons of CO2, according to BP’s Web site. Based on that figure, we calculate the average U.S. household contributes, at most, an infinitesimal 0.0000000000148 degrees Fahrenheit annually to average global temperature—a trivial contribution, even when multiplied by the 100 million U.S. households.

Moreover, the carbon offset hucksters have created such a price scale that, if you wanted to purchase enough offsets at the low-end price of $5.50/mt-CO2 to potentially avoid 1 degree Fahrenheit of warming based on the average household’s “carbon footprint,” they would reap about $7.04 trillion. An offset purchase price of $30/mt-CO2 makes the price of potentially avoiding 1 degree Fahrenheit of warming—and their potential gross revenue—a whopping $38.6 trillion.

In constructing the calculator, we decided to err on the side of overestimating temperature effects of CO2 emissions by assuming the relationship between atmospheric CO2 and global temperature is linear—that is, every unit increase in atmospheric CO2 causes a constant unit increase in global temperature.

We know this assumption greatly overestimates the CO2-temperature relationship since, in reality, the relationship is logarithmic in nature—that is, every unit increase in atmospheric CO2 actually has a diminishing effect on global temperature.

Our calculator also errs on the side of overestimating the potential temperature increase related to your carbon footprint by relying exclusively on land-based measurements of historical temperature data.

Land-based temperature data indicate the average global temperature increased about 0.07 degrees Centigrade per decade during 1880-2005. In contrast, when ocean-based measurements are factored in, average global temperature has increased by only 0.04 degrees Centigrade per decade.

We believe our calculator overestimates the temperature impact of your carbon footprint and underestimates the cost of potentially avoiding 1 degree Fahrenheit of warming by a factor ranging anywhere from 100 to 1,000.

We erred on the side of overestimating for two reasons. First, test users had trouble juggling too many leading zeroes in the temperature effect—so we limited the number of leading zeroes to 10, rather than 12 or 13. Next, we wanted to give the carbon footprint promoters every break possible in terms of potential temperature impact of CO2.

Your carbon footprint? Carbon offset-buyer, beware. It’s a gimmick designed to part you from your money without providing any measurable environmental benefit.

Steven Milloy publishes and He is an adjunct scholar at the Competitive Enterprise Institute.

From the May 24-30, 2006, issue

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